The use of sprinkler irrigation equipment to cool the aerial environment of crops is a recent innovation in agricultural production. The benefits to be gained include increased yield and quality of the crop and the extension through climatic modification of the normal geographic limits of specific crop production.
Plant water potential is influenced more by evaporative demand of the atmosphere than by soil water availability, particularly when soil water is maintained within the tensiometer range. The significance of changes in plant water potential is not well defined, but there is evidence that the changes can be controlled by mist irrigation with resultant beneficial yield response.
Plant growth is restricted by water deficits caused by excessive transpiration during the heat of midday. Simultaneous plant and air temperatures may differ widely and plants may respond to air temperature changes of only a few degrees Celsius. A change of a few degrees in leaf temperature can make a major difference in the biological functions of plants.
Beans, peas, potatoes, tomatoes, straw berries, and tree fruits are among the crops reported to have critical maximum temperatures from which they can be profitably protected. Little has been recorded about critical temperatures but 90°F (32.2°C) has arbitrarily been assumed to be the upper threshold temperature for most temperate zone crops.
Reductions in air and soil temperature produced by sprinkling with conventional and low-rate irrigation equipment have been documented. Improved crop growth has often been associated with these environmental modifications, but in many instances the degree of modification has not been interpreted on a climatic basis. None of the studies reported has been conducted in a climate similar to that of southern Alberta, which features hot dry summers but which has few extreme maximum temperatures. The sprinkler method of irrigation has been increasing in popularity to the extent that it predominates in some localities and many systems currently in use have a crop-cooling capability. This paper describes crop-cooling experimental techniques used in southern Alberta and relates the results to prevailing meteorological conditions.
Most crop cooling described in the literature has been done when air temperature exceeded 90°F (32.2°C). At Lethbridge, on the average, air temperature has exceeded 90°F (32.2°C) on only five occasions for a total of 16 h annually. It was considered impractical to assess cooling on the basis of so few extreme temperatures. But, on the average, air temperatures exceed 80 F (26.7 C) 6, 16, 14, and 5 d in June, July, August, and September, respectively, or a total of nearly 150 h. Consequently, this lower temperature was used as a base.
The experimental design consisted of 12 plots, each 40 ft (12.2 m) square, arranged in three replications. Each fourplot replication contained two irrigation treatments on which cooling was super imposed and two on which cooling was not superimposed. Irrigation and cooling treatments were randomized within each replication. The replicated design was provided for the physiological assessment of the crops being studied under the cooling regime. Equipment availability limited the instrumentation for cooling assessment to one complete replication.
Sprinklers for both irrigation and cooling were of full-circle, single-nozzle, conventional design and were located at the corners of the plots. Sprinkler nozzles of two sizes, 13/64 inch (5.2 mm) and 7/64 inch (2.8 mm), were used for the two irrigation treatments but for cooling 4/64-inch (1.6-mm) nozzles were used.
For cooling, a temperature controller located near the pump actuated an electric pumping unit as long as the ambient temperature exceeded 80°F (26.7 C). Cooling was arbitrarily cycled 16 min on and 14 min off by a time clock. The purpose of the on-off cycling and the use of small-sized nozzles was to reduce the amount of water applied to the plots. The cooling treatment was intended only to cool the crop environment and not to supply water for crop use… <more>
(Source: - E.H. Hobbs, Member CSAE, Research Branch, Agriculture Canada Research Station http://www.csbe-scgab.ca/docs/journal/15/15_1_6_ocr.pdf)