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1 PRESSURE AND SPACING EFFECT OF SPRINKLER IRRIGAT I ON FOR COLD PROTECTION IN STRAWBERRIES By MARIA ISABEL ZAMORA RE A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIRE MENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2013
2 2013 Mara Isabel Zamora Re
3 To God And to my parents Ricardo Zamora and Claudia Re
4 ACKNOWLEDGMENTS Foremost, I would like to tha nk God for giving me the life and strength to live this opportunity, but even more for letting me successfully finish this step in my life. Moreover, I would like to thanks my parents and family, who had been a shoulder of support in which I could found un conditional love and encouragement; regardless the distance between us. And also, I want to thank my friends who have listened, advised and encouraged me to keep strongly walking reaching the goal. I would like to acknowledge and thank Danny Burch for his magnanimous contributions, hard work and outstanding support during these two years working to express my gratefulness towards Dr. Hubert Werner, who gave me statisti cal advice, for all his patience and guidance. I would like to thank my research group and also ABE members: Pat Rush, Michael Gutierrez, Sara Winn, Eliza Breder and PSREU personnel, who were involved in the development of this research: Buck Nelson, Patri ck Penny, Timothy Pedersen, Leonard Novinger, Peter DuBose, Mark Kann, Dave Carson, Joel Berry and any other member who made this project possible. I would like give thanks to the graduate committee members: Dr. Kati Migliaccio, Dr. Craig Stanley and Dr. L incoln Zotarelli for their guidance. And last but not least, I want to give thanks to my advisor, Dr. Michael Dukes, for the opportunity of letting me work in this project, for his patience and for helping me grow throughout this learning process. This pro ject was supported by the Southwest Florida Water Management District. I really appreciate and give my sincere thanks to everyone who has given some time, support and helped me grow during this time.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 L IST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF ABBREVIATIONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 13 CHAPTER 1 LITE RATURE REVIEW: SPRINKLER IRRIGATION COLD PROTECTION ........... 15 Introduction ................................ ................................ ................................ ............. 15 Strawberry Description ................................ ................................ ............................ 16 Strawberry Production and Importance ................................ ................................ ... 17 Strawberry Irrigation Systems ................................ ................................ ................. 19 Drip Irrigation ................................ ................................ ................................ .... 19 Sprinkler Irrigation ................................ ................................ ............................ 19 Microsprinklers ................................ ................................ ................................ 20 Historical Data Temperatures ................................ ................................ ................. 20 Energy Balance ................................ ................................ ................................ ...... 21 Latent Heat Transfer ................................ ................................ ............................... 23 Evaporative Cooling ................................ ................................ ......................... 23 Psychrometrics ................................ ................................ ................................ 24 Frost Damage ................................ ................................ ................................ ......... 25 Cell Injury ................................ ................................ ................................ ......... 25 Plant Sensitivity ................................ ................................ ................................ 26 ................................ ................................ ........ 26 Crop Sensitivity and Critical Temperatures ................................ ...................... 27 Advective Frosts ................................ ................................ ............................... 29 Radiative Frosts ................................ ................................ ............................... 29 Economic Im portance of Cold Protection ................................ ................................ 30 Sprinkler Irrigation Cold Protection ................................ ................................ ......... 30 Sprinkler Uniformity ................................ ................................ .......................... 32 Frequency of Application ................................ ................................ .................. 34 Application Rate ................................ ................................ ............................... 36 2 EFFECT OF SPRINKLER TYPE AND PRESSURE ON IRRIGA TION UNIFORMITY ................................ ................................ ................................ ......... 54 Introduction ................................ ................................ ................................ ............. 54 Space Pro for Simulation ................................ ................................ .................. 56 Over Head Sprinkler Irrigation for Cold Protection ................................ ........... 57
6 Materials and Methods ................................ ................................ ............................ 60 Uniformity Testing Analysis ................................ ................................ .............. 60 Sprinklers ................................ ................................ ................................ ......... 61 Collectors and Spacing ................................ ................................ .................... 61 Irrigation System Supply Pressure ................................ ................................ ... 62 Irrigation Simulation Using SPACE Pro ................................ ............................ 63 Data Analysis ................................ ................................ ................................ ... 64 Results and Discussion ................................ ................................ ........................... 65 Wind Conditions ................................ ................................ ............................... 66 Sprinkler Irrigation Uniformity Testing ................................ .............................. 66 Citra ................................ ................................ ................................ ........... 66 Hastings ................................ ................................ ................................ ..... 69 Application Rate ................................ ................................ ............................... 71 Citra ................................ ................................ ................................ ........... 71 Hastings ................................ ................................ ................................ ..... 74 Space Pro Results ................................ ................................ ............................ 75 Distribution Uniformity ................................ ................................ ................ 76 Application Rate ................................ ................................ ......................... 82 3 EFFECT OF SPRINKLER PRESSURE AND SPACING ON STRAWBERRY YIELD DURING COLD PROTECTION ................................ ................................ 104 Introduction ................................ ................................ ................................ ........... 104 Critical Temperatures and Cold Damage ................................ ....................... 107 Sprinkler Irrigation for Cold Protection ................................ ............................ 109 Materials and Methods ................................ ................................ .......................... 110 Treatments ................................ ................................ ................................ ..... 110 Strawberry Field Experiment ................................ ................................ .......... 111 Plot Description and Harvest Protocol ................................ ............................ 112 Temperature ................................ ................................ ................................ ... 113 Thermocouples ................................ ................................ ........................ 113 Wireless sensors ................................ ................................ ...................... 114 Experimental Design ................................ ................................ ...................... 115 Results ................................ ................................ ................................ .................. 115 2011 2012 Season ................................ ................................ ......................... 116 Yields ................................ ................................ ................................ ....... 116 Volume of water applied for cold protection ................................ ............. 119 Cold events according to the crop stage critical temperature ................... 119 2012 2013 Season ................................ ................................ ......................... 120 Volume of water applied for cold protection ................................ ............. 124 Cold events according to the crop stage critical temperature ................... 124 Low Quarter Distribution Uniformity and Application Rate Scenarios for the Treatments ................................ ................................ ................................ .. 125 Uniformity ................................ ................................ ................................ 125 Application rate ................................ ................................ ........................ 125 Conclusions ................................ ................................ ................................ .......... 126
7 4 CONCLUSIONS AND FUTURE WORK ................................ ............................... 141 Conclusions ................................ ................................ ................................ .......... 141 Future W ork ................................ ................................ ................................ .......... 145 APPENDIX : IRRIGAT ION COMPARISON DURING CRITICAL TEMPERATURES FOR STRAWBERRIES ................................ ................................ ......................... 146 LIST OF REFERENCES ................................ ................................ ............................. 153 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 16 0
8 LIST OF TABLES Table page 1 1 Citrus cold protection application rate recommendation according to minimum temperature expected and wind speed conditions. ............................. 45 1 2 Critical temperature of the blossoms, and young fruits of strawberries for four stages. ................................ ................................ ................................ ................ 45 1 3 Assessment of survival and minimum temp erature reached for exposed and covered blossoms in open and popcorn stages for four strawberry cultivars. .... 45 1 4 Comparison of heat consumed through evaporation with heat released through free zing. ................................ ................................ ................................ 46 1 6 Application rates for overhead sprinkler protection of tall and short crops. ......... 47 1 7 Application rates comparison betw een computer model and AR from Gerber and Martsolf ................................ ................................ ................................ ........ 48 1 8 Minimum starting and stopping air temperatures for frost protection with sprinklers as a function of wet bulb and dew point temperature ......................... 49 1 9 Dew point temperature corresponding to air temperature and relative humidity. ................................ ................................ ................................ ............. 50 2 1 Expected DU lq values according to the sprinkler type. ................................ ........ 88 2 2 Manufacturer recommendations for sprinklers tested at Citra and Hastings, FL. ................................ ................................ ................................ ...................... 88 2 3 Uniformity and application rate averages for four sprinkler types measured over three irrigation system supply pressures at 14.6 m sprinkler spacing and variable wind cond itions. ................................ ................................ ................... 89 2 4 Uniformity and application rate averages for four sprinkler types measured over three irrigation system supply pressures at 12.2 m sprinkl er spacing and variable wind conditions. ................................ ................................ ................... 89 2 5 The p values of the F test statistic for interactions from the ANOVA for distribution uniformity performed near Citra, FL ................................ ................. 90 2 6 Uniformity and application rate for WR 32 impact sprinkler performed n ear Hastings, FL. ................................ ................................ ................................ ...... 90 2 7 The p values of the F test statistic for interactions from the ANOVA for irrigation uniformity and application rate near Citra and Hastings, FL. .............. 90
9 2 8 The p values of the F test statistic for interactions from the ANOVA for application rate obtained from catch can tests performed near Citra, FL. .......... 91 2 9 WR 32 sprinkler distribution uniformity and application rate mean comparison among values resulted from the SPACE Pro sprinklers simulations and field data ................................ ................................ ................................ ................... 92 2 10 Nelson R33 sprinkler distribution uniformity and application rate mean comparison among SPACE Pro s imulations and field data ............................... 92 3 1 Citrus cold protection application rate recommendation ................................ .. 128 3 2 Strawberry critical temperatures at different crop stages calculated using dew point and wet bulb temperatures ................................ ................................ ..... 128 3 3 Treatments evaluated 2011 2013. ................................ ................................ .... 129 3 4 Summary water applied and water savings per tre atment during two years of field results. ................................ ................................ ................................ ...... 129 3 5 Treatment means and pairwise comparison tests. 2011 12 season. ................ 129 3 6 P values of the ANOVA F test statistic for treatment means through recovery cold periods during 2011 2012 season. ................................ ............................ 130 3 7 Mean Comparison on recovery cold periods with significant differences between irrigated treatments during the 2011 12 season. ................................ 130 3 8 Amount of water applied during the cold events and percent water savings per treatment. 2011 12 season. ................................ ................................ ....... 131 3 9 Treatment yield means and pairwise comparison tests 2012 13 season. ....... 131 3 10 P values of the ANOVA F test statistic for treatment means through recovery cold periods during 2012 2013 season. ................................ ............................ 132 3 11 Me an Comparison on recovery cold periods with significant differences between irrigated treatments during the 2012 13 season. ................................ 132 3 12 Amount of water applied during the cold events and percent w ater savings per treatment. 2012 13 season. ................................ ................................ ....... 133
10 LIST OF FIGURES Figure page 1 1 Amount of hours below freezi ng point and below irrigation system turn ON temperature Historical Seasonal Years from 1979 to 2013 ............................... 51 1 2 Effect of three frequencies of application over leaf temperature ........................ 52 1 3 Short crops application rates. Over plant conventional sprinkler application rate requirements for frost protection of short crops ................................ .......... 53 2 1 Single leg profile tests performed using WR 32 and Nelson R33 sprinklers at three pressure levels in order to develop uniformity profiles throughout simulations performed i nto SPACE Pro program. ................................ ............... 93 2 2 Collector layout for testing area ................................ ................................ ......... 93 2 3 Testing areas at different sprinkler and collector spacing s . ............................... 94 2 4 Pressure regulator used to keep steady pressure conditions dur ing the performance of DU lq tests. ................................ ................................ .................. 94 2 5 Sprinkler pressure monitoring using pressure gauges and regulators placed at the bottom of the sprinkl er heads. ................................ ................................ .. 95 2 6 Low quarter distribution uniformity as a function of sprinkler and pressure interaction near Citra, FL ................................ ................................ .................. 95 2 7 Effect of sprinkler type and sprinkler spacing on low quarter distribution uniformity near Citra, FL .. ................................ ................................ .................. 96 2 8 Effect of irrigation pressure and sprinkler spacing over low quarter distribution uniformity near Citra, F L ................................ ................................ 96 2 9 Low quarter distribution uniformity (DU lq ) as a function of pressure location interaction tested near Citra, and Hastings FL. ................................ ................... 97 2 10 Effect of wind speed over low quarter distribution uniformity of the WR 32 impact sprinkler evaluated near Citra and Hastings FL. ................................ ..... 97 2 11 Application rates as a function of the sprinkler type pressure interaction, near Citra, FL.. ................................ ................................ ................................ ............ 98 2 12 Effect of sprinkle r type spacing interaction over application rate near Citra, FL.. ................................ ................................ ................................ ..................... 98 2 13 Application rate as a function of the sprinkler pressure spacing interaction evaluated near Citra, FL. ................................ ................................ .................... 99
11 2 14 Application rate obtained from using WR 32 impact sprinklers at 14.6 m spacing at three pressure levels evaluated near Citra, and Hastings FL.. ........ 100 2 15 Distribution uniformity and application rate comparisons among SPACE Pro simulations and field data using WR 32 and R33 sprinklers ............................ 101 2 16 WR 32 sprinklers distribution uniformity densograms from the SPACE Pro sprinklers simulations ................................ ................................ ...................... 102 2 17 Nelson R33 sprinklers distribution uniformity (DU lq ) densograms resulted from the SPACE Pro sprinklers simulations ................................ .................... 103 3 1 Strawberry harvesting layout. ................................ ................................ ........... 134 3 2 Weighted marketable weight per treatment during harvest season 2011 12. . 135 3 3 Cumulative marketable weight per treatment during harvest season 2011 12 136 3 4 Weighted marketable weight per treatment during harvest season 2012 13. ... 137 3 5 Cum ulative marketable weight per treatment during harvest season 2012 13. 138 3 6 Low quarter distribution uniformity values for WR 32 impact sprinklers corresponding to the SPC AC, GROW and LOW treatments evaluated under cold conditions at Citra, FL. ................................ ................................ .............. 139 3 7 Application rates resulted from the evaluation of WR 32 impact sprinklers corresponding to the SPC, AC, GROW and LOW treatments evaluated under cold conditions at Citra, FL. ................................ ................................ .............. 140
12 LIST OF ABBREVIATIONS AC Automated t reatment. Treatment with an automatic i rrigation system control for cold protection based on average air temperature and dew point temperature measured by w ireless sensors AR Application rate DP Dew point Temperature DU LQ Low Quarter Distribution Uniformity GROW GROW Treatment replic protection using a 345 kPa at the irrigation system pressure, 14.6 m s prinkler spacing and a thermostat/thermocouple to turn on/off the irrigation system for cold protection LOW LOW t reatment Treatment that c ons ists of reducing the pressure in the ir rigation system supply to 207 kP a and following the other strawberry generally used conditions for cold protection ( 14.6 m sprinkler spacing and the use of a thermostat/thermocouple to turn on/off the irrigation syste m ) NO NO treatment. Treatment without sprinkler irrigation for cold protection (non irrigated plots) which constituted the control treatment for comparison SPC SPC treatment. Treatment which consisted on reducing the sprinkler spacing to 12.2 m and followi ng the other strawberry generally used conditions for cold protection (345 kPa irrigation system pressure and the use of a thermostat/thermocouple to turn on/off the irrigation system) TC Critical damage temperature. Temperature at which plant damages can occur if reached
13 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science PRESSURE AND SPACING EFFECT OF SPRINKLER IRRIGATION FOR COLD PROTECTION IN STRAWBERRIES By Mara Isabel Zamora Re December 2013 Chair: Michael D. Dukes Major: Agricultural and Biological Engineering The United States is the largest strawberry producing country in the world and Florida ranks as the second la rgest producing state To achieve high profits, strawberries are planted during the winter and protection from cold damage is needed. Irrigation is the primary means for cold protection. During extreme cold events recently experienced in the Dover/Plant Ci ty area of Florida, high volumes of irrigation used to protect the plants caused the aquifer level to drop 18.3 meters, about 750 residential wells were impacted and over 140 sinkholes were reported. Although sprinkler irrigation for cold protection has be en effective for the past several decades, the reco mmended application rate (AR) of 6.35 mm hr 1 has not been revised neither t he effectiveness of alternative rates for satisfactory p rotection The objectives for this project were to: (i) investigate cold protection practices in ARs (ii) assess the effect of sprinkler type spacing, irrigation system pressure variations and varied wind conditions over irrigation distrib ution uniformity (DU lq ) and AR, and (iii) evaluate the effect of varying sprinkler spacing and pressure on strawberry yield quality and quantity under cold conditions. Four sprinkler types: Wade Rain WR 32 impact
14 sprinklers and three Nelson rotators: R33 R 33LP and R2000WF were evaluated at three pressures (345, 276 and 207 kPa), two spacings (14.6 and 12.2 m) and varied wind conditions. The interactions sprinkler type pressure, sprinkler type spacing and pressure spacing had a significant effect on DU lq an d AR as well as the presence of high wind conditions. Significantly higher DU lq values were obtained by R2000WF and WR 32 at 345 kPa and 12.2 m spacing, by contrast uniformity was significantly reduced at 207 kPa and 14.6 m spacing. Higher wind speed redu ced significantly the uniformity. Nelson R33 and R33LP obtained significantly higher AR at all pressure levels and 12.2 m spacing. By contrast, the lowest AR were obtained by WR 32 and R2000WF at 207 kPa and 14.6 m spacing. Under cold conditions five treat ments were evaluated : AC (automatic control system), GROW (345 kPa at 14.6 m spacing), LOW (207 kPa at 14.6 m spacing), SPC (345 kPa at 12.2 m spacing), and NO (non irrigated). T hermocouple s controlled the irrigation system for GROW, LOW and SPC treatments Results showed significant yield differences between the irrigated treatments and the control. Recovery capability from the cold events among the irrigated treatments did not differ significantly show ing a linear increase in the yield after cold event s Water savings of 5% and up to 23% were obtained by using an automated irrigation system (AC treat.) during the 2011 12 and 2012 13 seasons correspondingly. Reducing the irrigation system pressure resulted in lower DU lq but without yield differences and ach ieving water savings of 19.3 billion liters of water per harvest season on average considering the strawberries planted in Florida in 2010.
15 CHAPTER 1 LITERATURE REVIEW: SPRINKLER IRRIGATION COLD PROTECTION Introduction Irrigation is the primary method use d for fruit, vegetable and nursery cold protection. Low growing crops, such as strawberries, and deciduous fruit trees can be protected from cold damage by using overhead sprinkler irrigation (Snyder and de Melo Abreu 2 005a) If the application rates and uniformity are adequate, this method of protection can be effective under windy conditions and temperature as low as 7C (Snyder and de Melo Abreu 2005a) Sprinkler irrigation for cold protection has been used to protect strawberries for several decades (Locascio et al. 1967) Two sprinkler application rates of 3.3 and 6.6 mm hr 1 were evaluated under various weather conditions. Both resulted equally effective under low wind conditions (0 0.9 m s 1 ) and relatively higher temperatures (>4.4C). When air and dew point temperatures reached as low as 8.9C and 17.8C, respectively, only the 6.6 mm hr 1 application rate was effective to protect th e crop (Locascio et al. 1967) A table of application rates based on sprinkler irrigation model for cold protection of citrus developed by Gerber and Harrison ( 1964) was publish ed by Gerber and Martsolf ( 1965a) This table provides the precipitation rate according to a range of wind conditions and minimum leaf temperature, e.g. for a minimum leaf temperature of 5.6C and wind speed ranging from 0.9 to 1.8 m s 1 an application rate of 6.1 mm hr 1 was recommended (Table 1 1). Even when this recommended table of application rates has been generally accepted, it has been overestimated (Perry 1979) and rec ent
16 investigation on lower application rates which may achieve adequate cold protection has not been thoroughly tested. The U.S. is the largest strawberry producer in the world, and Florida represents the second largest harvesting state (FAOSTAT 2013a) In Florida, strawberries are planted using raised beds with drip irrigation under plastic mulch. Sprinkler irrigation is used for plant establishment and also to protect the crop from cold damage duri ng the winter (Albregts and Howard 1984) In recent years, severe cold conditions were experienced in the Dover/Plant City area, which resulted in several resource problems believed to be caused by irrigation used f or crop protection. Associated problems such as resource depletion, nutrient leaching, and increased plant diseases may be consequence of overwatering plants. In spite of reducing the volume of water applied through irrigation and optimizing the best manag ement practices followed by the growers, this project found an opportunity to investigate current irrigation cold protection practices with the intent to identify ways to enhance and optimize irrigation for crop cold protection. This included current appli cation rates and possible changes to system uniformity under varying conditions. This first chapter consists of a literature review on cold protection used in Florida agriculture focusing on strawberry production. Strawberry Description Strawberry belongs to the Rosaceae family and it is a cross between Fragaria x ananassa Although other crops such as apples, pears, cherries and plums are included in this family, it is the only vegetable crop in this family (Peres et a l. 2010) Even when other wild Fragaria species had been spread in the Americas, Europe and Asia, as the main zones in the world, F. x ananassa is the species more
17 commercially recognized around the world due to its economic significance (Davis 2008) Some of the varieties grown in Florida are: Camarosa (developed by UC Davis), Carmine (UF), Camino Real (UC Davis), Gaviota (UF), Strawberry Festival (UF), Sweet Charlie (UF), Treasure (JP Research), Ventana (UC Davis) and Winter Dawn (UF) (Peres et al. 2010) The common transplant types used in Florida for planting are bare root green top plants and containerized transplants (plugs), generally coming from Canada or places where low er temperatures are predominant and can provide beneficial conditions for transplant adaptation. The former is the most broadly available type of transplant; however, it presents more difficulties to be established in the field and requires higher overhead irrigation during the first seven to twelve days. The latter requires less overhead irrigation for establishment. Recommended planting dates range from 15 Sept ember to 15 October for North Florida, 15 September to 25 October for Central Florida and 1 Octo ber to 1 December for South Florida (Peres et al. 2010) In Florida the two row bed system is used with a distance of 1.2 1.5 m between beds, 30.5 40.6 cm between plants and 30.5 35.6 cm between rows. The first ripe f ruit can be harvested 40 100 days from transplanting. Typical plant populations range, 39,500 54,300 plants per hectare (Peres et al. 2010) Strawberry Production and Importance The United States of America has been t he largest strawberry producing country in the world, followed by Spain, Turkey, Republic of Korea and Japan on average during the last decade (FAOSTAT 2013a) Even when some of these countries stil l have had a high production, their growth is relatively stable or negative in some cases during the ten years. However, in the same period Turkey, Egypt and Mexico had experienced
18 high growth in production that positioned them also between the top five pr oducers (FAOSTAT 2013b) In 2011, the total U.S. strawberry production was 1,312,960 Mg harvested in 23,260 hectares (FAOSTAT 2013a) and the total value of strawberry production (fresh and processed) in the United States was estimated as $2.4 billion (USDA 2012) in the same year. Mexico was the fifth top producer for the pe riod 2004 11 (FAOSTAT 2013b) ; it plays an important role in the strawberry exportation market to the U.S., which increased dramatically since 2010. Mexico exported around 178,800 Mg of fresh and fro zen strawberries in 2011 (U.S. Department of Commerce and U.S. Census Bureau 2012) Strawberries represent an important crop to the state of Florida, where around 17% of the total U.S. crop is harvested, second only to California (USDA, Economics, Statistics and Market Information System 2012) The production value in 2011 was estimated to be $366 million from about 4,000 ha ha rvested in open field systems (USDA 2013) The west central counties, Hillsborough and Manatee, represent the strawberry fields are l ocated in the Plant City Dover production area in Hillsborough County. Other southwestern counties represent the remainder (USDA, Economics, Statistics and Market Informati on System 2012) However, strawberry growers have faced big challenges in recent years due to the high production and marketing costs, which average over $67,126 per hectare (VanSickle et al. 2009) but more impor tantly, the dramatic increase in U.S. imports from
19 Mexico, which caused an oversupply in the market and a staggering pressure in the strawberry industry (Wu et al. 2012) Strawberry Irrigation Systems Florida strawberries are produced in annual hills using bare root transplants under open field conditions with a two component irrigation system that includes drip and sprinkler. Drip Irrigation Typically, strawberries in Florida are grown in polyethylene mulch beds which are irrigated and fertilized through a single drip line per bed. Drip irrigation is used to produce strawberries improving water and nutrient management; hence reducing crop production costs (Hochmuth et al. 2011) Drip irrigation may contribute to keep the moisture and compaction in the soil, storing more heat during the day, than a loose dry soil. Therefore, during a freeze/frost night, more heat can be transferred from the soil to the crop, reducing the incidence micro climate (cold spots formed by cold air drainage) in the field and keeping the temperature from falling below critical damage (Perry 1998) Sprinkler Irrigation Sprinkler irrigation is typically used by Florida strawb erry growers for crop establishment and frost protection (Albregts and Howard 1984) Bare root strawberry transplants may require 10 to 14 days of frequent intermittent overhead irrigation after transplanting for 12 and 14 hours per day giving an approximately of 406 610 mm just for plant establishment prior the irrigation with the drip system (Santos et al. 2010) The effectiveness of sprinkler irrigation f or cold protection in strawberries has been proven for several decades (Locascio et al. 1967) The heat loss from the plant to its immediate environment is substituted by the sensible heat and the heat of fusion
20 ass ociated with the water from the sprinkler irrigation, protecting the plants from frost or freeze damage (Harrison et al. 1987) Microsprinklers Microirrigation systems have been evaluated for frost protection. The goal of this type of irrigation system is to reduce the volume of water used compared to sprinklers. Therefore, only the ground under the plants is kept near 0C with the intention of concentrating and enhancing radiation and sensible heat transmitted upwa rds into the plants (Snyder and de Melo Abreu 2005a) Protection from cold damage in strawberries was evaluated using microsprinklers as well as using an automated pulsed irrigation system developed for frost protecti on intended to reduce the amount of water used without compromising crop protection (Stombaugh et al. 1992) The effectiveness of using this type of irrigation showed less than 3% blossoms killed from frost when c onstant sprinkling was applied compared to 52% blossoms killed for the control treatment without cold protection under a minimum air temperature at 1.5 m above the soil surface of 2.16C and 4C measured at the lowest unsprinkled bud. Furthermore, an 89% of water savings was achieved by using the automated irrigation system when mild frost conditions occur red (Stombaugh et al. 1992) Historical Data Temperatures In Florida, strawberry growers use sprinkler irriga tion as a common method to protect their crop from cold damage during the winter. However, variations in temperature and duration of cold events have resulted in an increase of irrigation pumping, endangering the water supplies, affecting the groundwater a nd resulting in sinkholes in many cases, which have affected the surrounding areas and communities.
21 An example of this situation occurred in the winter of 2010 as a result of cold protection pumping, when the aquifer level dropped nearly 18.3 meters in som e locations, around 750 residential wells were impacted and more than 140 sinkholes were reported (SWFWMD 2012) During the last 34 strawberry seasons the amount of hours where the temperature has dropped below freezing po int (0C) and the temperature at which normally the growers turn on the irrigation system (1.1C) has varied dramatically (1 1; NOAA 2013a; NOAA 2013b) The average number of hours below 0C is 69.5 hours and below 1.1C is 107.5 hours. The seasons with the most hours below freezing were 2008 09, 2009 10 and 2010 11 with a total amount of hours below 0C of 129, 145 and 147 hours respectively for each season. However, the last two seasons 2011 12 and 2012 13, only 50 and 30 hours were below freezing, respectively, and both seasons were below the average number of hours compared to the last 34 years (1 1). Energy Balance It is important to understand the energy balance occurring in the atmosphere in order to pro vide enough energy for the plant to be protected from cold damage. During daytime, radiation from the sun and sky adds energy to the surface (Snyder and de Melo Abreu 2005a) through direct rays to all objects remaini ng with lower temperatures. However at nighttime, particularly during clear nights, no heat through radiation is coming from the sky and more energy is lost through radiant cooling (Braud and Hawthorne 1965) Althoug h, when clouds are present, some radiant energy is retained and reflected back to the earth reducing the rapid cooling that may produce cold damage (Snyder and de Melo Abreu 2005a)
22 Four main weather conditions influe nce the occurrence of cold damage in plants: air temperature, relative humidity, wind speed and net radiation. When water vapor in the atmosphere changes phase to liquid through condensation, it results in dew formation on cold surfaces. However, if air te mperature drops, this moisture can be transformed into ice crystals leading to the formation of frost over solid objects such as flowers, buds or berries. These solid surfaces can reduce their temperature to freezing point or lower as a result of a high ra diant cooling rate and no wind conditions (Braud and Hawthorne 1965) During a clear night, high radiant cooling rate from solid objects occur when low wind conditions are present; therefore the surface at the ground cools forming a cold temperature continues dropping and little or no wind is present (Braud and Hawthorne 1965) The process desc from atmospheric conditions being inversed to the normal daytime condition at which air temperature is reduced according to the height (Haman 2006) By the contrast, if wind is present the inversion process is dissipated when wind blows the cold layer and combines it with the warmer upper air without resulting in frost occurrence due to the equalization of air and surface temperatures when warmer air exchange s heat to the solid surfaces (Braud and Hawthorne 1965) Although energy fluxes from the soil and air moderately balances the energy losses, the temperature falls due to a decrease in the sensible heat of the air: th erefore, a net loss of radiation is present during a radiation frost night (Snyder and de Melo Abreu 2005a)
23 Hence, the purpose of the cold protection methods is to intentionally modify the energy balance components i n order to decrease the magnitude of energy changes stored in the crop. Therefore, by cooling or freezing water, it converts latent to sensible heat, raises the surface temperature and decreases the rate of temperature drop at the plant surface (Snyder and de Melo Abreu 2005a) Latent Heat Transfer The chemical energy stored in the bonds that join water molecules together is called latent heat, whereas the sensible heat is the heat measured with a thermometer. Latent h eat is released to the atmosphere and is converted to sensible heat when water condenses, cools or freezes, increasing the temperature of the surrounding environment. Vice versa, when water changes phase and melts, warms or evaporates sensible heat is chan ged to latent heat decreasing the air temperature (Snyder 2000) The heat released through fusion is 80 calories per gram and the temperature when water is freezing will be close to 0C, even though the surroundings may be colder. Therefore, an equilibrium temperature state will be established as long as the mixture of water and ice is present and the temperature remains close to 0C. This equilibrium between vapor, liquid and ice is known as triple point temperature (Harrison et al. 1987) Evaporative Cooling Evaporative cooling occurs when a gas flows over a liquid. Evaporation takes place when liquid molecules near the surface collision increasing their energy above that nee ded to overcome the surface binding energy. The latent heat of vaporization of the liquid is the energy relative with the phase change. The internal energy of the liquid will maintain the evaporation, which will experience then the cooling effect when a
24 re duction in the temperature occurs. However, the latent energy lost by the liquid due to evaporation has to be replenished by energy transfer to the liquid from its surroundings, if steady state conditions are to be constant. Psychrometrics The study of phy sical and thermodynamic properties of air water mixtures is called psychrometrics, which can be useful to predict freezing and frost conditions to apply cold protection methods (Bucklin and Haman 2009) An estimation of the potential for frost and the determination of the best time to start and stop the sprinklers for frost protection can be done by using the dew point temperature and the wet bulb temperature. Generally, the nighttime low temperature is determined by t he heat lost to the sky and the dew point temperature. As heat radiates to the sky at night, the dry bulb temperature decreases, and if enough heat is lost it will reach the dew point temperature and stabilizes as moisture starts condensing from the air as dew or frost. Therefore, with this information, the lowest possible night time low air temperature can be estimated. However, on clear nights with low humidity, the radiation losses from plant surfaces can provoke lower temperatures than air temperature (Bucklin and Haman 2009) Sprinklers can be used to avoid cold damage by evaporative cooling when they are turned on at the correct time. At the moment of turning the sprinklers on, the air around them will reach the w et bulb temperature. Damage can result from the sprinkler system if the wet bulb temperature is below 0C. Hence, in order to avoid plant damages it should be started when t he wet bulb temperature is 1.1 C or higher ( Bucklin and Haman 2009)
25 Frost Damage Many crops such as apples, peaches, grapes and strawberries are susceptible to frost damage. When the freezing temperatures threaten the plants, flowers and fruits, heat must be applied by an effective frost protecti on management strategy. Consequently, crop loss and susceptibility to diseases will be reduced, and profitability will be increased. However, frost protection management requires timely and accurate monitoring of environmental variables and proper frost pr otection measures (Heinemann et al. 1992) Cell Injury Essentially, the main cause of plant injury is not the cold temperature. Direct damage in the plants is due to the intracellular freezing, when ice crystals f orm inside the protoplasm of cells. However, when the ice crystals form inside the plants but outside of the cells (i.e. extracellular freezing), indirect damage can occur (Westwood 1978) Intracellular ice formati drop and the super cool level before freezing (Levitt 1980) As a consequence o f the extracellular ice mass growth, cells will gradually be killed due to the evaporation of the liquid water (vapor pressure gradient) inside the cells and the increased solute concentration which reduces the freezing possibilities, but increases the cel (Levitt 1980) In injured plants, normally the desiccation provokes dead cells in the surrounding of the ice crystal. Hence, frost damage is mainly caused by extracellular ice formation that produces se condary water stress to the surrounding cells (Snyder and de Melo Abreu 2005a)
26 Plant Sensitivity Frost damage varies within varieties and species at the same temperature and ation to cold temperatures prior to a frost night, is developed in the plants against freeze injury after the cold periods due to an increase in solute content of the plant tissue, or a decrease in ice nucleation active (INA) bacteria concentrations, or a combination of both during those periods. Therefore, the freezing temperature can vary noticeably according to the hardening level of the plants (Snyder and de Melo Abreu 2005a) Avoidance and tolerance are the two di fferent forms used by the plants to support low temperatures and both are involved in hardening. The freezing temperature of tissues (e.g. in olive and citrus tree leaves) gets lower due to the accumulation of sugars or sugar alcohols and supercooling incr eases in many deciduous and evergreen fruit trees in response to low air temperature. Also, an increase of fatty acids of plasma membrane lipids may harden some cells and so, it would increase membrane stability in desiccation. However, hardening will be r educed whenever the assimilate level in the tissues is depleted or if exposed to warm temperatures (Snyder and de Melo Abreu 2005a) efinitions interchangeably with an Those terms have been used to express a meteorological event, which causes crops and plant freezing injury (Snyder and de Melo Abreu 2005a) refers to the formation of ice crystals on surfaces either by freezing of dew or a phase change from vapor to ice (Blanc et al. 1963; C uhna 1982)
27 According to Snyder and de Melo Abreu (2005 air temperature of 0C or lower, measured at a height of between 1.25 and 2.0 m above Some avoidance factors (e .g. supercooling and concentration of ice nucleating bacteria) might provoke a freezing of the water within plants. Damage to the plant tissue depends on some tolerance factors (e.g. solute content of the cells). When extracellular ice forms inside of the plants, the frost event is converted into a freeze event. However, freeze injury is present when an irreversible physiological condition occurs causing death or malfunction of the plant cells after falling below a critical value (Snyder and de Melo Abreu 2005a) Crop Sensitivity and Critical Temperatures g plant tissue temperature. Subzero air temperatures are the consequence of the reduction in sensible heat content of air near the surface, due to the following main factors: (i) net radiation loss from the surface to the sky (i.e. radiation frost), (ii) o r warmer air replaced by wind blowing in subzero air temperature (i.e. advection frost), or (iii) a combination of both. Commonly, frost damage occurs to crops from extracellular ice formation inside plant tissue, consequently damages to the cells are pres ent due to water withdrawing and dehydration. As a protective response to cold events, the plants tend to harden against freeze injury. This is one of the factors which determine the temperature at which ice forms within the plant tissue and when damage oc curs. Therefore, frost injury
28 the temperature related to a specific level of damage (Snyder and de Melo Abreu 2005a) The critical te mperature is defined as the temperature below which, a percent of the plant part will be killed when hold for 30 minutes or more, i.e. if for blossoms T 90 = 2.2C thus, 90% of the blossoms will be killed if blossoms are exposed below 2.2C during 30 minu tes or more (Perry 1979; Perry 1986) The critical temperature is determined according to the plant variety and stage of development. In general, the critical temperature is below 0C considering that the water within a plant is at a water potential lower than that of free water, thus intracellular freezing, which causes damages in the plant, does not occur (Perry 1979) Some approximate values based on obse rvations and opinions of leading small extension personnel, not on controlled research, showed critical temperature of the blossoms, and young fruits of strawberries (Table 1 2; Phillips et al. 1962) A critical tem perature of 3.11C was observed for two stages of blossom development: open blossom and buds with visible petals of four strawberry cultivars. The minimum air and blossom/bud temperature recorded was 3.7C and 3.8C, correspondingly, and the minimum win d speed was 1 m s 1 occurring damage during that night of frost protection on strawberries (Table 1 3; Perry and Poling 1986) Perry and Poling ( 1986) suggested a critical tempe rature within all this criteria and it was supported by Boyce and Strater ( 1984) demonstrating that popcorn and open blossoms damage increased from 0% to 7% and from 0% to 20%, respectively, when the freezing tempera ture changed from 3.0 to 4.0C for eight cultivars. Usually,
29 the critical temperature increases with time after the developing of the buds until the fruit stage, since this crop stage the most sensitive to freezing. (Kalma et al. 1992; Snyder et al. 1992) Nevertheless, in some cases a combination of both conditions will occur. Advective Frosts These frosts occur when cold ai r blows into an area to replace warmer air present before the weather changes. Large scale incursions of cold air with a well mixed, windy atmosphere, no temperature inversion, low humidity and a subzero temperature generally present even during daytime; a re conditions associated to advective frosts (Snyder et al. 1992) In these frosts the lowest temperatures are usually observed on the middle and higher portions of hillsides that are open and exposed to the wind. Hi gher night time temperatures are observed on the down wind sides of hills and in low spots that are sheltered from the wind (Snyder and de Melo Abreu 2005a) Radiative Frosts In general, the radiative frosts are pres ent on a clear and calm night, as the result of the cooling attributable to the energy loss through radiant exchange, with temperature inversions (i.e. temperature increases with height), low dew point temperatures and air temperatures that usually fall be low 0C during the night but above 0C during the day (Snyder et al. 1992) Cold air accumulates in depressions, where the air becomes vertically stratified with temperature increasing with height; therefore, higher night time temperatures are observed on hilltops and on upper middle sections of hillsides that are free from obstacles to block cold air drainage (Snyder and de Melo Abreu 2005a)
30 Economic Importance of Cold Protectio n In strawberries market the prices usually determine the length of the strawberry season; however, most Florida growers concur that early yields provide the highest profits per unit, with prices generally declining at the end of February. Sprinkler Irriga tion Cold Protection Increases or decreases in water temperature are dependent on changes in the sensible heat content. Three processes cause a reduction on water temperature: (i) sensible heat in the water is transferred to its surroundings; (ii) evaporat ion, when sensible heat consumption occurs to break the hydrogen bonds between water molecules; or (iii) when there is net radiation loss. Part of sensible heat is lost by radiation as water droplets fly from a sprinkler head to a plant and soil surfaces, some will transfer from the warmer water to the cooler air and some will be lost to latent heat as water evaporates from the droplets (Snyder and de Melo Abreu 2005b) Sprinkler irrigation is used for frost or freeze protection under the principle of latent heat transfer. The heat loss from the plant to its surrounding environment is replaced by the sensible heat and the heat of fusion associated with the water. Cold protection is supplied due to the release of the la tent heat of fusion when water changes phase from liquid to solid (water to ice) (Harrison et al. 1987) The amount of heat 4. The water freezing poi nt is at 0C and the heat released as the water freezes keeps this temperature approximately constant although the surroundings may be colder. Therefore, cold protection will be provided as long as the mixture of water and ice is present and the temperatur e continues close to 0C. However, greater damage can occur than experienced by an unprotected crop when windy conditions are present
31 or when temperature falls so low that the application rate is inadequate to provide greater heat than it is lost to evapor ation (Snyder and de Melo Abreu 2005b) Momentarily plant temperature rises when water droplets strike a flower, bud or small fruit, due to latent heat release when water freezes; however, energy is lost as latent he at when water vaporizes from the ice coated plant tissue. These two processes and radiation losses, cause the temperature to drop until the sprinklers rotate and strike the plant again. Therefore, to prevent the plant temperature from falling too low betwe en pulses, it is necessary to re apply water frequently at a sufficient application rate (Snyder and de Melo Abreu 2005b) A frequency of application no longer than 60 s (Wheat on and Kidder 1965) and a variable application rate according to minimum temperature and wind speed are recommended (Gerber and Martsolf 1965a) A wet leaf may be colder than a dry leaf due to cooling by evaporatio n. The heat consumption during evaporation is approximately 7.5 times more than is liberated by freezing, hence at least 7.5 times as much water must be frozen as is evaporated. Therefore, the temperature on a sprinkled leaf will be lower than a non sprink led leaf if less heat is released by freezing than is used for evaporation of water and ice from the leaf (Gerber and Martsolf 1965b) Thus, it is important when irrigation is used for freeze protection that an applic ation rate can be maintained over the entire irrigated area during a cold event based on minimum temperature and wind speed conditions (Gerber and Martsolf 1965a; Gerber and Harrison 1964) The leaf temperature may be 1.6 to 2.2C below air temperature under no wind and clear conditions due to the process of inversion; in which temperature drops faster as a result of radiation cooling. When a light wind is present, the leaf temperature will be
32 o nly slightly below air temperature (Gerber and Martsolf 1965b) ; as a result of breaking the inversion process and increasing the exchange of heat among air and surface temperatures (Braud and Hawthorne 1965) Sprinkler application rates of 3.3 mm h 1 and 6.6 mm h 1 were evaluated for cold protection in strawberries during four severe freeze events (Locascio et al. 1967) These evaluations recorded a minimum air temperature of 8.8C, variations in wind speed ranging from 0 to 5.4 m s 1 and 0C as the lowest dew point temperature. Under low wind conditions (0 0.9 m s 1 ) and minimum temperature of 4.4C, both 3.3 mm h 1 and 6.6 mm h 1 applic ation rates were equally effective. However, no protection was provided using 3.3 mm h 1 under air temperatures lower than 4.4C, and wind speeds greater than 0.9 m s 1 The injury reported with a no irrigation treatment was 87% of the flowers, 98% of the immature fruit and 100% of the mature fruit. As a comparison, the corresponding results for injury of flowers, immature fruit and mature fruit were 41, 59 and 50% in plots receiving the 3.3 mm h 1 rate and 13, 19 and 6% in plots receiving the 6.6 mm h 1 r ate. In another study strawberry conducted during the winter of 1985 1986 using overhead sprinkler irrigation showed satisfactory protection of early fruits with 15% fruit losses to freezing. Using a 6.4 mm h 1 application rate, 889 mm of water w ere applie d to the uncovered sprinkler irrigated p lants. This amount represented approximately 50% of the total water used by many commercial strawberry growers at that time (Hochmuth 1993) Sprinkler Uniformity Since uniform coverage must be accomplished for effective plant protection from cold damage; sprinkler irrigation uniformity must be achieved accordingly. Irrigation
33 efficiency is defined as a ratio between the total volume of irrigation water beneficially used over the total water applied minus the outflow (the total volume of irrigation water that leaves the boundaries). This term is defined as a performance indicator and is generally expressed as a percentage (Burt et al. 1997a) Distribution Uniformity (DU lq ) measures the variation of the irrigation water applied to different areas in a field. In general, DU lq is defined as a ratio of the smallest accumulated depths of water in the distribution, to the average depth accumulated. A lthough high DU lq values can be obtained, irrigation efficiency may not be achieved due to under or over irrigation. Nevertheless, irrigation cannot be non uniform and efficient. Therefore, DU irrigation system performance is a tool which can provide infor mation of the potential irrigation efficiency (Burt et al. 1997a) The average low quarter depth, is the average of the depths accumulated in the quarter of the field area receiving the smallest depths (ASAE 2001) An e mph asis on the areas receiving the least irrigat ion is given by focusing on a minimum value range (the lowest quarter) instead of using the absolute minimum value (zero) (Burt et al. 1997a) The low quarter distribution uniformity, DU lq is a ratio defined by the fo llowing formula (Merriam and Keller 1978) : (1 1 ) Where is the average of lowest quarter of catch can measurements (mL) and is the average depth of application over all catch cans measurements (mL). The Coefficient of Uniformity (CU) is another indicator which can assist in system design and/or selection, and can be used to quantify certain aspects of system
34 performance in the field. However, the entire irrigation system performance can be affected by multiple factors such as: win d, application rates, water applied, runoff, pump performance and overall system management (ASAE 2001) The Christiansen Uniformity Coefficient (Christiansen 1942) ; (ASAE 2001) is defined by the following formula: ( 1 2) Where V i is the individual cat ch can measurement (mL) and is the average volume of application over all the catch can measurements (mL). Frequency of Application The system design and operation; e.g. the frequency of application or sprinkler rotational speed, also influence the ef fectiveness of cold protection (Wheaton and Kidder 1965) The temperature of a wet plant rises as water freezes, but it falls as water vaporizes and radiative losses occur; therefore frequent rotation rate is requir ed to reduce the interval when the plant temperature falls below 0C ( 1 2). Three scenarios may result from the frequency in which water is applied to the plants: (i) a fast frequency of application in which the plant remains wet at the next sprinkler rot ation, (ii) a rotation rate in which the duration allows the water applied just to turn into ice, and (iii) the sprinkler could revolve too slow so that the water is frozen and the temperature of the ice falls below freezing temperature (0 C ) (Niemann 1957 1958) Ideally, the second condition should be accomplished, in which a good coverage is provided during enough time to keep the temperature near 0C to avoid damage to the plant.
35 The greater the wind speed, th e greater the evaporation rate, which causes a temperature decrease to near the wet bulb temperature before another pulse of water hits the plant. As well, the lower the dew point temperature, the greater the evaporation rate (Snyder 2000) Hence, higher sensible heat losses from the plant surfaces and more water needs to be frozen to compensate for these losses. When the unprotected minimum temperature is lower, more energy is needed from the freezing process to make up for the sensible heat deficit; hence, a higher application rate is needed (Snyder and de Melo Abreu 2005b) Consequently, a frequent interval of time for a rotation rate no longer than 60 seconds is required in o rder to shorten the period the temperature is below the critical damage temperature (Wheaton and Kidder 1965) Experiments conducted by Wheaton and Kidder ( 1965) were perform ed in order to determine the effect of repeat frequency of application under windborne freeze conditions in the absence of radiation cooling. The tests were conducted with rotation speeds of 20 s, 60 s and 120 s for application rates of 2.8 mm h 1 and 5.1 mm h 1 Under low wind conditions (0.4 m s 1 ) the 2.8 mm h 1 application rate gave protection to about 1.1C using 20 s repeat frequency; while the 60 s frequency protected to a minimum temperature around 3.6C. Hence, changing the frequency from 120 s t o 60 s allowed to decrease the secure temperature in more than 2.2 degrees. When tests were performed under higher wind conditions (1.3 m s 1 ) using the same application rate, protection was provided nearly 2.8C using the 20 s frequency and only down to 1.1C when 60 s frequency was tested (Wheaton and Kidder 1965) Leaf cold protection to a temperature of 2.2C was provided using 60 s frequency and 5.1 mm h 1 application rate under wind speed of 1.3 m s 1 ; where as the
36 20 s frequency protected down to 3.6C under the same wind conditions (Wheaton and Kidder 1965) It was determined that the safe temperature level was lowered from 0.8 to 2.2 degrees by increasing the rotati onal speed from 120 s to 60 s or from 60 s to 20 s (and other factors remaining constant) under windborne freeze conditions. Furthermore, fewer temperature variations at the leaf surface resulted from shorter frequencies of repeat application without ice m ass formation (Fig 1 2; Wheaton and Kidder 1965) Results may vary if i ce layer is formed at the leaf. Application Rate (AR) The requirement rate for cold protection is defined as the amount of water needed to be ap plied to provide enough heat by freezing in order to compensate for the heat loss by other means, i.e. radiation, convection and evaporation (Braud and Hawthorne 1965; Businger 1965; Gerber and Martsolf 1965a; Gerber and Harrison 1964; Perry 1979; Perry et al. 1980; Perry and Poling 1986; Snyder and de Melo Abreu 2005a; Wheaton and Kidder 1965) AR Models The application rate for over plant irrigation with conventional sprinklers depends on the rotation rate, wind speed, dew point temperature and unprotected minimum temperature (Snyder 2000; Snyder and de Melo Abreu 2005b) The following section presents previous models which were developed in order to determine the amount of water needed to provide enough heat release and freezing of ice water film on the plant to be protected. Businger Model and Gerber and Harrison Model. Application rates had been developed for different crops. In order to determine a sprinkling application rate (AR), two main models were established initially by Businger ( 1965) and then by Gerber and Harrison ( 1964) (Perry 1979)
37 These models are based on theoretical computations and were developed from given set of atmospheric conditions (i.e. plant part temperature, minimum tolerable or critical temperature, wind speed and plant part size). The AR requirement was calculated taking into consi deration the heat balance of a single horizontal leaf. Also, the minimum The results from the models of Businger ( 1965) and Gerber and Harrison ( 1964) for sprinkler irrigation for cold protection of citrus showed an extreme low temperature of 9.4C under wind speed ranges between 2.2 4.5 m s 1 The application rates were strongly dependent on wind speed. However, d ue to rapid ice formation, ice with a milky white appearance was observed and with inclusion of air bubbles; hence, a deficient sprinkling and a temperature depression were more likely. The variability of exposure, water distribution and height above the s urface (affected by the wind) produced variability in temperatures. The results of using this general theory of irrigation, to estimate the amount of water required based on the lowest anticipated temperature and wind speed, showed that the drop size used was too small for most efficient water use of heat in the water (Gerber and Harrison 1964) As a consequence of the deficient sprinkling, more damage occurred by increasing the killing temperature of citrus leaf and decreasing the temperature below ambient (Gerber and Harrison 1964) Gerber and Martsolf Application Rate Table. Based on the Gerber and Harrison ( 1964) model, Gerber and Martsolf ( 1965a) developed a suggested application rate table for expected minimum temperatures and wind conditions during a freeze/frost
38 night (Table 1 1). Successful protection using this AR table had been achieved; ho wever, it has been shown that is generally overestimated (Perry 1979) Gerber and Martsolf ( 1979) showed a sprinkler application rate theoretical model of a 20 mm diameter tre e leaf. This model used the following simple empirical equation for the application rate ( ): ( 1 3) Where, u is the wind speed (m s 1 ), and is the temperature of a dry unprotected leaf (C). To estimate the temperature difference between air and a leaf of 20 mm diameter on a typical frost night, where a high stomatal resistance is present, the approach outlined by Campbell and Norman ( 1998) can be used as follows: ( 1 4) u 1 If these two equations are combined, a new simple equation is developed for the AR in terms of wind speed ( u ) and air temperature (C) described as follows: ( 1 5) This equation is valid for wind speeds in the range of 0.5 and 5.0 m s 1 It is recommended an additional application amount ranging from 0 mm h 1 for sprinkler systems with a uniform cov erage over a thin crop canopy, to 2 mm h 1 for canopies with dense foliage or for sprinkler systems with less uniform coverage. The ARs generated in Equation 5 are recommended for tall crops such as grapevines; however, lower ARs are needed for smaller cro ps such as strawberries due to the less surface area to cover, less evaporation and better uniformity reached when shorter vegetation is wetted. An AR comparison between short and tall crops is
39 described in Table 1 6 and 1 3 shows the application rates for short crops such as strawberries. Barfield (et al.1981) Model. The sprinkler AR model described by Gerber and Harrison ( 1964 ) assumes no humidity effect in the calculation. Consequently, Barfield et al. (1981) showed through their model that ignoring humidity can cause an underestimation of the required AR, hence, a larger error which may result in subsequent damage to valuable horticultural crops. Th erefore, this parameter has to be taken into account. An example given by Barfield et al. ( 1981) for air temperature 5 C, wind speed 2 m s 1 and relative humidity 50%, estimated an application rate of 6.9 mm h 1 By ignoring the relative humidity term, a large error would have been made resulting in 1.97 mm h 1 lower AR, equivalent to an underestimation of 28% of the requirement (Barfield et al. 1981) Perry K.B. Model. F urther analyses of these models were performed by Perry ( 1979) who incorporated humidity and ice accumulation into amended versions of former models creating a new refined model to predict variable sprinkler AR in ti me and space. The results from Gerber and Harrison ( 1964) model were published in a table of AR for various wind speed and minimum temperature of a dry leaf by Gerber and Marts olf ( 1965a) (Table 1 1). Furthermore, Perry ( 1979) made a detailed comparison among the published values in the AR table by Gerber and Martsolf ( 1965a) and these values repro grammed in FORTRAN (program used by Businger 1965 in initial model). Even when the AR table had been used successfully, an overestimation was determined throughout this comparison (Table 1 7). In the AR table, a lo wer limit of 2.5
40 mm hr 1 was established and rounding was applied. The reprogrammed values were supposed to be equal to the AR table by Gerber and Martsolf ( 1965a) (GM) if they were (i) < 2.5 mm hr 1 when GM used 2. 5 mm hr 1 (ii) within 10% of GM value, and (iii) rounded off to the nearest tenth of a cm. However, only 21% of their values were verified (Table 1 7 values with asterisk (*)). Although the AR table has been successful, its usage is limited due to overesti mations for wind speeds over 5 m s 1 which are very extreme and unusual frost/freeze conditions. Only 33% of the values were validated for lower wind conditions (Perry 1979) Turning irrigation systems on and off ca n achieve water conservation and greater efficiency by an adjustment of the sprinkling AR according to the atmospheric conditions (Perry et al. 1980) In order to conserve water P erry et al. ( 1980) added intermittent sprinkling as another component to the model. A calculation of the maximum off period can be made through the sum of time required to freeze the applied water plus the time in which the ice coated plant parts cool to the critical temperature. This off period has to be long enough to conserve water, but not excessive that the plant parts cools below the critical temperature (Perry et al. 1980) Experimental results conducted by Perry et al. ( 1980) indicated that the optimum off time is between 1.5 and 4.0 min, concluding that water consumption for frost protection can be reduced by intermittent sprinkling. The initial model developed by Perry ( 1979) was SPAR79 (Sprinkling Application Rate model developed in 1979), then it was transformed into SPAR81 and furthermore it enhance into FROSTPRO (Perry 1986) This microcomp uter program calculated irrigation rates for frost/freeze protection of orchards based on given atmospheric and
41 crop parameters, including the relative humidity parameter suggested by Barfield et al. ( 1981) The ra te of heats lost by the plant was determined using an energy budget approach at the actual plant part temperature and at the critical temperature. Therefore, the difference of these two rates of heat loss is the rate at which heat must be applied by the la tent heat of fusion liberated as the applied water freezes (Perry 1986) Martsolf J.D. (Minimum diameter of pattern). Under different meteorological conditions, models have been used for decades to predict the minimum precipitation rate that will provide protection. However, it is necessary to determine whether the specific sprinkler system can be expected to deliver enough water. Therefore, the minimum diameter of pattern that will provide the minimum precipitation ra te was defined by Martsolf ( 1993) as follows: ( 1 6) Where d is the minimum diameter of the cylinder of protection (ft.), R is flow rate through the nozzle (gph), P is the minimum precipitation rate given by the sprinkler model (in hr 1 ). Stombaugh et al. ( 1990) Automated pulsing system. As a consequence of the development of mathematical models to predict the application rate to provide effective prote ction from frost damage (Perry 1986) and due to the achievement of a desired application rate by an intermittent irrigation (Perry et al. 1980) ; an automated pulsing irrigation s ystem for frost protection of strawberries was developed by Stombaugh et al. ( 1990) This automated system determines, based on three models, when to start the irrigation, (according to the atmospheric conditions and before dropping below critical
42 based on FROSTPRO) and when to stop the system (ice layer on the plant is melting).Tests were performed under actual frost conditions and it was concluded that a microcomputer can effectively control a pulsed irrigation system for frost protection for strawberries (Stombaugh et al. 1990) Start and Stop Sprinklers According to the literature the temperature to start and stop the sprinklers for frost protection is based on the dew point and wet bulb temperatures (Table 1 8). To use Table 1 8, first, find in the top row the wet bulb temperature, which is greater than or equal to the critical damage temperature, for the crop. Second, locate the dew point temperature in the left hand column and match the air temperature that corresponds. The sprinklers should be operating before the air temperature measured upwind from the crop falls to the selected a ir temperature. When frost alarms are used, it is recommended to set it about 0.4 C higher than the starting air temperature identified in Table 1 8 to safeguard sufficient time to start the sprinklers and when the irrigation is started using a thermostat it should be set 0.4 2 C, according to its accuracy (Snyder and de Melo Abreu 2005b) Table 1 9 can be used in order to determine dew point temperature (C) when air temperature and relative humidity are known. To estimate the start and stop temperature, the vapor pressure (e d in kPa) at the dew point temperature (T d in C) is estimated from the wet bulb temperature (T w in C) as: ( 1 7) Where the saturation vapor pressure at the wet bulb temperature (e w ) is: ( 1 8)
43 And the barometric pressure (P b ) as a function of the elevation (E L in meters) is: ( 1 9) Therefore the corresponding air temperature (T a ) can be calculated as: ( 1 10) Where the saturation vapor pressure at the dew point temperature (e d ) is ( 1 11) The variation in exposed leaf temperature can be higher than air temperature due to exposure to the sky and should be used to determine the moment to start the system. It is less risky when the irrigation system is started when the wet bulb temperature reaches 0 C (Snyder and de Melo Abreu 2005a) A freezing ice/wa ter mixture remains near 0 C, as equal as a melting ice/water mixture; therefore, the irrigation is typically stopped when ice is melting from the crop. The longest time the plant can support being without rewetting is the time required to freeze a film o f water adhering to the leaf (Harrison et al. 1987) However, the sprinklers should not be turned off if the air temperature is above 0 C and the sun is shining unless the wet bulb temperature measured upwind from the crop is above the critical damage temperature. Therefore, the highest freezing temperature for strawberries is 0.8 C. A psychrometer can be used to directly measure wet bulb temperature or it can be estimated from the dew point and air temperatures (Snyder and de Melo Abreu 2005a) Over the past years, higher interest in irrigation management throughout the application of better practices and technology has risen in response to the occurrence of
44 unusual cold eve nts in the Dover Plant/City area, which caused hydric resource problems believed to be caused by irrigation for cold protection. Therefore, this research found as a main goal to optimize irrigation management practices for cold protection in strawberries t hat could conserve water. The objectives for this project were to: (i) current strawberry irrigation cold protection application rates (ii) assess the effect of sprinkler t ype, sprinkler spacing, irrigation system pressure variations and varied climatic wind conditions over irrigation distribution uniformity (DU lq ) and application rate (AR), and (iii) evaluate the effect of varying sprinkler spacing and pressure on strawberr y yield quality and quantity under cold conditions.
45 Table 1 1. Citrus cold protection application rate (mm hr 1 ) recommendation according to minimum temperature expected and wind speed conditions (Gerber and Martsolf 1965a; Gerber and Harrison 1964) Min. Temp. Expected Wind Speed (m s 1 ) 0 0.4 0.9 1.8 2.2 3.6 4.5 6.3 8.0 9.8 13.4 (C) Application Rate (mm hr 1 ) 2.8 2.5 2.5 2.5 2.5 5.1 7.6 3.3 2.5 2.5 3.6 5.1 10.2 15.2 4.4 2.5 4.1 7.6 10.2 20.3 40.6 5.6 3.0 6.1 12.7 15.2 30.5 45.7 6.7 4.1 7.6 15.2 20.3 40.6 61.0 7.8 5.1 10.2 17.8 25.4 50.8 76.2 9.4 6.6 12.7 22.9 33.0 66.0 101.6 11.7 8.6 17.8 30.5 43.2 86.4 127.0 Table 1 2. Critical temperature (C) of the blos soms, and young fruits of strawberries for four stages (Phillips et al. 1962) Crop Tight Bud Balloon Bud Full Bloom Green Fruit Strawberries 5.6 2.2 0.56 2.2 Table 1 3. Assessment of survival and minimum t emperature ( C ) reached for exposed and covered blossoms in open and popcorn stages for four strawberry cultivars (Perry and Poling 1986) Cultivar Open Popcorn Exposed Covered Exposed Covered Assess. Min temp Assess. Min temp Assess. Min temp Assess. Min temp. Atlas 0 3.7 0 3.8 0 3.3 0 3.4 Apollo + 3.1 0 3.2 + 1.7 + 2.2 Chandler 0 3.6 + 1.9 + 2.7 0 3.6 Douglas + 2.6 + 3.1 0 3.7 + 0.9 0=Dead + Alive
46 Table 1 4. Comparison of he at consumed through evaporation (heat of vaporization) with heat released (heat of fusion) through freezing (Gerber and Martsolf 1965b) Unit of water Heat of vaporization (at 0C) Heat of fusion 1 gram 596 Calories or 2.4 BTU 80 Calories or 0.32 BTU 1 Pound 1072 BTU 144 BTU 1 Gallon 8100 BTU 1200 BTU Table 1 5. Results obtained at the Archer Road Unit with irrigation for citrus cold protection (Gerber and Harrison 1964) Date Min temp (C) Wind (m s 1 ) Application rate (mm h 1 ) Results Dec 6 7 3.1 0.0 0.5 2.54 Good Dec 10 11 2.4 0.0 0.5 2.54 Good Dec 12 13 9.2 2.2 4.5 2.54 Trees killed Dec 13 14 5.6 0.0 0.5 2.54 Trees killed Dec 14 15 5.1 0.0 0.5 2.54 Trees kille d Jan 24 25 4.2 1.8 2.7 2.79 Variable Note: The operating pressure for the system was 413.7 kPa at the nozzle of the sprinkler. The nozzle size was 3.2 mm, except for January 24 and 25 when 3.97 mm. nozzles were used.
47 Table 1 6. Application rates (m m h 1 ) for overhead sprinkler protection of tall (orchard and vine) and short (field and row) crops depending on the minimum temperature and rotation rate, for wind speeds between 0 and 2.5 m s 1 Minimum temperature Tall crops Short crops (C) 30 s rotat ion 60 s rotation 30 s rotation 60 s rotation 2.0 2.5 3.2 1.8 2.3 4.0 3.8 4.5 3.0 3.5 6.0 5.1 5.8 4.2 4.7 Note: Application rates are about 0.02 mm hr 1 lower for no wind and about 0.02 mm hr 1 higher for wind speeds near 2.5 m s 1 with canopies similar in size to strawberries. Taller field and row crops (e.g. potatoes and tomatoes) require intermediate application rates (Snyder and de Melo Abreu 2005c)
48 Table 1 7. Application rates (mm hr 1 ) comparison between composite table of AR from the computer model (GH) ( Gerber and Harrison 1964) and AR from Table 1 6 of Gerber and Mar tsolf (1965a) (GM) at different wind conditions (m s 1 ). Application Rate Comparison (mm hr 1 ) Wind Speed (m s 1 ) 0 to 5 1 to 2 2.5 to 4 5 to 7 9 to 11 15 Temp of Dry Leaf (C) GM GH GM GH GM GH GM GH GM GH GM GH 2.8 2.5 1.8 2.5 3.9 2.5 5.6 2.5 7.5 5.0 9.6 7.5 11.7 3.3 2.5 2.1 2.5 4.6 3.6 6.7 5.1 8.9 10.1 11.5 15.3 14 4.4 2.5 2.8 4.1 6.1 7.6 8.8 10.2 11.8 20.4 15.1 40.6 18.4 5.6 3.0 3.4 6.1 7.6 12.7 10.9 15.2 14.6 30.4 18.7 45.6 22.7 6.7 4.1 4.1 7.6 9.0 15.2 12.9 20.3 17.3 40.6 2 2.2 60.9 27.0 7.8 5.1 4.7 10.2 10.4 17.8 14.9 25.4 20.0 50.8 25.6 76.2 31.2 9.4 6.6 5.6 12.7 12.5 22.9 17.9 33.0 24.0 66.0 30.7 99.0 37.4 11.7 8.6 6.9 17.8 15.2 30.5 21.8 43.2 29.2 86.4 37.3 129.6 45.4 Table comparison established by Perry (1979 ) (*) Designates th at th e pair comparison (AR from the computer model (GH) and Table 1 6 (GM)) is considered equal (Perry 1979)
49 Table 1 8. Minimum starting and stopping a ir temperatures (C) for frost protection with sprinklers as a function of wet bulb and dew point temperature (C) at mean sea level (Snyder and de Melo Abreu 2005b) Dew point temperature Wet bulb temperature C C 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.0 0.5 0.5 0.3 1.0 1.0 0.2 0.6 1.5 1.5 0.7 0.1 1.0 2.0 2.0 1.2 0.4 0.4 1.2 2.5 2.5 1.7 0.9 0.1 0.7 1.5 3.0 3.0 2.2 1.4 0.6 0.2 1.0 1.8 3.5 2.7 2.0 1.2 0.4 0.4 1. 3 2.1 4.0 2.5 1.7 0.9 0.1 0.7 1.5 2.3 4.5 2.2 1.4 0.7 0.1 1.0 1.8 2.6 5.0 2.0 1.2 0.4 0.4 1.2 2.0 2.8 5.5 1.7 1.0 0.2 0.6 1.4 2.2 3.1 6.0 1.5 0.7 0.1 0.9 1.7 2.5 3.3 6.5 1.3 0.5 0.3 1.1 1.9 2.7 3.5 7.0 1.1 0.3 0.5 1.3 2.1 2 .9 3.7 7.5 0.9 0.1 0.7 1.5 2.3 3.1 3.9 8.0 0.7 0.1 0.9 1.7 2.5 3. 4.1 8.5 0.5 0.3 1.1 1.9 2.7 3.5 4.3 9.0 0.3 0.5 1.3 2.1 2.9 3.7 4.5 9.5 0.1 0.7 1.5 2.2 3.1 3.9 4.7 10.0 0.1 0.8 1.6 2.4 3.2 4.0 4.9 Note : Select a wet bulb temperature th at is above the critical damage temperature for your crop and locate the appropriate column. Then choose the row with the correct dew point temperature and read the corresponding air temperature, from the table to turn your sprinklers on or off. This table is for the mean sea level, which should be reasonable accurate up to about 500 m elevation.
50 Table 1 9. Dew point temperature (C) corresponding to air temperature and relative humidity (Snyder and de Melo Abreu 2 005b) Dew point Temp. Wet bulb temp. C C 2.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 100 2.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 90 3.4 1.4 0.5 2.5 4.5 6.5 8.4 10.4 80 5.0 3.0 1.1 0.9 2.8 4.8 6.7 8.7 70 6.7 4.8 2.9 1.0 1.0 2.9 4.8 6.7 60 8.7 6.8 4 .9 3.0 1.2 0.7 2.6 4.5 50 11.0 9.2 7.3 5.5 3.6 1.8 0.1 1.9 40 13.8 12.0 10.2 8.4 6.6 4.8 3.0 1.2 30 17.2 15.5 13.7 12.0 10.2 8.5 6.8 5.0 20 21.9 20.2 18.6 16.9 15.2 13.6 11.9 10.2 10 29.5 27.9 26.4 24.8 23.3 21.7 20.2 18.6 Note: Select a relative humidity in the left column and an air temperature from the top row. Then find the corresponding dew point temperature in the table.
51 Figure 1 1. Amount of hours below freezing point (0C) and below irrigation sys tem turn ON temperature (1.1C). Historical Seasonal Years from 1979 to 2013 for Gainesville (1982 1991; (Florida Climate Center 2012a) and Citra, FL (1992 2000; Florida Climate Center 2012b; 2000 13 (FAWN 2013; Florida Climate Center 2012b) ). 0 25 50 75 100 125 150 175 200 225 250 1979-80 1980-81 1981-82 1982-83 1983-84 1984-85 1985-86 1986-87 1987-88 1988-89 1989-90 1990-91 1991-92 1992-93 1993-94 1994-95 1995-96 1996-97 1997-98 1998-99 1999-00 2000-01 2001-02 2002-03 2003-04 2004-05 2005-06 2006-07 2007-08 2008-09 2009-10 2010-11 2011-12 2012-13 Hours Year season Hours < 0 C Hours < 1.1 C
52 Figure 1 2. Effect of three frequencies of application (120 s, 60 s and 30 s) over leaf temperature using a sprinkler application rate of 2.8 mm hr 1 und er 1.53 m s 1 wind conditions, a wet bulb and air temperatures near 2.0C and 0C (Wheaton and Kidder 1965) -3.0 0.0 3.0 6.0 0 20 40 60 80 100 120 140 160 Leaf Temperature ( C) Time (s) 30 s 60 s 120 s Wet bulb Temp. Air Temp. AR= 2.8 mm hr 1 Wind speed= 1.53 m s 1
53 Figure 1 3. Short crops application rates. Over plant conventional sprinkler application rate requi rements for frost protection of short crops with head rotation rates of 30 s and 60 s. Wind speed ranges from 0.0 m s 1 at the bottom to 2.5 m s 1 at the top (Schultz and Lider 1968; Snyder and de Melo Abreu 2005b) 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 0 -2 -4 -6 -8 Application Rate (mm hr 1 ) Minimum Temperature ( C) Short Crops 30 s Short Crops 60 s Wind Sped 0.0 2.5 m s 1
54 CHAPTER 2 EFFECT OF SPRINKLER TYPE AND PRESSURE ON IRRIGATION UNIFORMITY Introduction Agricultural irrigation has been used since ancient times to meet the crop water needs for evapotranspiration (ET) (Ali 2010a) However, when calculating the amount of water required, other parameters must be considered, e.g. soil and plant type, plant stage of growth and climatic conditions, in order to optimize the irrigation management (Ali 2010a) Irrigation management also depends on the irrigation system characterization, management practices and soil characteristics of the area irrigated. In Florida, during the strawberry season, water is required for successful establ ishment; growth and development, system maintenance, chemical delivery (fertigation), and cold protection among others (Albregts and Howard 1984) When water is applied through irrigation, there are two main issues: how well is the applied water used? And how uniformly is the water distributed to the plants? (Burt et al. 1997b) Therefore, an irrigation system can be evaluated based on two metrics: (i) uniformity and (ii) efficienc y (Irrigation Association 2011) Uniformity. Irrigation distribution uniformity (DU) is a parameter to measure the evenness of water application to a crop over an area, and it is negatively affected when va riation increases (Ali 2010b) Generally it is expressed as a decimal; therefore a value of 1.00 would represent an ideal and perfect uniformity, meaning an equal amount of water received at any point within the irrigat ed area, which is unlikely to occur in reality (Ali 2010b; Irrigation Association 2011) One of the conventional methods to describe irrigation uniformity is the lower quarter distributio n uniformity (DU lq ), which compares the driest 25% of an irrigated area to the total area. This method is preferred since it is
55 easily measured in the field (Irrigation Association 2011) A table of expect ed DU lq values according to the sprinkler type used is shown in Table 2 1.Three main components are required in order to achieve high sprinkler uniformity: nozzle type, pressure level according to the selected nozzle, and spacing, based on the two previous components. Uniformity will decrease proportionally if one or more of the elements changes or reduces its optimal performance (Irrigation Association 2011) The distribution of water will depend on the sp rinkler nozzle distribution to the soil or crop, and afterward its distribution in the soil profile from the soil surface (Pair 1968) Therefore, non uniform sprinkler water applications may result from different sprin kler rotation speeds, diameter changes due to wear, asymmetrical trajectory angle due to non vertical risers and wind influence over the aerial water distribution (Irrigation Association 2011) Efficiency. Even when efficiency is related to u niformity, and sometimes both terms are used interchangeably, but are different concepts. When water is applied to crops, the percent of the water applied beneficially used by the plants, is referred as efficiency. Lower efficiency values will be obtained as a result of water unavailability (lack of water at the root zone) or not used (plant water needs satisfied) after its application (Irrigation Association 2011) Uniformity and efficiency can be affected by different factors, which can be categorized as: irrigation system components (installation, maintenance and repair) and water management human factors (scheduling and timing). Uniformity is associated with the former components, while efficiency is ass ociated with both factors. More factors and the combination influence the efficiency and distribution of water use (e.g. soil,
56 water, plants, irrigation system and scheduling which also are related to the design, installation, maintenance and adequate mana gement of the crop and the irrigation) (Irrigation Association 2011) Therefore, a combination of these factors: the distribution patterns from adjacent sprinklers, the pressure in the system as well as th e nozzle size variation throughout the field influence the sprinkler irrigation DU (Burt et al. 1997b) Some studies have determined wind effects over irrigation distribution uniformity when it exceeds 1.8 2 m s 1 (Mateos 1998) A non uniform distribution may result in yield damage due to lack of water, or on the other hand, over irrigation can occur causing plant injury, water logging, salinization and chemical transport leading to groundwater pollution (Solomon 1983) Although high uniformity can be achieved; water applied can be excessive and may cause runoff and deep percolation, resulting in low application efficiency (AE). However, hig h AE (with minimal under irrigation) only can be accomplished when (DU) is high (Burt et al. 1997b) reducing consequently, over and underwater areas (Irrigation Associat ion 2011) Hence, maximum efficiency can be achieved when using irrigation systems with adequate design and maintenance that can apply uniform irrigation application (Ali 2010b) Space Pro for Simulation Irrigation sim ulation models had been developed aiming to achieve irrigation system uniformity. This means to minimize the difference among maximum and minimum wetted areas, without irrigating the dry zones, thus, avoiding over irrigation of the rest areas (Zoldoske 2007) Irrigation system uniformity is derived from single leg profile tests. These tests consist of an evaluation of the radius of single sprinklers. The application volume or
57 of throw are measured using equidistant open containers (catch cans) placed one touching each other) starting from the nearest sprinkler head and extending through the sprinkler wetted radius. When these tests are performed, only one sprinkler head is ope rated during a time enough to collect an average of 12.7 ml of water in the catch cans (Zoldoske 2007) Sprinkler Profile And Coverage Evaluation (SPACE PRO) is a computer program developed by the Center of Irrigat ion Technology (CIT) to support irrigation designers in the process of sprinkler and spacing selection (Oliphant 2005) spacing design to ge nerate a matrix, simulating a field test developed in a grid pattern (2 1). The catchment spacing of the test and the sprinkler spacing in the irrigation system design will determine the matrix size (Oliphant 2005) This program allows testing the uniformity of irrigation system designs by assessing different spacing designs, sprinkler nozzles and pressure combinations which can be compared in order to define the best efficient design, coverage and lower cost (Oliphant 2005) SPACE Pro can be an useful tool for growers to implement, in order to select optimum sprinkler type, spacing and pressure, thus achieving higher irrigation uniformity application and potentially reduce y ield damage due to over or under irrigation, or deficient sprinkler coverage when used for cold protection. Over H ead S prinkler I rrigation for Cold P rotection Low growing crops, such as strawberries, have been effectively protected from cold damage by over head sprinkler irrigation when a sufficient and uniform application rate (AR) is used. Nevertheless, under high wind conditions or very low temperatures that water application is not enough to provide greater heat than what is lost due to
58 evaporation, and in that case less damage may occur on non irrigated plants (Snyder and de Melo Abreu 2005c) Conventional impact sprinklers have been used for cold protection in strawberries; however, these sprinklers are most effe ctive when plants are coated uniformly (Snyder and de Melo Abreu 2005c) A frequency of application no longer than 60 s (Wheaton and Kidder 1965) and a variable application rat e according to minimum temperature and wind speed are recommended (Gerber and Martsolf 1965a) Higher application rates (AR) are needed for longer rotation intervals, lower minimum temperatures and higher wind speed s. AR requirements will depend on variability on conventional rotating, variable rate, or low volume targeted sprinklers. However, protection will be provided if plants are coated with a liquid ice mixture and water dripping off the icicles. Cold damage ca n occur due to insufficient AR or slow rotation rates, so the water may freeze and the temperature from ice coated plants can get lower than the non irrigated plants (Snyder and de Melo Abreu 2005c) The efficiency o f sprinkler irrigation is reduced by non uniform water application. Therefore, in conjunction with uniformity, sprinkler types and irrigation system pressure play an important role in providing high AE which might be reached through lower application rates (Burt et al. 1997b) Even when pressure, discharge and wetted diameter are the most important parameters to select a specific sprinkler type, some other parameters such as nozzle size, wind, sprinkler overlap and sprink ler rotation speed should be taken in consideration when this decision is made in order to determine the AR, the spacing
59 between sprinklers and the size of water droplets (Parker 2009) Sprinkler overlapping is a param eter to be achieved; therefore, spacing usually is designed in a matter that around 50 60% of the wetted areas overlap (Parker 2009) In Florida, two main purposes are accomplished by using sprinkler irrigation in stra wberry production fields: establishment and cold protection (Albregts and Howard 1984) Strawberry transplant establishment may require 10 to 14 consecutive days of frequent intermittent overhead irrigation, totalin g approximately 165 to 247 ha mm (1 ha mm~ 10,000 L) (Santos et al. 2010) During both establishment and cold protection, growers generally use impact sprinklers spaced at 14.6 m and a pressure of 345 kPa. However, ot her impact sprinklers are being used recently by strawberry growers: Nelson F32 at 14.6 m spacing and Rain Bird L20 at 12.2 m spacing. As well as two sprinkler rotators: Nelson R33 and Nelson R2000WF with 14.6 m and 12.2 m sprinkler spacing, respectively, are being used lately for the same purposes in strawberries (Whidden 2013) The optimum recommendations for highest survival rate and lowest amount of water were determined by evaluating controlled irrigation settin gs to protect crops under cold conditions. A two phased approach was implemented for the whole study and this chapter corresponds to the first phase accomplished. DU lq tests were performed in order to potentially maximize crop yield and efficiently use the available water supplies. The objective of this experiment was to evaluate the effect of sprinkler type, sprinkler spacing, pressure variation in the water supply line and varied wind conditions on irrigation distribution uniformity and application rate.
60 Materials and Methods In order to replicate Florida grower irrigation practices for cold protection in strawberries, one conventional impact sprinkler (WR 32) and three rotator types (Nelson R33, Nelson R33LP, Nelson R2000WF) were tested using two sprinkle r spacings (14.6m and 12.2 m) and three irrigation pressure levels (345, 276 and 207 kPa) under variable wind conditions. The most likely sprinkler used by the strawberry growers (WR 32) under the conditions generally performed (345 kPa and 14.6 m sprinkle r spacing) was evaluated at two locations (Citra and Hastings); and the results were analyzed for the specific location separately. Details are described as follows: Uniformity Testing Analysis The measure of how uniform irrigation water is distributed to different areas in the field is defined as DU This term is generally defined as a ratio of the average of the smallest accumulated depths in the distribution (DU lq ), to the average depth accumulated in all the elements (DU total avg ) (Burt et al. 1997b) The fraction defined for the formula could vary; however, the lowest quarter (1/4) depth (DU lq ) has been used by irrigation (ASCE 1978) DU lq is a ratio defined by the following formula (Merriam and Keller 1978) : (2 1) Where: : Average of the lowest quarter of catch can measurements (mL). : Average depth of application over all catch cans measurements (mL). DU lq tests were performed at two University of Florida, Institute of Food and Agricultural Sciences (UF IFAS) facilities. Initial tests were performed from March 10
61 until May 26, 2011 at Cowpen Branch Facility located near Hastings, Florida; however, most tests were completed within the period June 2011 until May 2013 at the Plant Science Research and Education Unit (PSREU) near Citra, Florida Two overhead sprinkler systems were setup at 14.6 and 12.2 m sprinkler spacing and discharge was measured for uniformity under three different operating pressures and variable wind conditions. Sprinklers A visit to strawb erry farmers at Hillsborough, FL was done in April 2011 with the aim to investigate the types of sprinklers, spacing and pressure used to protect their crops. In accordance with this visit, it was concluded to evaluate four types of sprinklers in order to replicate the most common grower irrigation practices for cold protection in strawberries. Among them the sprinkler traditionally used by growers was evaluated: Wade Rain WR 32 brass impact sprinkler aluminum arm with 3.6 mm nozzles for low volume applications (Wade Rain Inc. 2007) and also three Nelson rotators: R33 and R33LP (low pressure model) both using 3.6 mm nozzles (Nelson Irrigation Corporation 2003) and Nelson R2000WF (Nelson Irrigation Corporation 2009) with 3.18 mm nozzle and a purple diffuser. At the Cowpen Branch Facility located near Hastings, only the WR 32 impact sprinklers were tested, while at PSREU, near Citra, all sp rinklers were evaluated (Table 2 2). Collectors and Spacing The distribution uniformity of the sprinkler irrigation system was tested using applied after an irrigation even t (2 2). The sprinkler types were evaluated under two different spacings: 14.6 m by 14.6 m, being this the more used spacing by strawberry
62 growers and 12.2 m by 12.2 m (2 3), being close to the manufacturer recommended spacing in order to achieve optimum o verlapping. Two uniformity testing areas were established which consisted of seven lines of plastic collectors, seven collectors per line, totaling 49 collectors per test. The catch cans of approximately 20 cm diameter, and 16 cm tall were spaced uniformly along the straight lines forming an equally spaced grid. The collector spacing along each line was 1.8 m when the 14.6 m sprinkler spacing was tested, and 1.5 m collector spacing when the 12.2 m sprinkler spacing was evaluated (2 3). Irrigation System Sup ply Pressure In order to test the recommended cold protection application rate of 6.35 mm hr 1 tests were performed under different pressures during one hour irrigation event, time enough to collect the minimum volume of water recommended for cold protect ion. A control valve was manually operated and a pressure regulator series 25AUB Z3 and LF25AUB (Watts Regulator Co. 2009) was used to keep the pressure under steady conditions (2 4). This pressure r egulator was adjusted according to the pressure to be assessed (345, 276 or 207 kPa). Prior the test, the pressure desired was specified as 345, 276 or 207 kPa. According to the pressure under evaluation, Senninger pressure regulators (Senninger Irrigation Inc. 2010) and pressure gauges were placed at the bottom of each sprinkler head, in order to keep the pressure stable and monitor it at the beginning and at the end of the test (2 5). Graduated cylinder s (250 ml) were used to measure catch can volumes.
63 Irrigation Simulation U sing SPACE Pro Single leg profile tests. These tests consist of the evaluation of a single sprinkler application volume values, which can be measured using catch cans, in order to ob tain water application profiles along one or more radial leg. Therefore, for this experiment, the catch cans were placed next to each other, starting from the sprinkler head and extending beyond the sprinkler wetted radius of throw. During a one hour test duration, only one sprinkler was operated in order to capture the volume of water applied. Generally, the sprinkler water application rate defines the duration of the test; however, a minimum reading of 3 mm in the driest catch can is recommended (ASAE 1985) Once the profile tests were finished, the data was input into SPACE Pro to perform the simulations and obtain the uniformity profiles. The tests were executed in order to simulate the sprinkler type performance at three different pressures (345, 276 and 207 kPa), two nozzle types (3.6 and 3.18 mm) and two sprinkler spacings (12.2 m and 14.6 m). Using the data from the single leg profile tests, the program measures the distance from the sprinkler head to each catch can an d look up in the profile the theorem relates the three sides of a right triangle and states that the square of the hypotenuse (the side opposite to the right angle) is equal to the sum of squares of the two other sides. Therefore, using this theorem, SPACE Pro squares and sums the looked up volume values (from the single leg profile tests) at the distances where the catch cans are from the sprinkler head, and finally takes the square root of them to obtain the AR for that specific catch can (i.e. the hypotenuse). These calculations are
64 performed in the program filling a grid for the total area irrigated, which can be shown as a densogram. Calculations. SPACE Pro calcu (CU), DU lq AR, and scheduling coefficient, which normally is based on 1% of the covered area (Oliphant 2005) Densograms. A densogram is a pattern of dots showing th e irrigation coverage resulting from the combination of sprinklers, nozzles, pressure and spacing (Solomon 1988) The densograms and scheduling coefficient are SPACE Pro tools provided with the aim of testing sprink lers systems designs and solving potential problems before being installed. These tools provide a preview of irrigation using specific sprinkler types and spacing combinations (Solomon 1988) For this study, an expe riment was conducted in order to simulate full field sprinkler irrigation uniformity obtaining water distribution profiles at three pressure levels and two sprinkler spacings of two different sprinkler types: WR 32 impact sprinklers and Nelson R33 rotator sprinklers. Single leg sprinkler profile tests were performed at PSREU, near Citra under near no wind conditions to avoid measurement distortions. SPACE Pro was used to simulate the DU lq and AR using the data from single leg sprinkler profile tests descri bed previously. Data Analysis At two different UF IFAS facilities, two main response variables were analyzed: Low quarter distribution uniformity (DU lq ) Application rate (AR)
65 These response variables were analyzed in a three factorial experiment with diff erent sprinkler types, pressures and spacings. Data recording for the different treatments resulted in unequal sample sizes. The GLM procedure of Minitab was used to compute the ANOVA for unequal sample sizes, the adjusted least square means and their cor responding standard error (SE) (Minitab Inc. 2013) In most cases, the H 0 was rejected for the first order interaction, so the specific treatment means had to be compared using the Bonferroni simultaneous multiple co mparison procedure. Also to test the differences between treatment main effects the LSD according to Bonferroni (LSD Bon ) was applied. This comparison was performed using the 95% CI based on their specific SE. In order to reduce the error variation (MS Erro r) the wind speed (m s 1 ) as a co variable was recorded. This co variable was always highly significant and contributed means. DU lq and AR results are presented in this document. Results and Discussion A total of 339 catch can tests were conducted under a variety of wind conditions at the two locations. Due to the variability present at the test locations, separate analyses were performed for PSREU and Cowpen Branch UF IFAS facilities, respectively. The former shows the comparison between four sprinkler types, two sprinkler spacings, and three irrigation system pressures tested at Citra, FL; while the latter analysis evaluated only the WR 32 impact sprinklers 14.6 m spaced, (sprinkler type and spacing conditions more likely used by strawberry growers in Florida) evaluated at the three different irrigation system pressures (345, 276 and 207 kPa) at Hastings, FL. The procedure of LSD Bon was applied for main treatment me an comparisons.
66 Wind Conditions wind conditions are unpredictable, DU lq tests were conducted under different wind conditions from nearly no wind (min avg. 0.2 m s 1 ) to hig h wind speed conditions (max. avg. 7.2 m s 1 ) in a few tests. However, the overall average wind speed during the tests was 2.7 m s 1 A FAWN weather station located on site was used to monitor wind speed at 10 m height, giving an output average every 15 mi nutes. Wind conditions were categorized as low < 1.73 m s 1 medium 1.74 to 3.53 m s 1 and high > 3.53 m s 1 However, quantitative wind measurements (m s 1 ) were included in the ANOVA analysis as a covariate. Sprinkler Irrigation Uniformity Testing (DU lq ) Citra A total of 297 catch can tests were evaluated at PSREU, near Citra, FL from Jun. 2011 to May 2013. During the catch can tests performed, the average wind speed ranged between 1.74 to 3.53 m s 1 being categorized as medium wind speed conditions (Tabl e 2 3, Table 2 4). Among the four sprinklers when 14.6 m sprinkler spacing was used over the three pressure levels, maximum and minimum DU lq was 0.89 and 0.49, respectively (Table 2 3), while 12.2 m sprinkler spacing yielded respective maximum and minimum DU lq of 0.90 and 0.43 under medium wind speed conditions (Table 2 4). Overall average DU lq was 0.73 and 0.76 for 14.6 m and 12.2 m sprinkler spacing; respectively under medium wind speed conditions (1.74 to 3.53 m s 1 ) (Table 2 3, Table 2 4). Irrigation lo w quarter distribution uniformity tests (DU lq ) were significantly influenced by wind speed (Table 2 5). Furthermore, the interactions among factors:
67 sprinkler pressure, sprinkler spacing, and pressure spacing resulted in a statistically significant effect on uniformity (Table 2 5) and are explained as follows: Sprinkler type pressure interaction The interaction among sprinkler and pressure resulted in a significant effect on DU lq (2 7). The four sprinklers tested under three irrigation supply pressures re sulted in five different statistical groups. The two highest DU lq values were achieved by Nelson R2000WF at 345 kPa and at 276 kPa under low wind conditions (DU lq = 0.81 for both conditions), followed by WR 32 at the same pressures under medium wind conditi ons (DU lq = 0.80 and 0.79, accordingly). High DU lq values were achieved by Nelson R33LP (DU lq = 0.79) and R33 (DU lq = 0.77) only at the 345 kPa pressure. No significant differences were present among any of these sprinkler pressure interactions and neither be tween those sprinkler types and pressures. However, significantly lower distribution uniformity resulted when using WR 32 and Nelson R2000WF at 207 kPa irrigation system pressure versus the same sprinklers evaluated under higher pressures. Nelson R33 at 27 6 kPa resulted in significantly lower uniformity (DU lq = 0.70) than the sprinklers previously described, but significantly higher than Nelson R33LP and R33 when used at the lowest pressure (DU lq = 0.66 and 0.61, respectively). The lowest Du lq was obtained by the Nelson R33 when a 207 kPa irrigation system pressure was tested, which is not surprising, since this sprinkler should be used under higher pressures according to manufacturer recommendations (Table 2 2). Comparing the sprinkler types separately under each pressure, all the sprinkler types achieved the highest low quarter distribution uniformity (DU lq ) values when using the highest pressure (345 kPa), and vice versa, also obtained the lowest DU lq values
68 when tested under the lowest pressure (207 kPa) (2 6). Highest pressures (345 and 276 kPa) were significantly different versus the lowest pressure (207 kPa) within each sprinkler type. However, no significant differences were found when the two highest pressures were compared among them for all the sprink ler types, with the exception of the R33 sprinkler rotator. The effect of the sprinkler Nelson R33 and variable pressures resulted in significantly different DU lq values at each pressure level, obtaining the lowest DU lq among the sprinklers at 207 kPa pres sure. Sprinkler type sprinkler spacing interaction The sprinkler type spacing interaction resulted in a significant effect on distribution uniformity (2 7). The highest DU lq was achieved by the impact sprinkler WR 32 at 12.2 m sprinkler spacing, followed by the rotator Nelson R2000WF using the same spacing (DUl q = 0.81 for both) Likewise, the effect of spacing over uniformity was not significant when using the R2000WF, which reached a similar irrigation uniformity at the two sprinkler spacings (DUl q = 0.76 and 0.81, respectively) This same pattern was followed by the Nelson rotator sprinklers, which did not differ significantly when tested under different sprinkler spacing. Nevertheless, this was not the case of the WR 32 impact sprinkler, which resulted i n significantly higher DU lq at 12.2 m versus 14.6 m spacing (DUl q = 0.81 versus 0.74) (2 7). However, when a comparison among the sprinkler types at each spacing is performed, significant differences among them are present in some cases (2 7). At the 12.2 m spacing, WR 32 and R2000WF (DU lq = 0.81 and 0.81) were significantly higher on uniformity than the Nelsons R33LP and R33 (DU lq = 0.74 and 0.68), the latter representing the lowest overall uniformity value. When the sprinkler types were
69 performed under 14.6 m spacing, a similar uniformity pattern was more likely to occur, where the R2000WF and the WR 32 obtained the highest DUlq values (DU lq = 0.76 and 0.74), but only the former obtained a significantly higher uniformity than the rest of the sprinklers (DUl q R 33LP= 0.72 and R33= 0.70) (2 7). Irrigation system pressure sprinkler spacing interaction Significant effect over distribution uniformity resulted from the irrigation system pressure and sprinkler spacing interaction (2 8). When the irrigation system pres sure was 345 kPa, the highest uniformity values were obtained at either sprinkler spacing (DU lq 0.80 and 0.80). However, when the irrigation system pressure was reduced to 276 kPa and at a sprinkler spacing of 12.2 m DU lq of 0.78 was not statistically diff erent. In contrast, when the sprinkler spacing was increased to 14.6 m, DU lq was significantly lower (0.74). The same trend occurred at the lowest irrigation system pressure (207 kPa), which differs statistically when the sprinkler spacing is increased to 14.6 m (DU lq at 12.2 m= 0.71 versus 14.6 m= 0.66), these being the lowest overall uniformity values (2 8). Hastings At the Cowpen Branch UF/IFAS facility, near Hastings, at total of 50 DU lq tests were performed within the period of March May 2011. These t ests only evaluated the impact sprinklers WR 32 at 14.6 m sprinkler spacing under three irrigation supply pressures (345, 276 and 207 kPa). Generally, wind conditions predominant during the tests performed at Hastings were high (> 3.53 m s 1 ). The sprinkle r type and sprinkler where high wind conditions are common.
70 A comparison among DU lq values obtained at Hastings was performed versus the results acquired at PSREU, near Cit ra by using the same sprinkler type, pressure levels and spacing settings (Table 2 6). Irrigation system pressure location interaction. The results from the interaction among the pressure and location caused a significant effect on distribution uniformity (2 9). The highest mean uniformity value (DU lq = 0.75) was achieved when 345 kPa was used as irrigation pressure supply at the PSREU, Citra, presenting significantly higher differences when compared to 207 kPa at Hastings and Citra (DU lq = 0.66 and 0.63, acc ordingly). Higher DU lq values were obtained under higher pressure levels. However, when using the highest pressure (345 kPa) at Hastings, distribution uniformity was lower than the DU lq value obtained when decreasing the irrigation pressure to 276 kPa at H astings (DU lq = 0.68 and 0.71, correspondingly). Nevertheless, differences were not significant when this interaction was compared to the three pressure levels at Hastings (DU lq at 345 kPa= 0.68, 276 kPa= 0.71 and 207 kPa= 0.66), or when it was compared to 276 kPa at Citra (DU lq = 0.69) (2 9). A similar pattern was followed when the pressure was set on 276 and 207 kPa at the two locations, where none of those interactions resulted in significant effects on uniformity. Wind Speed. During the catch can tests pe rformed at Hastings, the average wind speed ranged between 5.15 and 4.59 m s 1 category which presented speed conditions lower than 2.31 m s 1 on average (Table 2 6). The maximum DU lq value was obtained when using the highest pressure (345 kPa) under medium wind conditions at Citra; while the lowest DU lq value was obtained when
71 using the same pressure under high wind speed conditions at Hastings (Table 2 6). Wind speed, which was included as a co variable, resulted in high significant effect on distribution uniformity (Table 2 7; 2 10). Application Rate (AR) Water application rates (AR) were evaluated at the two different locations (Citra and Has tings), using three irrigation supply system pressures (345, 276 and 207 kPa) and two sprinkler spacings (14.6 m and 12.2 m). Results from the two locations are explained in the following section. Citra Overall average application rates using four sprinkle rs types and three pressure levels were 2.9 mm hr 1 for the 14.6 m sprinkler spacing and 4.5 mm hr 1 when 12.2 m spacing was evaluated (Table 2 3, Table 2 4). Therefore, a 55.2% overall increase in the application rate resulted as an effect of shortening t he sprinkler spacing. It was determined through an ANOVA that the wind speed (as co variable) and the interactions among factors: sprinkler pressure, sprinkler spacing and pressure spacing resulted in a statistically significant effect over application rat e when evaluated at the PSREU, near Citra, FL (Table 2 8). Each interaction is described as follows: Sprinkler type irrigation system pressure interaction. The mean application rates, as a result of the pressure levels and sprinkler types interactions, var ied almost 50% in some of the cases. The AR means ranged between a minimum of 2.3 mm hr 1 when using the rotator sprinkler R2000WF at 276 kPa, to a maximum of 4.7 mm hr 1 applied by R33LP at a 345 kPa system pressure. The rotators R33 and R33LP produced th e highest water application rates at a pressure of 345 kPa and then at 276 kPa (AR= 4.6, 4.5, 4.2 and 4.1 mm hr 1 respectively), without significant differences
72 among them (2 11). Both sprinkler types applied lower water rates when using the higher pressu res than the recommended by the manufacturer (Table 2 2). The impact sprinkler WR 32 using the highest pressure did not differ significantly on water application rate (AR= 4.0 mm hr 1 ) from the previous rotators when both were performed at 276 kPa (2 11). However, it was lower than the recommendation at 345 kPa (i.e. 5.5 mm hr 1 at 13.0 sprinkler spacing; Table 2 2). A similar pattern occurred comparing the WR 32 application rate of 3.5 mm hr 1 at 276 kPa system pressure, and the 3.2 and 3.0 mm hr 1 AR of t he rotators R33LP and R33 at 207 kPa pressure, without statistical differences between them (2 11). All these AR were lower than the recommended AR (when applicable). The AR values by using R33 at 207 kPa pressure might be as a result of not recommendation s applicable for this sprinkler at low pressure levels (Table 2 2). The lowest mean application rate was obtained by the R2000WF sprinkler performed under the three pressure levels (345 kPa= 2.9, 276 kPa= 2.3 and 207 kPa= 2.5 mm hr 1 ). The R2000WF evaluate d at 276 and 207 kPa obtained statistically lower water AR than the rest of the sprinkler types at the three pressure levels (Fig 2 11). The results at 207 kPa are not surprising since not recommendations have been developed for this sprinkler at the lowes t pressure level evaluated at the experiment (Table 2 2). Sprinkler type sprinkler spacing interaction. Variations on sprinkler type and spacing leaded to statistical differences in application rate. A value of 5.1 mm hr 1 was the highest mean AR acquired by R33LP sprinkler at 12.2 m spacing, which resulted significantly different from the other sprinkler spacing interactions. However, this AR value is close to the lower limit for low pressure applications range recommended by the
73 manufacturer; this meaning that higher application rates could be reached if it is used on higher pressures (e.g. 5.6 mm hr 1 at 345 kPa 12.8 m spacing; Table 2 2). Similar mean water application rate values were obtained by the Nelsons R33 and R33LP performed at 14.6 m spacing (AR = 3.3 mm hr 1 for both sprinklers); however, statistically different AR were obtained when 12.2 m spacing was used (AR= 4.7 and 5.1 mm hr 1 for R33 and R33LP, respectively; 2 12). Thus, application rates were more likely to follow the manufacture recommenda tions achieving similar ARs when using the lower spacing for these two sprinkler types. The impact sprinkler WR 32 mean application rates were 2.6 and 4.4 mm hr 1 using 14.6 m and 12.2 m spacing (2 12). These results showed lower AR than the manufacturer r ecommendations; however, when using the 12.2 m sprinkler spacing similar results could be obtained at the lowest pressure level (e.g. 4.7 mm hr 1 at 207 kPa, 12.3 m spacing; Table 2 2). Thus, the Nelson R33, R33LP and impact WR 32 sprinklers obtained 30%, 33% and 40% increase in mean application rate when the sprinkler spacing was reduced from 14.6 m to 12.2 m. All of them were statistically different within and among sprinkler types. However, this was not the case for the R2000WF rotator, which obtained AR of 2.4 and 2.7 mm hr 1 (at 14.6 m and 12.2 m spacings, accordingly); not presenting statistical differences between them. The rotator R2000WF obtained an AR of 2.4 mm hr 1 at 14.6 m spacing representing the lowest AR versus the rest of the sprinkler spaci ng interactions (2 12). Also, it is important to note that AR values at 14.6 m may be as a consequence that there are not recommendations applicable for this sprinkler at large spacing (Table 2 2).
74 Irrigation system pressure sprinkler spacing interaction. Significantly higher water application rates were obtained when the 12.2 m sprinkler spacing was used compared to the 14.6 m spacing among all pressure levels. Reducing the sprinkler spacing to 12.2 m, resulted in an AR increase of 30%, 30% and 33% when ev aluated at the three irrigation pressure levels 207, 276 and 345 kPa, correspondingly (2 13). The highest application rate was obtained using the highest pressure level at the shortest sprinkler spacing, this being an AR of 4.8 mm hr 1 using 345 kPa at 12. 2 m spacing, which was statistically different from the rest of interactions. The succeeding highest ARs were obtained using the reduced sprinkler spacing of 12.2 m at 276 kPa followed by 207 kPa (AR= 4.1 and 3.7 mm hr 1 ). Lower application rates were achi eved when a large distance among sprinklers was used. Mean application rates of 3.2, 3.0 and 2.6 mm hr 1 were obtained at 345, 276 and 207 kPa (2 13). All the irrigation system pressure sprinkler spacing interactions were significantly different from each other, with the exception of the 345 and 276 kPa pressures at 14.6 m sprinkler spacing, which did not differ significantly. The lowest application rate was obtained by using the lowest irrigation system pressure at the larger spacing among sprinklers (2 13 ). Hastings At the Cowpen Branch facility, near Hastings only the impact sprinklers WR 32 spaced 14.6 m at three pressure levels (345, 276 and 207 kPa) were evaluated. A comparison was performed with the results achieved at the PSREU, near Citra with the s ame irrigation settings. The highest mean water application rate was obtained using 345 kPa system pressure (AR= 3.1 mm hr 1 ) at Hastings. However, this AR differed from the mean rate
75 obtained from using the same sprinkler type and pressure level at Citra (AR= 2.9 mm hr 1 ). A significantly lower AR was obtained at Citra when the highest pressure was performed. Nevertheless, only this application rate was significantly different using the same settings when evaluated at the two locations (2 14). Evaluating t he 276 kPa pressure level at both facilities resulted in a mean AR of 2.9 mm hr 1 at Hastings and 2.7 mm hr 1 at Citra, and when the lowest pressure level was tested, mean AR of 2.2 mm hr 1 for both locations were obtained. Thus, equal pressure levels (276 and 207 kPa) evaluated at both locations did not differ significantly. Statistical differences were only found only when pressure levels were compared within them (2 14). Space Pro Results Uniformity profiles were simulated using SPACE Pro for two sprinkl er types: WR 32 impact sprinklers and Nelson R33 rotator sprinklers. A comparison among sprinkler types was performed at the three pressure levels and two sprinkler spacings (Table 2 9, 2 15). Data from sprinkler single leg profile tests was input into the program previously described in order to develop distribution profile simulations for both sprinkler types. In order to describe the results from this section, a comparison was performed among individual sets of data, which are called treatments for this results section. The treatments evaluated consisted of: (i) Field data evaluated at PSREU, Citra and (ii) simulated data using SPACE Pro. The relationships between distribution uniformity and application rate at different sprinkler types, pressures, spacin gs and pressure variations were evaluated at Citra and simulated using SPACE Pro and are described as follows:
76 Distribution Uniformity (DU lq ) WR 32 sprinklers The interaction spacing treatment caused a significant effect on WR 32 distribution uniformity, as well as the irrigation system pressure on uniformity. WR 32 sprinklers SPACE Pro simulations vs. field data. Significantly higher DU lq values were obtained at 345 kPa using a 14.6 m spacing and at 276 kPa using 12.2 m spacing both evaluated a t Citra (D U lq = 0.87 and 0.85 respectively ). As well, significantly higher DU lq values were obtained when WR simulated using SPACE Pro at 345 kPa and 12.2 m spacing (DU lq = 0.84 ) in comparison to the two lowest pressures (276 and 207 kPa ) performed in both treatments at 14.6 m sprinkler spacing. No statistically differences were present between the treatments when WR 32 was compared at 12.2 m spacing and the same pressure level (DU lq = 0.84, 0.83, 0.77 at 345, 276 and 207 kPa for SPACE Pr o and DU lq = 0.83, 0.85 and 0.75 for the same pressure levels evaluated at Citra). However, WR 32 SPACE Pro simulations at 14.6 m sprinkler spacing resulted in significantly lower DU lq values of 0.66, 0.64 and 0.58 when evaluated at 345, 276 and 207 kPa, ac cordingly. I n comparison, the field data resulted in DU lq values of 0.87, 084 and 0.77 evaluated at the same spacing and pressure levels respectively. By instance, the simulations at 14.6 m in SPACE Pro were the lowest DU lq values obtained across all the s pacings settings and pressure levels at the two treatments (modeled and observed data) (Table 2 9, 2 15). WR 32 distribution uniformity was statistically influenced by the irrigation system pressure supply. The results showed significantly higher DU lq valu es as the pressure level increased, following the same pattern as the field data results (Table 2 9, 2 15). No significant differences were present between the SPACE Pro data and the field data
77 among the two highest pressures and at the two sprinkler spaci ngs, except for the data simulated at 14.6 m spacing which presented very low DU lq at the three pressure levels. However, significantly lower DU lq values were obtained when the lowest pressure was evaluated throughout the simulations and field assessments. The overall significantly lowest uniformity values were obtained throughout simulations in SPACE Pro using the 276 and 207 kPa pressure levels and 14.6 m spacing (Fig. 2 15). SPACE Pro simulations using WR 32 sprinklers at varied factors. WR 32 distributi on uniformity increased on average 30% at all the three pressure levels when the spacing among sprinklers was reduced to 12.2 m. Average DU lq values were 0.84, 0.83 and 0.77 at the pressures 345, 276 and 207 kPa; respectively (Table 2 9; 2 15). These resul ts also showed that very similar distribution uniformity values can be achieved when the highest pressure levels are used, but uniformity may be reduced by 8% if the lowest pressure (207 kPa) is used. Higher DU lq values achieved at 12.2 m sprinkler spacing might be attributable because a better overlapping among the sprinklers is achieved since the shorter sprinkler spacing is close to the manufacturer recommendations, in which a sprinkler spacing within 13.0 m and 12.3 m should be used at the highest and t he lowest pressures under evaluation; respectively (Table 2 2). A comparison of WR 32 sprinklers evaluated at the three pressure levels at the two spacings was performed and is shown in 2 16. WR 32 impact sprinklers simulated at 14.6 m spacing (2 16 A, C, E) showed the lowest water application values in the center area between the sprinklers (2 16 red square), demonstrating a possible lack of sprinkler overlapping even when the highest
78 pressure was performed (2 16 A). Low DU lq values might resulted since th e sprinkler spacing recommendation ranges from 13.0 m to 12.3 m for high and low pressure; respectively, both values being below the 14.6 m sprinkler spacing tested. However, when the sprinkler spacing was lowered to 12.2 m (2 16 B, D, F), which is closer to the manufacturer recommendations, distribution uniformity values increased and sprinkler overlapping occurred. The only exception was when the lowest pressure was performed, which presented the highest water values concentrated in the center area (green square) resulting in overall lower uniformity (DU lq = 0.77) compared to the highest pressures (DU lq = 0.84, 83 for 345 and 276 kPa, respectively) (2 16 F). Nevertheless, no significant differences in uniformity were present between any of those pressure lev els when using the 12.2 m sprinkler spacing. Nelson R33 sprinklers. Using R33 sprinkler significant differences on DU lq were present between the treatments (SPACE Pro and field data) and also among the pressure levels (Table 2 10; 2 15). Nelson R33 SPACE P ro simulations vs. field data. Field data resulted in significa ntly higher uniformity values in comparison to SPACE Pro simulations only when evaluated at 207 kPa pressure level at both spacings. The highest overall DU lq value was obtained at the highest p ressure and larger spacing evaluated at Citra (DU lq =0.80 at 345 kPa and 14.6 m spacing). However, no significant differences were found when it was compared to the two highest pressures at both spacings evaluated at Citra (DU lq = 0.75 at 345 kPa 12.2 m spa cing, DU lq = 0.72, 0.70 at 276 kPa evaluated at 12.2 m and 14.6 m spacings evaluated at Citra), neither compared to SPACE Pro
79 simulations at 345 kPa for both spacing (DU lq = 0.70 and 0.68 at 14.6 m and 12.2 m spacing, respectively) (Fig. 2 15). Irrigation sy stem pressure influenced uniformity, as significantly lower uniformity values were obtained under lower pressure levels evaluated. The DU lq values obtained at 207 kPa pressure were significantly lower (DU lq = 0.64, 0.63, at 12.2m and 14.6m performed at Citr a, and DU lq = 0.50 and 0.47 SPACE Pro simulations at the same spacings) than the uniformity values obtained at any higher pressures performed in the field (Table 2 10; 2 15). R33 uniformity tested at Citra at the two highest pressures and two spacings (DU lq = 0.80, 0.75 at 345 kPa 14.6 m and 12.2 m, DU lq = 0.72, 0.70, at 276 kPa at 12.2 m and 14.6 m spacings) did not differ from the simulated data input using the same pressure levels and spacing settings (DU lq = 0.70, 0.68 at 345 at 14.6 m and 12.2 m, according ly; and DU lq = 0.64 SPACE Pro at 276 kPa at 12.2 m spacing). The only exception was the SPACE Pro simulation using 276 kPa at 14.6 m sprinkler spacing, which presented significantly lower DU lq values (DU lq = 0.57) than the field assessments. Nevertheless, wh en the lowest pressure was tested, significantly lower DU lq values were obtained throughout SPACE Pro at 14.6 m and 12.2 m spacings (DU lq = 0.47 and 0.50, respectively) versus the field data measured at the two sprinkler spacings (DU lq = 0.63 and 0.64, accor dingly). These simulated outcomes were the lowest DU lq values obtained across the three pressure levels and two sprinkler spacings for Nelson R33 sprinkler (Table 2 10; 2 15). SPACE Pro simulations using Nelson R33 sprinklers at varied factors. Distributio n uniformity values obtained by Nelson R33 spaced 14.6 m with a pressure
80 level of 345 kPa resulted 6% higher than WR 32 DU lq values at the same spacing and pressure settings (DU lq R33= 0.70 and WR 32= 0.66). By contrast, when R33 was simulated using 276 an d 207 kPa pressure levels, the uniformity was reduced in 11.3% and 19.0% (DU lq = 0.58 and 0.47), accordingly, compared to the impact sprinklers. The lowest uniformity (DU lq = 0.47) was obtained when the lowest pressure was simulated, not surprising results since no manufacturer recommendations exist for this sprinkler at pressures below 276 kPa (Table 2 2). When Nelson R33 sprinklers were simulated at 12.2 m spacing, overall DU lq values were 0.68, 0.64 and 0.50 at the 345, 276 and 207 kPa pressure levels (Ta ble 2 10). Higher uniformity values were obtained at higher pressure levels. Alike DU lq values were achieved when the simulations were performed at the two highest pressures without significant differences among them, and by contrast, and significantly low er DU lq values resulted under the lowest pressure. Also, R33 at 12.2 m spacing showed similar values to the WR 32 performed at 14.6 m spacing. It is important to note that when reducing the sprinkler spacing, the R33 simulated DU lq increased about 12.3% an d 6.3% only at 276 and 207 kPa pressures, and by contrast, reduction of 2.9% in uniformity was obtained when 345 kPa pressure level was used. These outcomes possibly resulted since the sprinkler spacing recommended for the Nelson R33 is 13.1 m; thus, low u niformity values are expected if spacings below this value are used (Table 2 2). Nelson R33 densograms simulations performed at three pressure levels and two sprinkler spacings using SPACE Pro are shown in 2 16. The results from this sprinkler simulated at 345 kPa pressure level and spaced 14.6 m, showed that the region
81 receiving the lowest water applications was located in the center area between the sprinklers (2 17 A red square). While as lower irrigation system pressures were simulated, the distribution uniformity values were reduced, showing more color diversification among the densograms (2 17 A, C, E). When shorter sprinkler spacing was simulated, similar overall uniformity values were obtained at the two highest pressure levels (DU lq = 0.68 and 0.64 a t 345 and 276 kPa, 2 16 B, D). However, this was not the case when the lowest pressure level was simulated, which obtained on average 24.2% lower DU lq values than the two higher pressures (Table 2 9, 2 17 F). Thus, these outcomes might be the result from e xceeding or not reaching the recommendations, since the spacings under evaluation were higher or lower (14.6 m and 12.2 m) than the range recommended by the manufacturer (13.4 m and 13.1 m for 345 and 276 kPa), showing very low uniformity values and a lot of water application variations. As seen in densograms for the Nelson R33, when the sprinkler pressure is significantly below the recommended range, the water distribution profile presents high color diversification in which water falls within an annular r ing around the sprinkler, resulting in a relatively poor overlap pattern. As the sprinkler spacing increases, higher DU lq overall values were obtained and slightly less color variation is shown in the densograms (2 17 E, F). As the sprinkler pressure incre ases, the magnitude of water application color lessens and more water is distributed in the field along the radial leg. Therefore, the higher the sprinkler pressure, the larger the wetted radius using values within the manufacturer recommended range (2 17 A, B, C, D). Using a shorter spacing among the sprinklers showed the area within the radius near the sprinkler as the region
82 receiving the lowest water application values, while the higher water application values were located in the center area (2 17 B, D F). More uniformity variability was displayed under low pressures (2 17 D, F), which exceeded the pressure recommendations from the manufacturer (Table 2 2). Application Rate (AR) WR 32 sprinklers. The interaction spacing pressure as well as the interac tion spacing treatment resulted in significant effect on water application rates. AR values were significantly higher when the sprinkler spacing was reduced to 12.2 m and higher pressures were evaluated in comparison to the larger sprinkler spacing at the three pressure levels. Significantly higher AR resulted from the combination of the overall highest pressure and the lowest spacing (345 kPa and 12.2 m spacing). By contrast, the lowest AR was obtained by the lowest pressures at the largest sprinkler spaci ngs (207 kPa and 14.6 m spacing). Therefore, the highest mean AR resulted from the evaluation of R33 at Citra using 345 kPa at 12.2 m sprinkler spacing (AR= 5.3 mm hr 1 ). The following two highest ARs, but significantly lower than the previously described were obtained at 207 and 276 kPa both at 12.2 m spacing tested at Citra (AR= 4.4 and 4.2 mm hr 1 respectively). Significantly lower ARs were obtained throughout SPACE Pro simulations at the two highest pressures and the shorter spacing (AR= 3.4 and 3.2 mm hr 1 for 345 and 276 kPa, accordingly). However, none of them differed from the field data evaluated at 345 and 276 kPa at 14.6 m spacing (AR= 3.0 mm hr 1 for both pressures) neither from the SPACE Pro simulations at 207 kPa and 12.2m spacing (AR= 2.9 mm hr 1 ). The lowest AR were obtained by SPACE Pro simulations at 14.6 m spacing and 345, 276 and 207
83 kPa (AR= 2.4, 2.2 and 2.0 mm hr 1 accordingly), without significant differences among them. The spacing treatment interaction resulted in a significant effe ct on water application rate, in which the simulated and the evaluated in the field both significantly increased when shorter sprinkler spacing was performed, and vice versa (Table 2 9, Fig. 2 15).Field data using 12.2 m spacing produced statistically the highest water applications rates compared to SPACE Pro simulations at 12.2 m spacing and compared to field data and simulations at 14.6 m spacing. At each sprinkler spacing (12.2 m and 14.6 m), the observed AR field data resulted significantly higher than the AR simulated in SPACE Pro. Simulations of WR 32 impact sprinklers spaced 14.6 m resulted in AR of 2.4, 2.2 and 2.0 mm hr 1 when performed at 345, 276 and 207 kPa irrigation pressure levels. Alike AR were obtained at the two highest pressures; however, reductions of 16.6% on AR were achieved when pressure was decreased from 345 to 207 kPa. In comparison, the field data showed 3.0, 3.0 and 2.5 mm hr 1 which did not differ significantly from the simulations only at the highest pressure, but significant di fferences were present under lower pressure levels (Table 2 9, Fig. 2 15). SPACE Pro simulations reducing the sprinkler spacing to 12.2 m resulted in ARs of 3.4, 3.2 and 2.9 mm hr 1 at 345, 276 and 207 kPa pressure levels evaluated. Therefore, by decreasin g the spacing among the sprinklers led to an overall application rate increase of 42%, 45% and 45% at 345, 276 and 207 kPa pressures, compared to the larger spacing (Table 2 9). Also, the AR was reduced in 1% when the irrigation system pressure was lowered from 345 kPa to 207 kPa.
84 Nelson R33 sprinklers. Significantly higher ARs resulted from higher pressures and significant differences were present at each pressure level. As well, the spacing treatment interaction resulted in a significant effect on water a pplication rates. Therefore, statistically higher AR were applied at 12.2 m sprinkler spacing performed at Citra, followed by the AR obtained at 14.6 m spacing evaluated also in the field without significant differences compared to the AR resulted from SPA CE Pro simulations at 12.2 m spacing. The statistically lowest AR was produced by SPACE Pro simulated at 14.6 m sprinkler spacing. Also, the AR simulations and field data were significantly different from each other. Nelson R33 rotator sprinklers simulated at 14.6 m sprinkler spacing, obtained an overall AR of 2.8, 2.1 and 1.9 mm hr 1 performed at 345, 276 and 207 kPa irrigation pressures. Decreasing the pressure from the highest to the lowest achieved 32% less water application rates at this sprinkler spac ing simulation. In comparison, the field data results were 3.7, 3.4 and 2.9 mm hr 1 performed at the same pressure levels, accordingly (Table 2 10, Fig. 2 15). By shortening the spacing among the sprinklers resulted in about 43%, 48% and 42% increase in th e application rate (AR= 4.0, 3.1 and 2.7 mm hr 1 ) significantly higher compare to the AR obtained at 14.6 m sprinkler spacing. Reductions in about 33% were reached by Nelson R33 when the irrigation system pressure was reduced from 345 to 207 kPa. Similar p attern was followed by the observed values when the sprinklers were tested in the field at the three pressure levels, however they presented significantly higher water application rates (AR= 5.4, 5.0 and 3.8 mm hr 1 ) (Table 2 10; Fig. 2 15).
85 Application ra tes achieved by both sprinklers had in common an increase on AR at smaller spacings and a decrease in water application when the pressure was reduced from the highest to the lowest level. Uniformity and efficiency (i.e. how evenly is the water applied to a n area and how efficiently that water is used), are two main factors to be considered when irrigation is applied to the crops to meet their ET demand, as well as when used to protect them from cold damage. However, these two terms are affected by many fact ors (e.g. soil and plant type, water quality, pressure available, wind, irrigation system and scheduling, among many others) that also should be taken in consideration in order to reduce the risks of under or over irrigations, which lead to low application efficiencies and very likely to waste water. The interactions sprinkler type pressure, sprinkler type spacing and pressure spacing had a significant effect on distribution uniformity and application rate. Furthermore, including the wind speed as a co var iable in the analyses, resulted in significant lower distribution uniformities when high wind speed conditions were present. Significantly lower application rates can be obtained as a result of larger sprinkler spacings. However, not in all the cases optim um distribution uniformity was acquired when 14.6 m sprinkler spacing was used, since very poor overlap pattern occurred among the sprinklers. Generally, the lowest DU lq values were obtained under the lowest irrigation system pressure across all the sprink ler types. In some sprinkler types (e.g. WR 32 and R2000WF), no differences in uniformity were present when using the two highest pressure levels (345 kPa or 276 kPa); however, other sprinkler types (e.g. Nelson R33),
86 obtained significantly lower DU lq valu es at each pressure level. In some instances, increased pressure improves distribution uniformity. It was determined that the interaction among the pressure variation and sprinkler spacing caused different DU lq values mainly for the 276 and 207 kPa pressur e levels. The highest pressure at the two spacings did not differ significantly at the two sprinkler spacings. It was found that the effect of spacing on uniformity was higher than the effect of pressure on uniformity and on application rate. Impact sprink lers evaluated under high wind conditions resulted in distortions on uniformity very observable when evaluated at open fields located at Hastings. Simulating sprinkler distribution patterns, throughout the use of programs, i.e. SPACE Pro, may contribute in making decisions over the selection of sprinkler types, pressures, spacings, and other factors which impact the uniformity and efficiency of the irrigation systems before their installation or improve the systems if already installed. However, for this pa rticular study, SPACE Pro results showed that the simulation data did not correspond with real uniformity and water application rate values. SPACE Pro densograms can contribute to visually interpret uniformity profiles and track over and under irrigated ar eas and find possible solutions; however, more preciseness may be required to be able to compare simulated values with real field values. Even when the application rate recommended for cold protection of 6.35 mm hr 1 has been used largely and successfully the results of this research found that lower application rates can be achieved by using any of the sprinklers simulated and tested at the field, which on average, overall sprinklers applied lower than 5.63 mm hr 1 Therefore, more opportunities to reduc e the amount of water applied through irrigation
87 using lower pressures in the system, but achieving adequate irrigation uniformity can be accomplished.
88 Table 2 1. Expected DU lq values according to the sprinkler type (Irrigation Association 2011) Sprinkler type DU lq Achievable Target Historical Rotary 0.75 0.85 0.65 0.75 0.55 0.65 Spray 0.65 0.75 0.55 0.65 0.45 0.55 (*)= Asterisk denotes that if DU lq values obtained are lower than historical, then i t should be consider improving layout or changing components. Table 2 2. Manufacturer recommendations for sprinklers tested (WR32, R33, R33LP and R2000WF) at Citra and Hastings, FL. (Nelson Irrigation Corporation 2003; Nelson Irrigation Corporation 2009; Wade Rain Inc. 2007) Sprinkler type Description Plate Nozzle Flow rate, Radius of throw and App. Rate at different pressures (kPa) (mm) 345 276 207 WR 32 Brass Aluminum 3.6 (L h 1 ) 924 827 715 impact arm RAD (m) 13 13 12 *AR (mm hr 1 ) 6 5 5 Nelson Rotator Gold 18 LPH 924 827 715 R33 RAD 13 13 *AR 5 5 Nelson Rotator Gold 18 LPH 924 827 715 R33LP (Low Press. RAD 13 13 12 Model) *AR 6 5 5 Nelson Rotator Red 3.2 LPH 734 656 2000WF (w/purple WF16 RAD 12 12 diffuser) *AR 5 5 RAD= radius of throw. *AR= Application rate calculated from flow rate and radius of flow recommended by the manufacturer. ( )= Not available manufacturer recommendations for that specific pressure.
89 Table 2 3. Uniformity (DU lq ) and application rate (AR: mm hr 1 ) averages for four sprinkler types measured over three irriga tion system supply pressures at 14.6 m sprinkler spacing and variable wind conditions. PSREU, Citra, FL. Sprinkler Spacing 14.6 m Type Pressure DU lq Avg AR Avg wind (kPa) Max Min Avg n (mm h 1 ) Speed (m s 1 ) Category WR 32 345 0.89 0.64 0.79 31 2.96 2.31 med 276 0.87 0.53 0.74 21 2.70 1.95 med 207 0.82 0.49 0.68 21 2.24 1.91 med R33 345 0.83 0.50 0.76 5 3.68 3.27 med 276 0.73 0.63 0.71 5 3.36 3.17 med 207 0.66 0.61 0.64 5 2.85 2.45 med R33LP 345 0.86 0.74 0.79 5 3.70 2.43 med 276 0.77 0.65 0.73 5 3.39 3.10 med 207 0.67 0.57 0.65 5 2.95 0.28 low 2000WF 345 0.85 0.74 0.80 5 2.66 1.67 low 276 0.88 0.70 0.80 5 2.38 1.58 low 207 0.72 0.67 0.69 5 2.19 1.58 low Overall 0.89 0.49 0.73 2.92 2.14 Table 2 4 U niformity (DU lq ) an d application rate (AR: mm hr 1 ) averages for four sprinkler types measured over three irrigation system supply pressures at 12.2 m sprinkler spacing and variable wind conditions. PSREU, Citra, FL. Sprinkler Spacing 12.2 m Type Pressur e DU lq Avg AR Avg wind (kPa) Max Min Avg n (mm h 1 ) Speed (m s 1 ) Categor y WR 32 345 0.90 0.68 0.81 24 5.07 2.91 med 276 0.90 0.74 0.84 26 4.28 2.67 med 207 0.85 0.69 0.79 22 3.82 2.76 med R33 345 0.85 0.68 0.75 30 5.36 2.94 med 276 0.78 0.63 0.70 21 4.97 2.77 med 207 0.77 0.43 0.61 20 3.80 2.60 med R33LP 345 0.85 0.74 0.79 5 5.63 2.46 med 276 0.83 0.69 0.76 5 5.02 2.32 med 207 0.74 0.63 0.68 5 4.53 2.31 med R2000WF 345 0.84 0.79 0.83 5 3.12 2.23 med 276 0.86 0.80 0.83 5 2.20 2.11 med 207 0.77 0.72 0.75 5 2.74 2.13 med Overall 0.90 0.43 0.76 4.21 2.52
90 Table 2 5 The p values of the F test statistic for interactions from the ANOVA for distribution uniformity (DU lq ) obtained from catch can tests performed near Citra, FL. Source DU lq Wind s peed m s 1 (Co variable) ** Sprinkler*Pressure ** Sprinkler*Spacing ** Pressure*Spacing ** Sprinkler*Pressure*Spacing ns ns= not significant, *=p<0.05 and **=p<0.005 Table 2 6 Uniformity (DU lq ) and application rate (AR: mm hr 1 ) for WR 32 impact s prinkler measured over three pressures (345, 276, 207 kPa), one sprinkler spacing (14.6 m) and variable wind conditions near Hastings, FL. Press DU lq Tests (n) Avg AR Avg wind Location (kPa) Max Min Avg (mm h 1 ) Speed (m s 1 ) Categ. Hastings 345 0.79 0.44 0.68 AB 19 3.13 A 5.15 high 276 0.87 0.53 0.71 AB 16 2.89 BC 4.97 high 207 0.82 0.47 0.66 B 15 2.19 D 4.59 high Citra 345 0.89 0.64 0.75 A 31 2.90 B 2.31 med 276 0.87 0.53 0.69 AB 21 2.66 C 1.95 med 207 0.82 0.49 0.63 B 21 2.19 D 1.9 1 med Different letters within a column indicate significantly different means (LSD Bon 95%CI). Table 2 7 The p values of the F test statistic for interactions from the ANOVA for irrigation uniformity (DU lq ) and application rate (AR: mm hr 1 ), near Cit ra and Hastings, FL. Source DU lq AR (mm hr 1 ) Wind speed m s 1 (Co variable) ** ** Pressure*Location * ns= not significant, *=p<0.05 and **=p<0.005
91 Table 2 8 The p values of the F test statistic for interactions from the ANOVA for application rat e (AR: mm hr 1 ) obtained from catch can tests performed near Citra, FL. Source AR (mm hr 1 ) Wind speed m s 1 (Co variable) ** Sprinkler*Pressure ** Sprinkler*Spacing ** Pressure*Spacing ** Sprinkler*Pressure*Spacing ns= not significant, *=p<0.05 an d **=p<0.005
92 Table 2 9 WR 32 sprinkler distribution uniformity (DU lq ) and application rate (AR) means comparison among values resulted from the SPACE Pro sprinklers simulations and field data evaluated near Citra, FL at three pressure levels and two sp rinkler spacings. Sprinkler Spacing Pressure DU lq AR (mm hr 1 ) (m) (kPa) SPACE Pro Citra SPACE Pro Citra WR 32 12.2 207 0.77 ABC 0.75 CD 2.9 CDE 4.4 B 276 0.83 ABC 0.85 A 3.2 C 4.2 B 345 0.84 ABC 0.83 ABC 3.4 C 5.3 A 14.6 207 0.58 E 0. 77 BC 2.0 G 2.5 EF 276 0.64 E 0.84 AB 2.2 FG 3.0 C 345 0.66 DE 0.87 A 2.4 DEFG 3.0 CDE Different letters within a column indicate significantly different means (LSD Bon 95%CI). Table 2 10 Nelson R33 sprinkler distribution uniformity (DU lq ) and application rate (AR) means comparison among SPACE Pro simulations and field data evaluated near Citra, FL at three pressure levels and two sprinkler spacings. Sprinkler Spacing Pressure DU lq AR (mm hr 1 ) (m) (kPa) SPACE Pro Citra SPACE Pro Citra R33 12.2 207 0.50 DE 0.64 BC 2.7 DEF 3.8 B 276 0.64 BC 0.72 AB 3.1 DE 5.0 A 345 0.68 ABC 0.75 AB 4.0 BC 5.4 A 14.6 207 0.47 E 0.63 CD 1.9 F 2.9 DEF 276 0.57 CDE 0.70 ABC 2.1 EF 3.4 BCD 345 0.70 ABC 0.80 A 2.8 CDE 3.7 BCD Different lett ers within a column indicate significantly different means (LSD Bon 95%CI).
93 Figure 2 1. Single leg profile tests performed using WR 32 and Nelson R33 sprinklers at three pressure levels in order to develop uniformity profiles throughout simul ations per formed into SPACE Pro program. Ph oto courtesy of Ma r a Zamora Figure 2 2. Collector layout for testing area using 14.6 m sprinkler spacing and 1.8 m collector spacing. Second testing area follows the same pattern but with 12.2m sprinkler spacing and 1.5 m collector spacing. Experimental fields located near Citra and Hastings, FL. Ph oto courtesy of Ma r a Zamora 14.6 m Sprinkler head
94 Figure 2 3. Testing areas at different sprinkler spacing (14.6 and 12.2 m) and collector spacing (1.8 m and 1.5 m). Set up used at the experimental fields located near Citra and Hastings FL. Ph oto courtesy of Ma r a Zamora Figure 2 4. Pressure regulator series 25AUB Z3 and LF25AUB Z3 the control valve in order to keep steady pressure conditions during the performance of DU lq tests. Uniformity experimental field, PSREU near Citra. Ph oto courtesy of Ma r a Zamora 12.2 m by 12.2m 14.6 m by 14.6 m 1.8 m 1.5 m
95 Figure 2 5. Sprinkler pressure monitoring using pressure gauges and regulators placed at the bottom of the sprinkler heads. Experimental fields near Citra and Hastings, FL. Ph oto courtesy of Michael Gutierrez Figure 2 6. Low quarter distribution uniformity (DU lq ) as a function of sprin kler and pressure interaction, near Citra, FL. Means that do not share a letter are significantly different LSD Bon CI 95%. BCD A A E CD AB DE ABCD AB BC A A 0.55 0.60 0.65 0.70 0.75 0.80 0.85 207 276 345 DU lq Pressure (kPa) R2000WF R33 R33LP WR32 Sprinkler head Pressure gauge Pressure regulator
96 Figure 2 7. Effect of sprinkler type and sprinkler spacing on low quarter distribution uniformity (DU lq ), near Citra, FL. Means that do not share a letter are significantly different LSD Bon CI 95%. Figure 2 8. Effect of irrigation pressure and sprinkler spacing over low quarter distribution uniformity (DU lq ), near Citra, FL. Means that do not share a letter are significantly di fferent LSD Bon CI 95%. A AB C BC B BC A B 0.55 0.60 0.65 0.70 0.75 0.80 0.85 12.2 14.6 DU lq Spacing (m) R2000WF R33 R33LP WR32 A A B A C B 0.55 0.60 0.65 0.70 0.75 0.80 0.85 14.6 12.2 DU lq Spacing (m) 345 kPa 276 kPa 207 kPa
97 Figure 2 9. Low quarter distribution uniformity (DU lq ) as a function of pressure location interaction tested near Citra, and Hastings FL. Means that do not share a letter are significantly different LSD Bon CI 95%. Figure 2 10. Effect of wind speed (m s 1 ) over low quarter distribution uniformity (DU lq ) of the WR 32 impact sprinkler evaluated near Citra and Hastings FL. B B AB AB A AB 0.55 0.60 0.65 0.70 0.75 0.80 0.85 Citra Hastings DU lq Location 207 kpa 276 kPa 345 kPa 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 DU lq Wind speed (m s 1 ) Hastings Citra
98 Figure 2 11. Application rates (AR) as a function of the sprinkler type pressure interaction, near Citra, FL. Means that do not share a letter are significantly different LSD Bon CI 95%. Figure 2 12. Effect of sprinkler type spacing interaction over application rate (AR), near Citra, FL. Means that do not share a letter are significantly different LSD Bon C I 95%. GH H FG EF AB A CD ABC A F DE BC 2.0 2.5 3.0 3.5 4.0 4.5 5.0 207 276 345 Application rate (mm hr 1 ) Pressure (kPa) 2000WF R33 R33LP WR32 E E B D A D C E 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 12.2 14.6 Application rate (mm hr 1 ) Spacing (m) 2000WF R33 R33LP WR32
99 Figure 2 13. Application rate (AR) as a function of the sprinkler pressure spacing interaction evaluated near Citra, FL. Means that do not share a letter are significantly different LSD Bon CI 95%. C E B D A D 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 12.2 14.6 Application rate (mm hr 1 ) Spacing (m) 207 kPa 276 kPa 345 kPa
100 Figure 2 14. Application rate (AR) obtained from using WR 32 impact sprinklers at 14.6 m spacing at three pressure levels evaluated near Citra, and Hastings FL. Means that do not share a letter are significantly different LSD Bon CI 95%. D D C BC B A 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Citra Hastings Application rate (mm hr 1 ) Location 207 kPa 276 kPa 345 kPa
101 Figure 2 15. Distributi on uniformity (DU lq ) and application rate (AR) comparisons among SPACE Pro simulations and field data evaluated near Citra using WR 32 and R33 sprinklers at three pressure levels and two sprinkler spacings. WR 32 DU lq AR ABC CDE E A ABC CD ABC BC DE AB AB BC 345 276 207 Pressure (kPa) DE E E A AB BC ABC ABC ABC ABC A CD 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 345 276 207 DU lq Pressure (kPa) CDE EF F BCD BCD DEF BC DE DEF A A B 345 276 207 Pressure (kPa) DEFG FG G CDE C EF C C CDE A B B 0.0 1.0 2.0 3.0 4.0 5.0 6.0 345 276 207 Application rate (mm hr 1 ) Pressure (kPa) SPACE Pro 14.6m Citra 14.6 m SPACE Pro 12.2m Citra 12.2 m R33
102 A B C D E F Figure 2 16. WR 32 sprink lers distribution uniformity (DU lq ) densograms from the SPACE Pro sprinklers simulations at three pressure levels and two sprinkler spacings. Red and green squares mean the lowest and the highest water application values, respectively. 14.6 m 12.2 m 345 kPa 276 kPa 207 kPa
103 A B C D E F Figure 2 17. Nelson R33 sprinklers distribution uniformity (DU lq ) densograms resulted from the SPACE Pro sprinklers simulations at three pressure levels and two sprinkler spacings. Red and green squares mean the lowest and the highest water application v alues, respectively. 14.6 m 12.2 m 345 kPa 276 kPa 207 kPa
104 CHAPTER 3 EFFECT OF SPRINKLER PRESSURE AND SPACING ON STRAWBERRY YIELD DURING COLD PROTECTION Introduction The total value for strawberries accounted for $366.3 and $200.97 million in Florida during the 2011 12 and 2012 13 seasons, r espectively (USDA 2013a) After tomatoes, strawberries are the second ranked crop (vegetable and berry crops) in agricultural revenue in the state (NASS and FDACS 2012) The strawberry industry in Florida is mainly concentrated in the Dover/Plant City area in Hillsborough County. A normal season generally develops from mid October until mid March or April; however, early yields occurring in November, December and January represent the mo st important part of the season due to higher prices in the market ($16.7 $26.6 per 5.4 kg flat in 2012; USDA 2012 ). The strawberry industry has to face many challenges in this short production window in addition to t he intense competition and weather conditions. The biggest competitor for Florida is California, which ranks first in U.S. strawberry production (USDA, Economics, Statistics and Market Information System 2012) ; however, Mexico is playing an important role more recently due to its rapid increase in production, which makes it among the top five producers in the world (FAOSTAT 2013) Weather conditions may represent another threat for the production of this crop in Florida, considering that extreme weather conditions might result in significant damage to strawberries, Florida growers have used sprinkler irrigation for many yea rs to protect their crops from frost/freeze damage during the winter. The effectiveness of sprinkler irrigation on strawberries has been reported decades ago. According to Locascio et al. (1967) two sprinkler appli cation rates (3.3
105 and 6.6 mm hr 1 ) provided successful cold damage protection under low wind conditions (0 to 0.4 m s 1 ) and air temperature as low as 4 C The 3.3 mm hr 1 application rate resulted in 46, 39 and 50% less mortality on flowers, immature fru its and mature fruits versus the unprotected plots. The 6.6 mm hr 1 application rate resulted in 74, 79 and 94% lower injury on the same parts of the plant compared to the non irrigated plants. Nevertheless, under higher wind conditions and lower air tempe ratures, the 3.3 mm hr 1 application rate did not provide equivalent protection to the higher rate. Another study developed during the 1962 season, evaluated three sprinkler application rates for cold protection in Louisiana strawberries at two locations (Braud and Hawthorne 1965) At the first site, an application rate of 2.8 mm hr 1 was tested during nearly 1.5 h of clear sky and no wind. This rate successfully protected the irrigated plants keeping the temperature at berry plant height about 1.1 C above freezing. With the aim of proving effectiveness for cold protection using a lower application rate, Braud and Hawthorne (1965) evaluated a 2.3 mm hr 1 rate; however, results sh owed insufficient coverage mainly in the center of the sprinklers, when this application rate was used; hence, they recommended a greater application rate to achieve coverage and uniformity. Furthermore, a n application rate of 3.3 mm hr 1 proved effective protection for blossoms and fruits, keeping the sprinkled areas most of the time near 0.5 C when air temperature reached 4.4C (Braud and Hawthorne 1965) At the second site, during the 1963 season three sprinkler application rates: 1.3, 2.5 and 5.1 mm hr 1 were evaluated under mild cold conditions, in which temperature did not fall to critical damage level. The three application rates resulted equally effective
106 to protect strawberries in Louisiana under those condi tions (Braud and Hawthorne 1965) Other studies had shown effective protection from frost damage on strawberries by using microsprinklers controlled by an automated system during mild frost conditions. The average app lication rates of 3.8 and 5.8 mm hr 1 provided protection under 2.2 C and 4 C air and bud temperatures, respectively. Results showed less than 3% blossom damage when microsprinklers were used, versus 52% blossom damage in the non irrigated plots (Stombaugh et al. 1992) A specific protection rate during a normal frost has been found more through empiricism than though mathematical modeling (Bagdonas et al. 1978) The sprinkle r application rate will vary according to the weather conditions, mainly on minimum air temperature and wind speed (Locascio et al. 1967) As a result of evaporative cooling, the wind condition represents an importa nt parameter to consider for application rates (Perry and Poling 1986) The application rate recommended for citrus increases as air temperature is lower and wind speed is higher. When air temperature is about 5.6 C a nd low wind conditions (0.9 1.8 m s 1 ) are present, an application rate of 6.1 mm hr 1 is recommended in order to avoid cold damage in citrus (Table 3 1, Gerber and Martsolf 1965) Furthermore, according to the same parameters: minimum air temperature and wind speed conditions; these application rates has been adapted and used in the strawberry industry for many years to protect this crop from cold damage (O'Dell and Williams 2 009) Although this table of application rates has been used largely and successfully for cold protection, the values are generally overestimated (Perry 1979)
107 Critical temperature according to the stage of the stra wberries and suggested temperatures to start the irrigation are described in Table 3 2. Critical Temperatures and Cold Damage The damage that occurs in plants due to cold temperatures is attributed to the extracellular ice formation inside the plant tissue ; however, injury to the cells occurs due to dehydration when water is drained out once the crystals of the ice are formed (Snyder and de Melo Abreu 2005) (T C) is the temperature at which some injury is expected, it is associated with air temperatures (measured in standard instrument shelters) and varies among crops depending on their tolerance factors (Snyder and de Melo A breu 2005) The temperature difference among the plant tissue and its immediate environment plus the radiation balance, will determine the rate at which the plant tissue will cool (Snyder and de Melo Abreu 2005) Some standard recommendations have evolved from tree fruit (i.e. citrus) and low growing crops (i.e. strawberries) literature with the aim of avoiding damage on the plants based on the critical temperature. Some of these general recommendations are: start irri gation at a temperature of 1.1 C stop irrigation when ice is melted, avoid irrigation under windy conditions (more than 4.5 m s 1 ) and critical temperatures for strawberries varies among cultivars and stage of development (Perry and Poling 1986) Research on critical temperature is limited; however, some studies have defined it as the temperature at which a blossom will be damaged after 30 min exposure. Phillips et al (1962) showed variations in the critical temperature based on the stage of the crop: tight bud 5.5 C balloon bud 2.2 C full bloom 0.5 C and green
108 fruit 2.2 C based on observations and opinions of leading small fruits re search and extension personn el. Due to the uncertainties, years later Perry and Poling (1986) developed another study in order to assess critical temperatures on two blossom stages. Four different strawberry cultivars were tested using the standa rd recommendations and also, testing a microcomputer program to determine application rates. The results showed different critical temperatures on each cultivar; however, no differences occurred on critical temperature around 3.1 C for blossoms in open an d popcorn stages. Also, application rates calculated throughout the program ranged from 2.0 to 5.1 mm hr 1 during the night according to minimum temperatures and weather conditions present (Perry and Poling 1986) The critical temperature of a plant part at a certain stage of development differs from air temperature and as critical temperature is approached, the risk increases. In order to achieve energy conservation through irrigation for cold protection, it should be applied as critical temperature is reached and should be kept just above it during the whole cold event; however, if the temperature monitoring system is not accurate or reliable, very high risk results, in which the economic damage may be unrecoverable fo r the growers. Therefore, many times, irrigation is used for cold protection even when the need is questionable (Martsolf 1992) As a result, enhanced systems are needed to monitor temperature and determine the bes t time to turn on/off the irrigation systems for cold protection, hence reduce damage on strawberries and potentially increase water conservation by decreasing the irrigation requirements (Zotarelli et al. 2012)
109 Po llination and successive fertilization of all the pistils in strawberry flowers is the key to obtain maximum fruit size later in the season (Peres et al. 2010) Premium prices for strawberries are available for early y ields, making this period the most important for (Santos et al. 2007) Therefore, irrigation for cold protection is critical during the blooming period to safeguard the pistils to be pollinated thus enhancing early production and profitability. Sprinkler Irrigation for Cold Protection Sufficient water is needed when irrigation is applied for cold protection, but without resulting in excess water that can cause root rot, or other disease problem s (e.g. Botrytis fruit rot) affecting the yields (Perry and Poling 1986) In Florida, sprinkler irrigation has been used as a best management practice (BMP) in agriculture (i.e. citrus, strawberries, blueberries, nurse ries and aquaculture). However, when the crops are exposed to temperatures where damage can occur, irrigation is turned on for protection generally when air temperature is 1.1 C Thus, this BMP allows the growers to protect their crops, but can strain the aquifer and lower its level, impacting residential wells and causing sinkholes to form (SWFWMD 2012) An unusual 11 days of severe freezes occurred in the Dover/P lant City area in January 2010 caus ing water source problem s thought to be caused by irrigation for crop protection. The amount of water during one night of freeze/frost protection is estimated to be among 508 and 762 m 3 ha 1 when high volume sprinklers are used (Santos et a l. 2011) which may be converted into an estimate of 1.8 and 2.7 million m 3 needed to protect the 3,561 ha planted in Florida in 2010 (USDA 2013b) Thus, during those 11 unusual fr eezes nights an estimated 19.9 to 29.9 million m 3 of water was applied through irrigation to protect the strawberries in Florida. Around 750 residential wells
110 were impacted and more than 140 sinkholes were reported due to the reductions in groundwater leve l (SWFWMD 2012) Further investigation is needed on the strawberry sprinkler application rate recommendation and on alternate rates under lower volume of water which m ay provide adequate protection. In order to develop str ategies to decrease the groundwater impacts and therefore, provide an opportunity to conserve water through irrigation for cold protection, the objective of this chapter was to evaluate the effect of varying sprinkler spacing and pressure on strawberry yie ld quality and quantity under cold conditions. In addition, it has been hypothesized that an operational system which integrates air temperature, relative humidity, wind speed and blossom temperature, may provide a better control on when to start the irri gation for cold protection (Perry and Poling 1986) Hence, an alternate objective was to assess new technologies based on temperature and relative humidity in order to control irrigation for freeze/frost protection, po tentially reduce water requirements and assure yield. Materials and Methods Treatments This experiment evaluated WR 32 brass impact sprinklers (Wade Rain Inc. 2007) with varying irrigation system pressures and spri nkler spacings. Five different treatments were developed in order to replicate strawberry grower practices but with the aim to conserve water through irrigation. Therefore, the treatments consisted of (i) GROW treatment, which used 345 kPa as irrigation sy stem pressure controlled by a thermostat or thermocouple at 14.6 m sprinkler spacing. This treatment mimicked the irrigation practices used by strawberry growers for cold protection, therefore, it is also
111 hmark for treatment comparison, (ii) AC treatment, followed the same pressure and spacing settings as GROW treatment; however the irrigation was controlled automatically though wireless temperature and relative humidity sensors, (iii) LOW treatment had a r educed irrigation system pressure of 207 kPa and 14.6 m sprinkler spacing, (iv) SPC had 345 kPa irrigation system pressure and reduced the sprinkler spacing of 12.2 m, this being the manufacturer recommended spacing for WR 32 impact sprinklers to achieve o ptimum sprinkler overlapping, (v) the NO treatment consisted on a non irrigated plots which were used as a comparison against the irrigated ones. The GROW, LOW and SPC treatments were controlled by a thermostat in the 2011 12 season, and by thermocouples d uring the 2012 13 season (Table 3 3). Strawberry Field Experiment A field study was conducted from September to April in two seasons: 2011 12 and 2012 13. The experimental area was located in the University of Florida, Institute of Food and Agricultural Sc iences (UF/IFAS) Plant Science Research and Education Unit root transplants were planted for the 2011 12 season and only lants for the 2012 13 season. After planting, sprinkler irrigation was applied to the plants during 10 hours per day approximately 10 days in both seasons to reduce transplant shock. The experiment was established on an Arredondo Sand soil with 0.5% organi c matter and pH of 6.2 (USDAc, 2013 ) A Kennco Super Bedder was used to pre form the planting beds, which were approximately 66 cm wide at the base, 61 cm wide on the top, and 10 13 cm high. Th e soil was fumigated with methyl bromide and chloropicrin (50/50, v/v) and immediately covered with black
112 high density polyethylene mulch, 1.25 mm thickness. Pre plant fertilizer 10 10 10 at 448.3 kg ha 1 was incorporated into the bed before fumigation and mulching. Fertilization and pest control was done according to existing recommendations (Peres et al. 2010) A tractor mounted hole puncher was used to make approximately 4.8 cm wide openings at 40.6 cm intervals in t win staggered rows with each 20.3 cm from the bed center. Fertigation was applied through a 15.9 mm drip tape line 0.25 mm thickness with 30.5 cm emitter spacing with a flow rate of 113.6 L hr 1 per 30.5 m of tape buried 2.5 cm deep. Overhead irrigation wa s used for frost protection and crop establishment (Albregts and Howard 1984) Variations in the irrigation system pressure and in the sprinkler spacing were assessed using Wade Rain WR 32 impact sprinklers with 3.6 mm nozzles for low volume applications (Wade Rain Inc. 2007) In addition, 345 kPa or 207 kPa Senninger pressure regulators (Senninger Irrigation Inc. 2010) wer e placed at the bottom of the sprinkler to maintain the irrigation system pressure at the corresponding treatment. Shields were set on determined sprinklers to control water direct ion and avoid plot overlapping. Plot Description and Harvest Protocol The st rawberry field experiment consisted of 15 plots 15.2 m by 15.2 m. Strawberry bare root transplants from nurseries in Canada were planted in five rows. A set of five treatments with three replications each were tested. Strawberries were harvested twice per week following a protocol consistently during the strawberry season. The center of the three middle rows was used as harvest areas. The ends of the rows and outside rows were established to eliminate border effects (3 1). Yield data from the harvest area w as weighed at the field, separated and
113 strawberries firm, not overripe or undeveloped, and which are free of mold or decay and free from damage cause d by dirt, moisture, foreign matter, disease, insects or mechanical or other means Each strawberry has not less than three fourths of its surface showing a pink or a red color (USDA 2006) The minimum diameter required was 1.9 cm (USDA 2006) The strawberries not following the standards Temperature During the two years of experiment, temperature within each plot was mon itored using two different temperature devices: thermocouples and wireless temperature sensors (only placed at the automated control treatments). Thermocouples Air temperature was recorded below, within and above the plant canopy (at 3.6, 16 and 30 cm abov e ground) using cooper constantan thermocouples placed within each plot and connected to six Campbell Scientific dataloggers (Campbell Scientific 2013) riation in temperature data that usually showed the most extreme cold effect below the temperature at which growers turn on the irrigation for cold protection in general ( i.e. temperatures below 1.1 C for the longest periods of time). Cold events are defined as periods when air temperatures were consistently below 1.1 C for more than 2 hours according to the station showing the most extreme cold effect. A detailed explanati on of the cold events with temperatures below the "physiological critical temperature" for strawberries ( 0.56 C ) is shown in the appendix.
114 This temperature is lower than where damage occurs in the "open blossom" stage of the crop (Table 3 2). The irrigati on for cold protection on irrigated treatments (GROW, LOW and SPC) was controlled by two thermocouples directly connected to the solenoid valve. These were programmed to turn on the irrigation when any of them reached an air temperature of 1.1 C mimicking a grower turning on the system at this temperature; thus it is called when both thermocoupl es reach a temperature above 1. 7 C Other climatic data (e.g. minimum air temper ature, minimum dew point temperature, precipitation and average wind speed) was monitored and obtained from the Florida Automated Weather Network (FAWN) archived weather data from the station located at the PSREU. Wireless sensors The implementation of new technologies to control the irrigation valves to turn on/off during freezing nights was evaluated using wireless sensors nodes (Praxsoft, Orlando, FL) that integrated canopy level temperature and relative humidity. This technology was evaluated using the AC treatment, which irrigation was automatically activated based on the dew point temperature from the sensors and the strawberry stage critical temperature (Table 3 2). On site temperature and RH monitoring through the wireless sensors was used to calcula te an average dew point (DP) and therefore, determine the safest time to turn on/off the irrigation system automatically avoiding damage in the plant tissues and potentially saving water. Using Table 3 2, the wet bulb temperature corresponded to the strawb erry open blossom critical temperature, the dew point temperature was measured
115 and selected in the matching row, and then the corresponding air temperature to trigger the irrigation for cold protection was read from the table. Using this method, the irriga tion system was programmed to shut off when temperature exceeded 2.2 C This A complete weather station was installed at the strawberry experimental field in order to monitor temperature relative humidity (RH), solar radiation and wind speed. Data access from the wireless sensors and the weather station was available online at www.agnetlive.com Experimental Design The experimental design was a sp lit plot design in which different harvest times were treated as subplots, whereas the different irrigation systems were the main plots. This design with three repetitions (three blocks) was implemented during both crop seasons. In every block WR 32 impact sprinklers were used (Wade Rain Inc. 2007) Yield data were analyzed by an analysis of variance (ANOVA) and a regression analysis in order to investigate and model the relationship between yields and treatments. The Bonferroni procedure was used for the comparison of treatment means. Yield recovery development after freeze events was analyzed using linear and quadratic polynomial contrasts. Results Cold protection field experiment results from two seasons, 2011 12 an d 2012 13, are presented. Minimum leaf temperatures, air temperature, and other climatic data in the strawberry field were recorded from December through March in both seasons. Cold events occurring prior to December are not shown because temperature did n ot reach
116 (GROW treatment) was used for treatment comparison on yield and volume of water applied through irrigation during cold events on both year seasons (Table 3 4). In the first season, yield did not vary between the two cultivars; therefore, no distinction was made in yield from the harvest areas. In order to analyze the data, cold events (temperature below 1.1 C recovery occurrence of cold events. A total of five and four cold recovery periods were present during the 2011 12 and 2012 13 seasons, respectively. Data from twice weekly harvests was weighted in order to have equal number of days between harvests. Mean yields were analyzed using ANOVA and Least Square Difference method of Bonferroni (LSD Bon ) (Table 3 5 and Table 3 8 for 2011 12 and 2012 13 seasons, correspondingly). Treatment contrast s were performed using the coefficient of orthogonal polynomials for equally spaced intervals to analyze the treatment yield recovery after the incident of cold events. 2011 2012 Season Yields Initial cold events started 11 November 2011, but consecutive c old events until the harvest season were above strawberry bloom critical temperature. A total of 23 harvests were performed 21 December 2011 to 15 March 2012. Early yields were affected by continuous cold events starting 2 January until 5 January 2012, whe re minimum air and dew point temperatures of 8.5 C and 12.8C were reached. A total of protect the crop during those consecutive nights in which 27 hours of both irrigati on types occurred on temperatures below strawberry blossom critical temperature (A 1).
117 These cold events drastically impacted the control treatment (non irrigated), resulting in very low average marketable weight (0.80 Mg ha 1 ) during the harvesting period Even when the irrigated treatments were affected and produced lower yields after the cold events, succeeding production recovery increased significantly (3 2). Non irrigated plots had 70% lower marketable yield (0.80 Mg ha 1 ) than the grower practice (2. 66 Mg ha 1 ; 3 3). However, average yield among irrigated treatments did not differ significantly over the season (Table 3 5). The freeze events from December 2011 thru March 2012 were grouped into five recovery periods. Mean comparison results showed high significant yield differences between the irrigated treatments and the control (Table 3 5). The control was affected by the initial severe cold events and on average mean yield was 84% lower (0.12 Mg ha 1 ) during the 5 recovery periods (LSD Bon NO > critica l LSD Bon for each period). P values of the ANOVA F test statistic for treatment means showed no significant differences between the irrigated treatments (Table 3 6), except during the second and fifth cold recovery periods, when some irrigated treatments p resented slightly significant differences (Table 3 7). Prior the second cold recovery period, cold events occurred from 14 January until 16 January 2012 with minimum air and dew point temperatures of 4.9 C and 3.8 C correspondingly, and an average of 27. 4 hours of irrigation in which 26 hours were applied when temperatures reached the critical for strawberry blossoms. The yield recovery after these cold events presented significant differences between certain irrigated treatments (Table 3 6). LOW (1.20 Mg ha 1 ) was 16.7% higher in yield versus AC (1.00 Mg ha 1 ), (Table 3 7). For this recovery period, the GROW (1.25 Mg ha 1 ) was
118 4%, 14% and 20% significantly higher than the LOW (1.20 Mg ha 1 ), SPC (1.08 Mg ha 1 ) and AC (1.00 Mg ha 1 ) treatments respectively 1 ) showed a high significant difference (p<0.005) by a 95% higher yield when irrigation was used to protect the crops (Table 3 7). During the 3 rd and 4 th cold periods, no signif icant effect on yield was present when using different irrigation systems. However, significant differences were found when compared to the control (Irr / NO in Table 3 6). Previous the 3 rd cold recovery period, only an average of 2.4 hours were irrigated w hen temperatures got below critical temperature on January 30, while the majority cold events occurred above critical temperatures (A 1). However, consecutive cold events previous the 4 th cold recovery period reached 6.5 C and 10.6 C as the lowest air and dew point temperatures. For these cold events occurred on 12 and 13 February 2012, irrigation systems were activated under critical temperatures during 18.8 and 21.7 hours for the grower practice and AC irrigation, correspondingly. During the fifth cold r ecovery period, significant differences between the irrigated treatments were found when using the LOW (0.86 Mg ha 1 ) treatment, which obtained 40%, 58% and 14% greater yields than SPC (0.52 Mg ha 1 ), GROW (0.36 Mg ha 1 ) and AC treatments (0.74 Mg ha 1 ) co rrespondingly, and 83% higher in yield when compared to the non irrigated (0.14 Mg ha 1 ; Table 3 7). During the same recovery period, the 1 ) against the AC treatment (0.74 Mg ha 1 ) resulted in 51% signif icantly lower yield (Table 3 7).
119 Volume of water applied for cold protection From the 2011 12 season, AC irrigation was activated during ten cold events for was trig gered for 16 cold events totaling 98.4 hours. From the total hours of irrigation, temperatures fell below strawberry blossom critical temperature (A 1). Table 3 8 shows the amoun t of water applied per treatment during the cold events for the first season. The GROW treatment used a total of 18,324 m 3 ha 1, while AC used 17,473 m 3 ha 1 LOW applied 14,209 m 3 ha 1 and the SPC treatment applied 26,390 m 3 ha 1 Water savings of 5% was achieved by AC and 22% when using LOW as the irrigation system pressure. However, by decreasing the sprinkler spacing, 44% extra water application was obtained. Cold events according to the crop stage critical temperature ( 0.56 C ) During the 2011 12 seaso n, minimum air and dew point temperatures of 8.5 C and 12.9 C were reached. However, thru the season only eleven cold events presented temperatures below the blossom critical temperature (A 1). All of them were freeze protected by irrigation, with the ex ception of the critical hours occurred on 2 temperatures got below the blossom critical accounted for 77 and 78 hours for the GROW and the AC treatments, respectively. Both typ es of irrigation were activated during the same critical hours, however, an excess of 2% irrigation was applied thru AC irrigation. The amount of water applied during the critical temperature events was very similar across all the treatments. A treatment c for strawberry critical temperature is presented in Appendix 1 (A 1). AC used 222 m 3 ha
120 1 LOW resulted in 22% water savings (3,22 1 m 3 ha 1 ), while reducing the sprinkler spacing to SPC treatment increased the water use by 44% applying 6,314 m 3 ha 1 more than GROW treatment. 2012 2013 Season Yields A total of 22 harvests were measured at the strawberry field from 3 January. 2012 thru 18 while AC irrigation was triggered only 12 cold events throughout the strawberry harvest season. According to the cold episodes occurred between December 2012 and April 2013 four cold recovery periods were defined during the strawberry harvest season in order to evaluate the recovery in yield of the treatments after cold events. The NO treatment was strongly affected by certain cold events and thus produced lower average marketabl e weights (1.27 Mg ha 1 ) at the end of the harvest period. The AC (2.71 Mg ha 1 ) and SPC (2.63 Mg ha 1 ) treatments both showed only slight differences in yield compared to GROW treatment (2.59 Mg ha 1 ). However, the LOW treatment achieved water savings of 22% without an effect on yield throughout the harvest period, and presented a slightly higher yield at the middle end of the period (3 4). Irrigation showed a significant effect on cumulative yield against the control treatment (3 5); however, no significa nt differences between irrigated treatments on average yield were present (Table 3 9). According to the LSD Bonferroni procedure, AC (0.87 Mg ha 1 ) and SPC (0.91 Mg ha 1 (0.89 Mg ha 1 ; Table 3 9). Nevertheless, the average yield of the non irrigated
121 treatment (0.43 Mg ha 1 ) was significantly lower than the other treatments for all pairwise comparisons (AC: 0.87 Mg ha 1 GROW: 0.89 Mg ha 1 LOW: 0.92 Mg ha 1 and SPC: 0.91 Mg ha 1 ). No significant differences in average yield were found between irrigated treatments (Table 3 9). Table 3 10 shows the p values of the ANOVA F test statistic for treatment mean comparison according to the yield recovery from cold occurrence during the strawberry 2012 13 harvesting season. Only during the second cold period the NO treatment followed the trend of the irrigated treatments due to prevailing overall mild temperatures. However for the cold periods 1, 3 and 4, significant effect of irriga tion was found when compared to the non irrigated treatment (p<0.005). Recover capability from freeze events among the irrigated treatments differ significantly only during the third recovery cold period (Table 3 10). Among the irrigated treatments, differ ences in the irrigations systems were significant only during the third recovery cold period in which the LOW treatment showed higher yields (2.83 Mg ha 1 ) at the end of the season. In contrast, the GROW treatment (2.20 Mg ha 1 ) obtained 74% and 63% signif icantly lower yields than LOW (2.83 Mg ha 1 ) and SPC (2.09 Mg ha 1 ) respectively. Only irrigated treatments showed a linear increase in the yield after each freeze event. Cold events and yield recovery periods during the 2012 13 season is described as foll ows. However, a detailed explanation can be found in the Appendix section. Cold events initiated since 19 December 2012; however, the predominant temperatures were above critical blossom temperature. Nevertheless, subsequent six cold events occurred in lat e December (22 through 31 December 2012) previous to the ha rvest season in
122 which air temperature of cumulative hours and AC irrigation 33.9 cumulative hours to protect the plant s in the different nights of cold occurrence, however, only 13.3 hours of irrigation were applied when temperature was below critical (A 2). As a result, initial yields (1 10 January 2013) were impacted by freeze and rainfall events occurred in mid Decembe r previous to the harvesting season, affecting more considerably the control treatment (3 4). Consequently, this leaded to the appearance and impact of Botrytis cinerea, to the crop. Botrytis is a fungus known as Botrytis fruit root or gray mold that affec ts fruits in the field, occasioning severe pre harvest losses (Peres 2011) A combination of pesticides and cultural practices (i.e. removing decaying and infected plants and fruits) reduced the damage and successfull y controlled the disease in the strawberry field. After the cold events occurred pre harvest, mild temperatures predominated until cold events occurred in late January (22, 23 and 24 January 2013). The sum of 15.3 hours of grower irrigation and 5.7 hours o f AC irrigation were needed to protect the crop from low air and dew point temperatures of Nevertheless, only one hour of irrigation was triggered to protect the crops from physiological damage. The irrigation effect on the treatments during this first recovery cold period was high significantly different compared to the non irrigated treatment (Table 3 10). Five following cold events occurred from 31 January until 5 February 2013. The lowest air temperature reached was 2.3 period occurred on 31 January until 1 February. The following four cold events were
123 above hours for a total of 1,175 m 3 ha 1 and 28.8 hours AC irrigation, which applied 851 m 3 ha 1 Nevertheless, only 6.8 and 3.8 hours of grower practice and AC irrigation occurred when cold periods reached critical temperatures for the strawberry blossom stage; however, no significant differ ences on yield recovery between the irrigated treatments and the control were present (Table 3 10). Severe cold events occurred from 17 February until 18 February 2013 reaching the lowest air and dew point temperatures: and during the strawberry harvesting season 2012 13. Total hours of irrigation were 19.1 and 22 for the grower practice an d AC, correspondingly, and 18.8 and 18.3 hours applied when temperatures fell below critical damage for strawberries. Yields were impacted by these two severe events, showing high significant differences during the third cold recovery period between the ir rigated treatments and the control (Table 3 10). The non irrigated treatment presented the poorest yield recovery (0.95 Mg ha 1 ) when compared to the irrigated treatments, obtaining 66% 55%, 57% and 57% lower yield than LOW (2.83 Mg ha 1 ), SPC (2.09 Mg ha 1 ), GROW (2.20 Mg ha 1 ) and AC (2.21 Mg ha 1 ) treatments respectively (Table 3 11). However, the irrigation effect among irrigated treatments was significant as well. The LOW treatment presented significant difference in yield recovery in contrast to the i rrigated treatments (Table 3 11). The total weighted yield (2.83 Mg ha 1 ) achieved was 26%, 22% and 22% higher than SPC, GROW and AC treatments accordingly during that cold period recovery (Table 3 11). The fourth period of cold events affecting treatment yields initiated on 2 March until 8 March 2013. Minimum air and dew point temperatures fell down to
124 During this period grower practice irrigation applied 909, m 3 hr 1 for a cumulative of 38.7 hours, while the AC irrigation was turned on only 26.4 hours, applying 778 m 3 hr 1 For critical temperature protection, irrigation run during 16.5 and 13.3 hours of grower practice and AC irrigation, correspondingly. Cumulative marketable weight (kg) per treatment shows slightly differences among ir rigated treatments, but almost twice difference total cumulative yield when compared to the non irrigated treatment (3 5), which obtained 49.1% lower yield than Volume of water applied for cold protection A total of 23 cold events o while only 17 were irrigated by the AC irrigation (A 2). However, during the harvest season, AC sprinkler irrigation was activated for the duration of 12 cold events, while on, which activated also the rest of the treatments, was triggered for a total of 16 cold events (3 4). The amount of water applied per treatment during the cold events is shown in Table 3 12. The GROW treatment applied 1,931 m 3 ha 1 while AC used 23,202 m 3 ha 1 LOW applied 22,313 m 3 ha 1 and the SPC treatment applied 43,298 m 3 ha 1 Water savings of 22% were achieved using LOW in the irrigation system pressure, followed by 23% water savings when AC technology was used. However, 44% extra water applicatio n was obtained by decreasing the sprinkler spacing to 12.2 m. Cold events according to the crop stage critical temperature ( 0.56C) During the 2012 13 season, a total of 15 cold events were identified to occur below the critical temperature ( 0.56C) (A 2 ). All of them were freeze protected by
125 treatment, correspondingly. A comparison between the amount of water applied per treatment showed that AC used 823 m 3 ha 1 less water, t his being 8% of water savings (2,353 m 3 ha 1 ) water savings. The SPC treatment increased the water use by 44% applying 4, 613 m 3 ha 1 2). Low Quarter D istribution U niformity (DU lq ) and Application R ate ( AR ) S cenarios for the SPC, AC, GROW and LOW T reatments Scenarios for the SPC, AC, GROW and LOW treatments were performed for DU lq and AR (3 6 and 3 7) Uniformity (DU lq ) Very high uniformity values without significant differences were obtained between SPC (DU lq = 0.80) AC and GROW treatments (DU lq = 0.79 for both) which all were performed using an irrigation system pressure of 345 kPa and 14.6 m spacing, with the exception of SPC treatment w hich used 12.2 m spacing (3 6) By the contrast, when the irrigation system pressure was reduced to 207 kPa (LOW treatment), uniformity was significantly lower (DU lq =0.68) (3 6) Application rate (AR) When application rates were assessed for the differen t treatments, significantly higher mean AR was obtained by the SPC (AR= 5.1 mm hr 1 ) in comparison to the AC, GROW (AR= 2.9 mm hr 1 for both) and LOW (AR= 2.2 mm hr 1 ) treatments (3 7) LOW treatment obtained significant lower mean AR in comparison to all other treatments (AR= 2.2 mm hr 1 ), resulting in 57%, 24% and 24% lower AR compared to SPC, AC and GROW treatments (3 7)
126 Consequently, if the distribution uniformity values, application rates and yield data are compared, it can be shown that almost 70% un iformity and reductions in the AR up to 57% can be achieved using a lower irrigation system pressure (207 kPa) and 14.6 m spacing and resulting in no significant differences in yield (LOW treatment) compared to the SPC, GROW and AC treatments. By contrast, using a shorter spacing among the sprinklers and high irrigation system pressure (12.2 m spacing and 345 kPa) achieved up to 80% of irrigation uniformity and no significant differences in yield ; however, increas ed the AR by 2.3 times in comparison to the LOW treatment Conclusions During the two winter seasons of 2011 12 and 2012 13, both seasons were about 28% and 57% respectively, below the average number of freezing hours compared to the last 34 years of historical data. The results during these se asons affirmed the effectiveness of sprinkler irrigation for cold protection on strawberries since unprotected strawberries resulted in significantly lower yield. However, during non severe cold events, the unprotected plants followed the trend of the irri gated treatments. Recovery capability from the cold events among the irrigated treatments differed randomly. Even when the irrigated treatments had a linear increase in yield recovery from cold events, there were no differences between them. Although there were similar total yields among irrigated treatments, there were differences in irrigation volume. Using an automated system based on dew point and air temperature reduced irrigation by 5% during the 2011 12 season and up to 23% during the 2012 13 season. However, increasing the application rate by reducing sprinkler spacing resulted in 44% extra water applied. Reducing the pressure in the irrigation supply to LOW resulted in 22% water savings. Reducing the pressure resulted in 4,115 m 3 ha 1 less irrigatio n during the first
127 season and 6,751 m 3 ha 1 in the second season without affecting yield under weather conditions below normal freeze years, in which this study was performed. Therefore, an estimated average of 19.3 billion liters of water per harvest seas on could be saved considering the 3 561 ha of strawberries planted in Florida in 2010 (USDA 2013b) Number that will proportionally increase with the area planted per season.
128 Table 3 1. Citrus cold protection application rate (mm hr 1 ) recommendation according to minimum temperature expected and wind speed conditions (Gerber and Martsolf 1965) Min. Temp. Expected Wind Speed (m s 1 ) 0 0. 4 0.9 1.8 2.2 3.6 4.5 6.3 8.0 9.8 13.4 (C) Application Rate (mm hr 1 ) 2.8 2.5 2.5 2.5 2.5 5.1 7.6 3.3 2.5 2.5 3.6 5.1 10.2 15.2 4.4 2.5 4.1 7.6 10.2 20.3 40.6 5.6 3.0 6.1 12.7 15.2 30.5 45.7 6.7 4.1 7.6 15.2 20.3 40.6 61.0 7.8 5.1 10. 2 17.8 25.4 50.8 76.2 9.4 6.6 12.7 22.9 33.0 66.0 101.6 11.7 8.6 17.8 30.5 43.2 86.4 127.0 Table 3 2. Strawberry critical temperatures at different crop stages calculated using dew point and wet bulb temperatures (C) *. Note: Table used to determin e turn on and turn off times for the AC treatment irrigation system. Strawberry Critical Temp. at Suggest Crop Stage Tight bud Popcorn Fruit Open Blossom Starting (Air) Temperatures Critical Temperature or Wet Bulb Temperature (C) Dew Point ( C) 5.0 2.2 1.7 0.6 (C)** 0.0 1.1 0.6 0.6 1.6 1.1 0.2 1.7 1.7 0.2 2.2 2.2 2.2 1.3 0.5 2.8 2.8 1.9 1.0 0.8 3.3 1.6 0.7 1.1 3.3 3.9 1.3 0.4 1.4 4.4 1.0 0.1 1.7 4.4 5.0 5.0 0.7 0.2 2 .0 5.6 4.7 0.4 0.4 2.3 5.0 6.1 4.4 0.2 0.7 2.6 6.7 4.2 0.1 0.9 2.8 5.6 7.2 3.9 0.3 1.2 3.1 7.8 3.7 0.6 1.4 3.3 6.1 Adapted from Snyder (2000) **Adapted from O'Dell and Williams (2009)
129 Table 3 3. Treatments evaluated in the experimental field at PSREU, near Citra, FL 2011 2013. Treatment Sprinkler Pressure (kPa) Sprinkler spacing (m) Control GROW WR 32 345 14.6 Thermocouple LOW WR 32 207 14.6 Thermocoupl e AC WR 32 345 14.6 Wireless sensors NO No sprinklers Non frost protected NA NA SPC WR 32 345 12.2 Thermocouple NA: Not Applicable Table 3 4. Summary water applied and water savings per treatment during two years of field results. PSREU, Citra, FL. T reat. Pressure (kPa) Irrigation m 3 ha 1 Mean yield (Mg ha 1 ) Water Savings (%) Yr. 1 Yr. 2 Yr. 1 Yr. 2 Yr. 1 Yr. 2 AC 345 17,473 23,202 3.11 a 2.71 a 5 23 GROW 345 18,324 30,06 2.66 a 2.59 a 0 LOW 207 14,209 23,313 2.99 a 2.76 a 22 22 NO 0. 80 b 1.27 b 100 100 SPC 345 26,390 43,298 2.68 a 2.63 a 44 44 Yr. 1 and Yr. 2 correspond to the 2011 12 and the 2012 13 seasons, correspondingly. Different letters correspond to significant differences between treatments. Table 3 5. Treatment means ( Mg ha 1 ) and pairwise comparison tests according to Bonferroni LSD during 2011 12 season. Treatment Pairwise comparison of yield means Yield means (Mg ha 1 ) LOW SPC GROW AC NO LOW 2.99 0.00 0.31 0.33 0.12 2.19** SPC 2.68 0.00 0.02 0.43 1.88** GR OW 2.66 0.00 0.45 1.86** AC 3.11 0.00 2.31** NO 0.80 0.00 ns: Not significant. (*) Denotes significant differences between treatments at each cold period (p< 0.05). (**) Denotes high significant differences between treatments at each cold pe riod (p<0.005).
130 Table 3 6. P values of the ANOVA F test statistic for treatment means through recovery cold periods during 2011 2012 season. 2011 12 Cold Period 1 2 3 4 5 Block ns ns ns ns ns Treat ** ** ** ** ** Irr. /NO ** ** ** ** Betw. Irrig.Treat. ns ns ns ns ns ns Interaction Time ** ** ** ** ns Irr./NO lin. ** ** ns ns ns Irr./NO sq. ns ns ns ns ns Error 2 ns ns ns ns ns ns: Not significant. (*) Denotes significant differences between treatments at each cold period (p< 0.05). (**) Denotes high significant differences between treatments at each cold period (p<0.005). Irr / NO: Comparison among irrigated treatments (Irr) versus non irrigated treatment (NO). Irr / NO lin. or Irr/NO sq.: Linear (lin ) or square (s q.) interaction among Irrigated versus non irrigated treatment. Table 3 7. Mean Comparison on recovery cold periods with significant differences between irrigated treatments during the 2011 12 season. Cold Period Treat 2 5 LOW /AC /other SPC n s ns GROW /other /AC AC ns ns NO /Irr. ** /Irr. ** ns: Not significant. (*) Denotes significant differences between treatments at each cold period (p< 0.05). (**) Denotes high significant differences between treatments at each cold period (p<0.0 05). /other: other irrigated treatments (AC, GROW and SPC) comparison. /Irr.: all irrigated treatments (AC, GROW, LOW and SPC) comparison.
131 Table 3 8. Amount of water applied during the cold events and percent water savings per treatment (compared to GRO W treatment). Citra, FL. 2011 12 season. Treatment Pressure (kPa) Irrigation Water savings (m 3 ha 1 ) Water Savings (%) (m 3 applied)* m 3 ha 1 AC 345 1,122 17,473 851 5 GROW 345 1,177 18,324 LOW 207 913 14,209 4,115 22 NO 18,324 100 SPC 345 1,177 26,390 8,066 44 Total irrigation (m 3 ) applied on the three repetition plots of the treatment, over all cold events. Table 3 9. Treatment yield means (Mg ha 1 ) and pairwise comparison tests according to Bonferroni LSD during 2012 13 seas on Treatment Pairwise comparison of yield means Avg. yield (Mg ha 1 ) LOW SPC GROW AC NO LOW 2.76 0.00 0.13 0.17 0.05 1.49** SPC 2.63 0.00 0.04 0.08 1.36** GROW 2.59 0.00 0.12 1.32** AC 2.71 0.00 1.44** NO 1.27 0.00 ns: Not signi ficant. (*) Denotes significant differences between treatments at each cold period (p< 0.05). (**) Denotes high significant differences between treatments at each cold period (p<0.005).
132 Table 3 10. P values of the ANOVA F test statistic for treatment me ans through recovery cold periods during 2012 2013 season. 2012 13 Cold Period 1 2 3 4 Block ns ns Treat ** ns ** ** Irr./NO ** ns ** ** Betw. Irrig.Treat ns ns ns Interaction Time ** ** ** ** Irr./NO lin. ns ns ** ns Irr./NO s q. ns ns ns ns Error 2 ns ns ns ns: Not significant. (*) Denotes significant differences between treatments at each cold period (p< 0.05). (**) Denotes high significant differences between treatments at each cold period (p<0.005). Irr/NO: Comparison a mong irrigated treatments (Irr) versus non irrigated treatment (NO). Irr/NO lin. or Irr/NO sq .: Linear (lin ) or square (sq.) interaction among Irrigated versus non irrigated treatment. Table 3 11. Mean Comparison on recovery cold periods with signific ant differences between irrigated treatments during the 2012 13 season. 2012 13 Treat Cold period 3 LOW /other SPC ns GROW ns AC ns NO /Irr ** ns: Not significant. (*) Denotes significant differences between treatments at each cold perio d (p< 0.05). (**) Denotes high significant differences between treatments at each cold period (p<0.005). /other: other irrigated treatments (AC, GROW and SPC) comparison. /Irr: all irrigated treatments (AC, GROW, LOW and SPC) comparison.
133 Table 3 12. Am ount of water applied during the cold events and percent water savings per treatment (compared to GROW treatment). Citra, FL. 2012 13 season. Treatment Pressure Irrigation Water savings Water Savings (kPa) (m 3 applied)* m 3 ha 1 (m 3 ha 1 ) (%) AC 345 1,4 90 23,202 6,862 23 GROW 345 1,931 30,06 LOW 207 1,497 23,313 6,751 22 NO 30,064 100 SPC 345 1,931 43,298 13,234 44 (*)=Total irrigation (m 3 ) applied on the three repetition plots of the treatment, over all cold events.
134 (a) (b) Figure 3 1. Strawberry harvesting layout. Subplots designed for (a) SPC at12 .2 m. sprinkler spacing and (b) for AC, GROW and LOW treatments at 14.6 m sprinkler spacing. H= harvest areas for data analysis, GR= guard rows and ENDS= end s of the rows, both established to eliminate border effect. Blue dots represent the sprinkler heads at the two sprinkler spacings. 12.2 m GR= 4.9m 12.2 m GR GR 0.6m ENDS ENDS H= 3.7m H= 3.7 m 14.6 m GR= 7.3m 1.8m G R G R ENDS ENDS 14.6 m
135 Figure 3 2. Weighted marketable weight (Mg ha 1 ) per treatment (lines) Irrigation cold protection events during harve st season 2011 12. PSREU, Citra, FL. Note: irrigation or cold events data points might be jointed through overnight and continued days cold events; therefore, only one data point is shown in 3 2, 3 3, 3 4 and 3 5. 0 10 20 30 40 50 60 70 80 90 100 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 12/3/2011 12/18/2011 1/2/2012 1/17/2012 2/1/2012 2/16/2012 3/2/2012 Rainfall (mm) Hrs < 1.1 C Marketable Wt (Mg ha 1 ) 2011 12 Rainfall Hrs <1.1 C AC GROW LOW NO SPC Cold Events Grower practice Irr. AC Irr.
136 Figure 3 3. Cumulative marketable weig ht (Mg ha 1 ) per treatment (lines). Irrigation cold protection events during harvest season 2011 12. PSREU, Citra, FL. 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 3-Dec 18-Dec 2-Jan 17-Jan 1-Feb 16-Feb 2-Mar 17-Mar Rainfall (mm) Hrs <1.1 C Cumulative marketable wt (Mg ha 1 ) 2011 2012 Rainfall Hrs <1.1 C AC GROW LOW NO SPC Grower practice Irr. AC Irr. Cold Events
137 Figure 3 4. Weighted marketable weight (Mg ha 1 ) per treatment (lines). Irrigation cold protection events during harvest season 2012 13. PSREU, Citra, FL. 0 10 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 6 7 8 9 10 19-Dec 3-Jan 18-Jan 2-Feb 17-Feb 4-Mar 19-Mar Rainfall (mm) Hrs. <1.1 C Marketable Weight (Mg ha 1 ) 2012 13 Rainfall Hrs. <1.1C AC GROW LOW NO SPC Cold events Grower practice Irr. AC Irr.
138 Figure 3 5. Cumulative marketable weight (Mg ha 1 ) per treatment (lines). Irrigation cold protection events during harvest season 2012 13. PSREU, Citra, FL. 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 19-Dec 3-Jan 18-Jan 2-Feb 17-Feb 4-Mar 19-Mar Rainfall (mm) Hrs. <1.1 C Cumulative marketable wt (ton ha 1 ) 2012 13 Rainfall Hrs. <1.1C AC GROW LOW NO SPC Cold events Grower practice Irr. AC Irr.
139 Figure 3 6. Low quarter distribution uniformity values (DU lq ) for W R 32 impact sprinklers evaluated at two irrigation system pressures ( 345 and 207 kPa ) and two sprinkler spacings (14.6 and 12.2 m ) corresponding to the SPC AC, GROW and LOW treatments evaluated under cold conditions at the different pressures and spacings at Citra, FL Different letters indicate significantly different means (LSD Bon 95%CI). A A A B 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 12.2 14.6 DU lq Spacing (m) 345 kPa 207 kPa LOW SPC AC
140 Figure 3 7. Application rate s resulted from the evaluation of WR 32 impact sprinklers at two irrigation system pressures (345 and 207 kPa) and two sprinkler spacings (14.6 and 12.2 m) corresponding to the SPC, AC, GROW and LOW treatments evaluated under cold conditions at the different pressures and spacings at Citra, FL Different letters indicate significantly different means (LSD Bon 95%CI). A C B D 0.0 1.0 2.0 3.0 4.0 5.0 6.0 12.2 14.6 Application rate (mm hr 1 ) Spacing (m) 345 kPa 207 kPa LOW SPC AC
141 CHAP TER 4 CONCLUSIONS AND FUTURE WORK Conclusions The main goal for this research was to optimize irrigation management practices for cold protection in strawberries that could conserve water. The primary objectives for this project were to: (i) investigate co industry and optimize current strawberry irrigation cold protection application rates (ii) assess the effect of sprinkler type, sprinkler spacing, irrigation system pressure variations and varied climatic win d conditions over irrigation distribution uniformity (DU lq ) and application rate (AR), and (iii) evaluate the effect of varying sprinkler spacing and pressure on strawberry yield quality and quantity under cold conditions. Sprinkler irrigation is a current practice widely used in Florida applied as an effort to reduce cold (freeze/frost) damage on strawberries during the winter. Two main parameters should be taken into consideration when water is applied for a successful protect ion: uniformity and applicati on rate A variety of sprinkler types are used for cold protection; however impact sprinkler s are most commonly used in Florida strawberry fields. Fo ur sprinkler types were tested as follows: Wade Rain impact sprinklers WR 32 and three Nelson rotators R33 R33LP and R2000WF The evaluation of the sprinkler types previously described, showed that all sprinkler s achieved higher uniformity under higher pressure levels and vice versa, the lower the pressure level, the lower the distribution uniformity for all sprinkler types However, if the irrigation system pressure was reduced to 276 kPa and 207 kPa, the WR 32 impact sprinklers and R2000WF rotator sprinklers resulted in significantly higher DU lq in comparison to the Nelson R33 and R33LP sprinklers
142 evaluated at the same pressure levels. By contrast, WR 32 and R2000WF water application rates (AR) were significantly lower than the R33 and R33LP performed at each pressure level and across all the irrigation pressure levels. Therefore, the former ones can achieve d higher uniformity profiles under lower application rates in comparison to the latter ones. In the s ame manner, WR 32 and R2000WF obtained significantly higher DU lq at a 12.2 m sprinkler spacing than the R33 and R33LP, but significant differences were not found in uniformity when all the sprinkler types were evaluated at 14.6 m sprinkler spacing. Nevertheless, significantly lower AR were applied by WR 32 and R2000WF in comparison to the two other Nelson rotators at both sprinkler spacings. All sprinkler ty pes applied significantly higher AR at a 12.2 m spacing, except R2000WF which applied statistically the same AR as WR 32 and R2000WF when evaluated at 14.6 m spacing. S ignificant differences in low quarter distribution uniformity (DU lq ) were not found when the sprinklers were tested at the two highest pressures and shorter spacing ( 345 and 276 kPa at 12.2 m spacing ) neither when evaluated at the highest pressure and largest spacing ( 345 kPa at 14.6 m s prinkler spacing ) However, significantly lower DU lq val ues were obtained at 207 kPa system pressure in comparison to the higher pressures. Significantly higher AR were obtained at the 12.2 m spacing in comparison to the 14.6 m spacing at all pressure levels, and overall significantly lower ARs were obtained as a result of using the 207 kPa when compared to the other pressure levels at the same spacing.
143 High wind speed resulted in significantly lo w er uniformity for the impact sprinklers. Therefore, sprinkler irrigation non uniform coverage may result during cold nights under high wind conditions, impacting the yields. Distribution profiles s imulation s can be performed by using programs e.g. SPACE Pro, which can be used to determine some irrigation system design components (i.e. sprinkler nozzle, pressure, spacing sprinkler type) which may impact uniformity and efficiency of the irrigation systems. For this particular study, SPACE Pro results showed that the simulation data di ffered from real uniformity and water application rate values The AR recommended for col d protection of 6.35 mm hr 1 has been used largely and successfully; though, the results of this research found that lower application rates can be achieved by using any of the sprinklers simulated and tested at the field, which on average, overall sprinkl ers applied AR lower than 5.63 mm hr 1 The commonly used WR 32 impact sprinklers had average reductions in water application rate s up to 37%, 45% and 52% using 345, 276 and 207 kPa compared to the 6.35 mm hr 1 recommended AR for cold protection and 57% an d 31% when these impact sprinklers are spaced at 14.6 m and 12.2m, respectively Therefore, more opportunities to reduce the amount of water applied through irrigation using lower pressures in the system, but achieving adequate irrigation uniformity can be accomplished. Even though the weather conditions for the 2011 12 and 2012 13 winter seasons, in which this research was performed were 28% and 57% respectively below the average number of freezing hours in comparison to the last 34 years of historical
144 da ta, the results affirmed the effectiveness of sprinkler irrigation for cold protection on strawberries since unprotected strawberries resulted in signific antly lower yield and a reduced recover y from the occurrence of cold events. By the contrast irrigate d treatments presented a linear increase in yield as a recovery from the cold events, without differences among them. Although no differences in yield were observed between the irrigated treatments, significant differences in irrigation volume occurred. Si mply reducing the irrigation system supply pressure to 207 kPa (LOW treatment) consistently produced 22% water savings during both year seasons even under lower distribution uniformity profiles. Using an automated irrigation system (AC treatment) based on dew point and air temperature with an AR of 2.9 mm hr 1 achieved 5% and up to 23% water savings during 2011 12 and 2012 13 year seasons in comparison to the GROW treatment, which had the same AR (2.9 mm hr 1 ). However, reductions in AC water savings can re sult if low dew point temperatures are reached in shorter periods of time, more likely to occur during severe cold events for longer periods. By contrast, reducing sprinkler spacing from 14.6 m to 12.2 m (SPC treatment) higher uniformity (DU lq = 0.80) and a pplication rate (AR= 5.1 mm hr 1 ) were achieved; however it resulted in 44% excess of water applied in both seasons in comparison to grower practices. This treatment may be effective under severe cold events since more water is applied, thus more heat coul d be released in order to keep the temperature around 0 C, higher uniformity can be achieved reducing possible damages in yield due to non uniformity applications. Under weather conditions below normal cold years, in which this study was performed, a total of 4,115 m 3 ha 1 less irrigation was applied by the LOW treatment during the first season
145 and 6,751 m 3 ha 1 in the second season without affecting yield. Therefore, substantial irrigation savings up to 19.3 billion liters of water per harvest season on av erage could be saved by reducing the pressure in the irrigation system, considering only 3,561 ha of strawberries planted in Florida in 2010 (USDA 2013b) Proportionally higher wate r savings can be achieved if larger strawberry areas are planted per season. Future W ork Higher uniformity was achieved by using shorter sprinkler spacings, however, under cold conditions only the irrigation treatment combination of a shorter spacing and h igher pressure was evaluated (SPC treatment: 12.2 m spacing at 345 kPa). Therefore, testing uniformity distribution and yield results under cold conditions using a shorter spacing and a lower pressure may result in substantial irrigation savings that may r esult in good yield quality and quantity. These irrigation treatments were evaluated under weather con ditions below average normal cold year s and were shown to save water under those conditions. However, severe cold conditions will prevail in normal cold y ears; therefore these treatments should be tested under normal and excessive cold year conditions in order to prove water savings without impacting yields.
146 APPENDIX IRRIGATION COMPARISON DURING CRITICAL TEMPERATURES FOR STRAWBERRIES A 1. Comparison of v olume applied by AC and other treatments during the physiological critical temp. events 2011 12. AC treatment Cold event Treat Wind Speed Irr. Time Irr. H Flow rate Vol applied Irr. Time Irr. H Flow ra te Vol applied Date Time Air Leaf DP (m s 1 ) (h) (m 3 h 1 ) (m 3 ) (h) (m 3 h 1 ) (m 3 ) 1/2/2012 AC 4.8 1.5 2.9 0.9 0 0 1.0 0.0 Start time 19:30 GROW 1.0 1.0 End Time 20:45 LOW 1.7 0.8 NO 3.6 SPC 4.3 1.0 Sum 0.0 1/3/2012 AC 0.8 3.1 12.9 3.4 2 2 1.0 23.9 Start time 5:15 GROW 3.6 5:45 1.0 23.9 5:45 End Time 10:30 LOW 3.4 7:45 0.8 18.5 7:45 NO 2.9 SPC 2.9 1.0 23.9 Sum 66.4 23.9 1/3 4/12 AC 3.2 13.75 13.75 1.0 164.5 Start time 18:00 GROW 7.2 4.4 8.9 0.7 18:30 1.0 164.4 18:30 End Time 8:30 LOW 7.1 8:15 0.8 127.5 8 :15 NO 8.5 SPC 3.9 1.0 164.4 Sum 456.3 164.5 01/04 5/12 AC 3.3 2.1 7.5 0.5 11.25 11.25 1.0 134.6 Start time 18:15 GROW 2.6 20:45 1.0 134.5 20:45 End Time 8:00 LOW 3.9 8:00 0.8 104.3 8:00 NO 5.1 SPC 2.3 1.0 134.5 Sum 373.3 134.6
147 A 1. Continued AC treatment Cold event Treat Wind Speed Irr. Ti me Irr. H Flow rate Vol applied Irr. Time Irr. H Flow rate Vol applied Date Time Air Leaf DP (m s 1 ) (h) (m 3 h 1 ) (m 3 ) (h) (m 3 h 1 ) (m 3 ) 1/14/2012 AC 2.4 2.1 1.2 0.9 4.5 4.5 1.0 53.8 Start time 3:45 GROW 1.8 3:45 1.0 5 3.8 3:45 End Time 8:15 LOW 2.7 8:15 0.8 41.7 8:15 NO 4.9 SPC 2.4 1.0 53.8 Sum 149.3 53.8 01/14 15/12 AC 3.3 2.2 3.8 0.4 12.25 12 1.0 146.5 Start time 18:45 GROW 2.1 19:45 1.0 146.5 19:45 End Time 8:00 LOW 3.4 8:00 0.8 113.6 8:00 NO 3.4 SPC 2.6 1.0 146.5 Sum 406.5 146.5 01/15 16/12 AC 2.2 1.7 2.2 0.1 10 8.5 1.0 101.7 Start time 20:45 GROW 1.6 21:45 1.0 119.6 23:17 End Time 7:45 LOW 1.8 7:45 0.8 92.7 7:45 NO 3.3 SPC 1.8 1.0 119.6 Sum 331.8 101.7 1/30/2012 AC 0.1 1.5 0.5 0.3 2.5 2.3 1.0 26.9 Start time 4:30 GROW 0.5 4:30 1.0 29.9 4:45 End Time 7:00 LOW 0.6 7:00 0.8 23.2 7:00 NO 2.2 SPC 0.2 1.0 29.9 Sum 83.0 26.9
148 A 1. Continued AC treatment Cold event Treat Wind Speed Irr. Time Irr. H Flow rate Vol applied Irr. Time Irr. H Flow rate Vol applied Date Time Air Leaf DP (m s 1 ) (h) (m 3 h 1 ) (m 3 ) (h) (m 3 h 1 ) (m 3 ) 2/12/2012 AC 3.6 4.2 10.6 3.3 6.8 9.7 1.0 116.0 Start time 1:00 GROW 4.9 2:01 1.0 81.3 2:00 End Time 9:45 LOW 4.0 8:45 0.8 63.0 8:45 NO 4.0 SPC 4.0 1.0 81.3 Sum 225.6 116.0 2/12 13/2012 AC 6.3 3.0 7.3 0.5 12 12 1.0 1 43.5 Start time 19:15 GROW 3.7 19:45 1.0 143.5 19:45 End Time 8:30 LOW 6.2 7:45 0.8 111.3 7:45 NO 6.5 SPC 3.1 1.0 143.5 Sum 398.2 143.5 3/5/2012 AC 1.1 0.9 1.5 0. 3 2 2 1.0 23.9 Start time 1:30 GROW 0.3 2:30 1.0 23.9 2:30 End Time 6:15 LOW 0.9 4:15 0.8 18.5 4:15 NO 0.8 SPC 1.4 1.0 23.9 Sum 66.4 23.9 Total (L) applied in th e experiment. (All treat running at the same time) 77 2,556.7 78 935.4 GROW 921.2 LOW 714.3 NO SPC 921.2 Difference (GROW AC) (14.3) % A C Savings (1.5)
149 A 2. Irrigation comparison between grower practice and other irrigated treatments vs.AC irrigation during the strawberry physiological critical temp. events 2012 13. AC treatment Col d event Treat Wind Speed Irr. Time Irr. H Flow rate Vol applied Irr. Time Irr. H Flow rate Vol applied Date Time Air Leaf DP (m s 1 ) (h) (m 3 h 1 ) (m 3 ) (h) (m 3 h 1 ) (m 3 ) 12/22/2012 AC 1.6 1.7 1.8 2.2 5:15 2.5 2.5 1.0 29.9 Start time 3:30 GROW 2.1 7:45 1.0 29.9 3:30 End Time 7:45 LOW 0.9 0.8 23.2 7:45 NO 2.1 SPC 0.9 1.0 29.9 Sum Sum 83.0 29.9 12/22 23/12 AC 2.8 2.2 2.7 0.4 9.8 9.8 1.0 117.2 Start time 19:15 GROW 3.2 19:45 1.0 117.2 20:00 End Time 8:00 LOW 3.4 21:00 0.8 90.9 8:00 NO 4.4 23:30 SPC 2.1 8:00 1.0 117.2 Sum Sum 325.2 117.2 12/27/2012 AC 0 .6 1.7 0.0 1.2 0.5 0.5 1.0 6.0 Start time 6:30 GROW 0.6 6:30 1.0 6.0 6:30 7:30 LOW 0.4 7:30 0.8 4.6 7:30 NO 0.6 SPC 0.8 1.0 6.0 Sum Sum 16.6 6.0 12/27 28/12 A C 0.0 1.7 0.2 0.3 0.25 0.25 1.0 3.0 Start time 21:45 GROW 1.0 7:00 1.0 3.0 End Time 3:00 LOW 0.7 7:15 0.8 2.3 3:00 7:00 NO 1.3 7:00 7:15 SPC 0.1 1.0 3.0 7:15 Sum Sum 8.3 3.0
150 A 2 Continued AC treatment Cold event Treat Wind Spee d Irr. Time Irr. H Flow rate Vol applie d Irr. Time Irr. H Flow rate Vol applied Date Time Air Leaf DP (m s 1 ) (h) (m 3 h 1 ) (m 3 ) ( h) (m 3 h 1 ) (m 3 ) 12/30/2012 AC 1.7 1.1 1.2 2.3 0.25 0.25 1.0 3.0 Start time 7:15 GROW 0.3 7:15 1.0 3.0 7:15 End Time 7:30 LOW 0.4 7:30 0.8 2.3 7:30 NO 0.3 SPC 0.5 1.0 3.0 Sum Sum 8.3 3.0 1/23/2013 AC 0.7 1.7 0.7 0.4 1 1 1.0 12.0 Start time 1:00 GROW 1.0 2:00 2:30 1.0 12.0 2:40 End Time 7:45 LOW 1.7 5:30 5:45 0.8 9.3 7:45 NO 2.2 7:15 7:30 SPC 0.6 1.0 12.0 Sum Sum 33.2 12.0 2/1 2/13 23:30 AC 4.1 1.1 0.6 0.6 23:30 0.25 23:45 0.25 1.0 3.0 23:45 GROW 0.7 0.3 1.6 0.4 23:45 2.25 1.0 29.9 4:30 2.25 26.9 2/2/2013 4:30 LOW 1.2 0.8 23.2 6:45 6:45 NO 1.9 SPC 0.3 1.0 29.9 Sum Sum 83.0 29.9 2/3/2013 1:45 AC 1.4 1.1 0.1 0.2 1:45 3 1:45 0.25 1.0 3.0 2:00 GROW 0.0 1.3 0.1 0.4 2:00 1.0 35.9 2:00 2.75 32.9 4:15 LOW 1.6 4:15 0.8 27.8 4:15 7:00 NO 1.6 7:00 7:00 SPC 1.0 1:45 1.0 35.9 99.5 35.9
151 A 2 Continued AC treatment Cold event Treat Wind Speed Irr. Time Irr. H Flow rate Vol applied Irr. Time Irr. H Flow rate Vol applied Date Time Air Leaf DP (m s 1 ) (h) (m 3 h 1 ) (m 3 ) (h) (m 3 h 1 ) (m 3 ) 2/4/2013 0:15 AC 1.0 1.1 1.3 0.4 0:15 1.25 6:15 0.5 1.0 3.0 1:00 GROW 0.2 1:00 1.0 14.9 7:00 3.0 2:00 LOW 1.2 2:00 0.8 11.6 6:15 NO 1.4 6:15 7:00 SPC 0.1 7:00 1.0 14.9 Sum Sum 41.5 6.0 2/17/2013 AC 2.2 2.2 5.9 1.5 5.75 5.8 1.0 69.4 Start time 1:30 GROW 2.2 1:45 1.0 68.7 1:15 End Time 7:30 LOW 2.6 7:30 0.8 53.3 7:30 NO 2.0 SPC 0.8 1.0 68.7 Sum Sum 190.8 69.4 2/17 18/13 AC 5.5 2.2 8.3 0.3 13 12.5 1.0 149.5 Start time 19:00 GROW 4.1 19:00 1.0 155.4 19:30 End T ime 8:00 LOW 6.0 8:00 0.8 120.5 8:00 NO 6.0 SPC 3.1 1.0 155.4 Sum Sum 431.4 149.5 3/2/2013 23:45 AC 1.0 1.1 1.6 0.6 0.75 0.5 1.0 6.0 3/2 3/2013 0:15 GROW 0.2 0.2 2.3 1.5 23:45 0:15 1.0 9.0 0:15 3:30 LOW 0.2 3:30 3:45 0.8 7.0 3:30 3:45 NO 0.4 3:45 SPC 0.1 1.0 9.0 Sum Sum 24.9 6.0
152 A 2 Continued AC treatment Cold event Treat Wind Spee d Irr. Time Irr. H F low rate Vol applied Irr. Time Irr. H Flow rate Vol applied Date Time Air Leaf DP (ms 1 ) (h) (m 3 h 1 ) (m 3 ) (h) (m 3 h 1 ) (m 3 ) 03/3 4/13 AC 3.1 1.7 3.3 0.7 11.8 20:15 11 1.0 131.6 Start time 19:15 GROW 2.8 19:30 1.0 140.5 7:15 End Time 7:15 LOW 4.2 7:15 0.8 108.9 NO 4.2 SPC 1.6 1.0 140.5 Sum 389.9 131.6 3/7/2013 AC 0.0 0.7 1.1 0.9 1.75 3.25 5:54 1 1.0 12.0 Start time 5:00 GROW 1.1 1.5 1.0 38.9 6:45 End Time 6:45 LOW 1.1 0.8 30.1 NO 2.2 SPC 0.6 1.0 38.9 Sum Sum 107.8 12.0 3/8/2013 AC 0.9 0.1 1.2 0.4 1:00 0.75 0.75 1.0 9.0 Start time 1:00, GROW 0.6 2:00 1.0 9.0 3:00 End Time 2:00, LOW 0.1 3:00 0.8 7.0 3:15 3:00 NO 0.7 3:15 3:45 3:15, SPC 0.8 3:45 1.0 9.0 3:45 4:45 24.9 9.0 Total (gal) applied per treatment during cold events (Assuming start irrigation time at critical temperature) 56.3 GROW 673.1 51.9 LOW 522.0 NO SPC 673.1 Total UF plots 1,868.2 Total AC plots 620.2 2,488.4 620.2 Difference (GROW AC)= 52.9 % AC Savings 7.9
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, American Society of Agronomy, Sacramento, California, 111 119. Zotarelli, L., Dukes, M. D., Morgan, K., Zamora, M. (2012 ). Efficacy of Irrigation Management for Frost Protection using Temperature Sensors Provided by Praxsoft to Protect Strawberry Crops in Citra, Deliverable 8d Ed., Gainesville, Florida.
160 BIOGRAPHICAL SKETCH Costa Rica, the small Central American country g reat in biodiversity, surrounded by beaches and mountains, gave birth to Maria Isabel Zamora Re. Her younger educational stages, started at different schools, since her parents always were looking for higher education standards. Therefore, after switching from various elementary schools, she got accepted at the Marista High School, a small catholic school in which what she was doing. Therefore, she was recognized du ring her 5 years of high school for her academic excellence and participation in multicultural activities. Furthermore, Ms. Zamora chose to attend EARTH University to start her career, due to their learning process generated through experience and particip ation. During her third year, she performed an exchange program working on water efficiency and conservation by using soil moisture sensors at the Agricultural and Biological Engineering Department of the University of Florida. The experience acquired thro ughout this program let her apply into her senior project before finishing her career. After four intense years of knowledge and hard work, she received with honors her Bachelor of Science in Agricultural Engineering at EARTH University, Costa Rica. At thi s University she instilled values as leadership, sustainable development and management of agriculture and natural resources, managerial and entrepreneurial capacity and high sense of environmental conservation working together with the society. Soon after her graduation, she was hired by a g olf course company in Costa Rica as the turfgrass project manager in order to establish nine holes at the golf course. After accomplishing this goal, she decided to pursue a higher degree education, therefore,
161 she a ccep ted an assistantship for a m Biological Engineering Department at the University of Florida. Her main research project was focused on optimizing sprinkler irrigation for cold protection in strawberries; ho wever she led other research projects i.e. rain sensor testing and distribution uniformity. She enjoyed her stay at the Department and appreciated all the collaboration and support from different ABE faculty members and staff, friends and family. She will continue doing what she loves building her life path, contributing to the society and preserving the environment.