The Irrigation Dating Game:
Finding the Perfect Match for Plants and Water
The purpose of this lesson is to help students understand that a farmer or rancher must take into account multiple factors when choosing an irrigation method for crops or pasture. Topography, crop selection and soil types will be compared with the three main types of irrigation: gravity flow, pressurized sprinkler and micro-irrigation.
The students will:
- Describe and simulate the three main types of irrigation systems.
- Describe several subtypes of each of the three main irrigation system types.
- Explore the influence slope, crop and soil type variations on the choice of irrigation systems.
Science - Hypothesizing, critical thinking, analyzing, comparing similarities and differences, describing, demonstrating, observing, listening, recording, discussing
English/Language Arts: - Reading, writing (technical), speaking
Performing Arts: - Creating, building
History/Social Science/Geography - Comparing past to present, studying geographical and topographical regions, weighing science against social perceptions, economics
Mathematics - measuring, subtracting, adding
In 1940 the average farmer in the United States could produce enough food for 19 people. Today, an American farmer can produce enough food to feed 129 people -- 101 in the United States and 28 abroad. Technological advances have increased the productivity of farmers, particularly by improving their ability to provide water to their crops through irrigation. Irrigation is defined as the managed application of water to soil for the purpose of increasing crop production.
Irrigated agriculture has helped American farmers produce the most abundant and diverse supply of food, fiber and foliage products in the world. Irrigation plays an especially important role in the Western United States where growing seasons are longer but there is not enough rainfall to supply an optimum amount of water to commercial crops. California alone produces over 250 agricultural commodities, most of which could not be grown there without irrigation.
The following AgriWater information and activities will focus on the three main types of irrigation - surface gravity flow, pressurized sprinkler, and micro-irrigation. In order to select the appropriate type of irrigation, the farmer must make informed decisions. Water availability, economics, soil types and plant biology factors must all be studied before choosing an irrigation technique. Some of the major considerations for the farmer or rancher include:
The complexity of choosing an appropriate irrigation system and utilizing the available water productively has helped to insure that good farmers are true water managers, very aware of conservation principles. It is in their own best economic interest to use water wisely.
- proximity of the field or pasture to a water source
- adequate distribution system to the field (pumps, canals or pipes)
- amount of water required by selected crop
- quality of available water
- cost of water
- topography of the land
- soil type
- annual precipitation
- cost of irrigation supplies
- availability of labor to set-up and maintain irrigation system
- fertilization methods
- methods for recycling or handling excess irrigation water
Basin irrigation, pressurized sprinkle irrigation, micro-irrigation, soil type, drainage, water holding capacity, saturation, runoff, water conservation.
Activity 1: Comparing the water holding capacity and speed of water saturation:
Different soils hold different amounts of water in the spaces between the soil particles. The amount of moisture a soil can retain per unit of soil volume is called its holding capacity. Holding capacity determines the quantity of water an irrigator must apply to a particular field and the frequency of the water application. In this lesson students will observe and demonstrate the different water holding capacities of sand, silt and clay soils.
Sand, silt and clay are distinguished physically by the size of the particles: sand having the largest and hardest particles, silt having fine, gritty particles, and clay having extremely fine, powdery particles when dry. The difference in particle size results in a difference in the available pore space between particles. The smaller the pore spaces, the more strongly water will cling inside them. Most soils are various mixtures of sand, silt, and clay, and a given field may contain different types of soil.
- 1-2 Hours
- Estimated Teaching Time:
- Three 1-hour sessions
- Opportunities for up to five 1-hour session follow-ups
Questions for discussion:
For one class demonstration or one participation activity for 3-5 students:
- What are the physical differences between sand, silt and clay particles?
- Which soil will pass water through to the jar the fastest? Why?
- Which soil type will absorb the most water? Why?
- Three transparent wide-mouth jars, each 1 to 3 liters in capacity
- Three small clay flowerpots with holes in the bottom, each 1-2 cups in capacity, of a shape and size that will fit inside the jars
- 1 cup of each of the following types of soils: sandy, silty and clay (get these from a local nursery)
- 1 water measuring device (graduated cylinder or flask).
- 1 stop watch
- small magnifying lens
- optional: weight scale capable of measuring in fractions of an ounce or gram.
Water will saturate the sand rapidly and excess water will quickly flow through the pot into the jar. Silt will saturate a little more slowly. Clay will take the longest.
Conclusions: Sand wets rapidly, but holds the least water, allowing most of the application to flow quickly into the jar. Silt wets slightly slower, but holds more water. Clay soils wet slowest, but hold the most water.
Activity 2: Comparing the efficiencies and irrigation time of gravity flow, sprinkler and micro-irrigation systems
In addition to soil types there are several other factors which affect the choice of irrigation method. Labor and equipment cost, water costs, crop selection and topography all must be considered before a farmer implements an irrigation system. The following activity introduces students to the three main types of irrigation systems and the advantages and disadvantages of each.
Questions for discussion:
- Have the students take a pinch of each of the soil samples with dry fingers, feeling the particles, smelling them, then wetting them slightly, feeling and smelling the change.
- Ask them to examine the pinches of soil with the magnifying lens.
- Explain the differences between sand, silt and clay.
- Place individual soil samples into each of the clay pots.
- Place each clay pot into its own jar.
- Pour equal amounts of water (100 ml) into each of the three pots in succession (Start with the sand, then the silt, then the clay).
- Time the period from water application until the flowerpots stop leaking. (About 15 minutes)
- Measure the amount of water collected in each of the jars under the pots. Optionally, measure the before and after weight of each of the soil-filled flowerpots, to find the weight of water retained.
- Have the students complete a simple lab report on this activity. Be sure to have them include a description of the procedure, observations during the experiment and an explanation of results.
- What are the three main types of irrigation systems?
- List one advantage and disadvantage of each.
- What are the goals of irrigating?
- How does crop selection affect irrigation techniques?
Conduct the demonstration, or group activity models as follows:
- Three 24"x24" nursery flats with solid bases (or a cardboard insert to cover drainage holes)
- Three strips of rag cloth two feet long, one for the top end of each flat
- A metal or plastic gutter for collecting runoff at the bottom of each flat (can be reused)
- 3 liters water - minimum
- 1 water dropper
- 1 garden watering can or hose nozzle with spray diffuser
- 1 garden trowel;
- 1 graduated cylinder or measuring cup (should hold at least 1 liter)
- 1 "six-pack" flower or vegetable seedlings
- 1 flat of grass or ground cover material from local nursery (can be purchased or planted from seeds 3-4 weeks prior to lesson)
- 1 stop watch
- Samples of drip or micro-sprinkler components and sprinkler heads (contact local farmers, or university extension, or an irrigation equipment supplier, or a US Bureau of Reclamation Water Conservation Coordinator)
Simulate gravity flow irrigation:
- Lay each strip of rag cloth along an edge of each flat
- Measuring out the soil, fill the three nursery flats level to the top with sand, silt and clay, respectively.
- Lightly compress each soil, being careful to not overly compact it.
- Prop up the rag covered edge of each flat at approximately 5 degrees with bricks or boards on a desk or table. (Pop Quiz: to find the rise in inches, multiply the sine of 5 degrees by 24 inches. If the students have not had that level of math, calculate the rise by the grade of the slope. A 5 degree angle is the same thing as an 8.75% grade, meaning the rise equals 8.75% of the horizontal travel. Is the horizontal travel equal to or less than the length of the flat? Turns out that for such small angles, the difference can be ignored. If you don't want to bother with any of this math, prop up the 24" flat so the top is 2.1" higher than the bottom. Farmers figure this all out using laser beam surveys.)
- Position the gutter under the lower end of the flat to catch drainage water.
- Place the graduated cylinder or other catch device in a position to capture the water as it runs off of the gutter, or pour the gutter water into the graduated cylinder when drainage is complete.
Simulate furrow irrigation.
- Slowly pour a gallon of water directly from the container back and forth across the rag strip at the top of the flat containing sand. (Using the rag avoids having the soil wash away from the impact of the water.) Have the students time the intervals from water application to the first dribble of drainage. Repeat for silt and clay.
- Measure the amount of water drained from each of the types of soil.
- Record the drainage time and quantity of water drained from each soil type.
- Observe how much soil runs off of each of the three fields and is suspended in the drainage water. Pour the drainage water through a coffee filter or lab filter in order to isolate the lost soil. Compare the amounts to the measured soil in the flats. How does the percentage of loss vary?
Simulate sprinkler irrigation
- Reduce the slope to a 3 degree angle. That's a 5.24% grade, or a one and a quarter inch rise for a 24" long flat.
- Using the hand trowel, create furrows in the flat (field).
- Slowly pour a gallon of water across the head of the furrows. Record drainage time and quantity for each of the three soil types.
- Have the students carve a single furrow perpendicular to the existing furrows to demonstrate how a farmer can cross rip the field during the winter to reduce water speed and erosion. Repeat the irrigation to observe the difference.
- Have the students plant one or two flowers or vegetable seedlings in each flat.
- Using the grass-planted flat or the flower or vegetable flats, pour or spray water from diffuser head of the hose or sprinkler can. Although this only approximates a pressurized system, students can observe the differences in water application and runoff between surface gravity flow and sprinkler irrigation.
- Again, have the students time the interval between application and the first dribble of runoff, and measure the quantity of runoff.
- Show samples of different types of sprinkler heads. Compare them to the ones on the school's playground.
Different soils affect the amount of water required to irrigate a field and the time it takes to complete the irrigation. Other factors, such as crop selection and land topography affect the irrigation as well. Farmers must carefully choose which type of irrigation system-gravity flow, pressurized sprinkler or micro-irrigation-is best for a particular field. The best choice will maximize the useable water available to the plant and minimize labor and equipment costs, evaporation, runoff and soil erosion.
Gravity flow irrigation uses the most water but requires the least amount of labor or equipment investment. This system gets water to all parts of the field, thus meeting the needs of all the individual plants. Gravity flow irrigation is efficient for densely planted field crops like wheat, rice or pasture. It can waste a lot of water when used on row crops or orchards because the standing water between the widely spaced plants is open to evaporation. Furrow irrigation helps direct and contain the amount of water in a gravity flow system so that it is used more efficiently.
Pressurized sprinkler systems generally use less water than gravity flow, but much of the spray through the air and the water which falls on plant leaves evaporates on hot, dry days. Placement of water is not exact because of wind scatter. This type of irrigation requires significantly more labor and equipment than gravity flow methods, but it is the only practical way to irrigate uneven ground.
Micro-irrigation uses the least amount of water but is very costly in terms of labor, equipment and maintenance. This type of irrigation eliminates drainage erosion problems. However, it cannot adequately apply water to field crops or pasture at a reasonable cost.
- Pour a gallon of water into a container. Have a student fill an eyedropper full of water from the container and water one flower or vegetable with it. Use as many eyedropper applications as necessary.
- Have the student attempt to water the flat filled with grass by the same procedure.
- Put a little silt or clay in the eyedropper. Does it clog up?
- Show different components of micro-irrigation system to the class. The AgriWater graphics on the WaterShare site illustrate both micro-spray and dripline types, and there are varieties of each. What they all have in common is the delivery of small drops close to the plant roots, at a rate matching the uptake of water by the plant. Many micro-irrigation systems require water filtration, to prevent clogging of the small water openings, or "emitters."
- Lead the class in a discussion on the advantages and disadvantages of this type of system.
- Define irrigation.
- List two advantages and disadvantages for each type of irrigation system.
- Which irrigation system might be the most appropriate in the following situations:
- Large 50 acre pasture with grass and a nearby creek
- An almond orchard
- A wheat field
- A tomato field
- Forage grass on uneven ground
- Why is it important that the farmer not use too much water?
- Define erosion.
- After reviewing on the AgriWater home page the irrigation graphic showing the farm, sketch how water is moved from a river to a field.