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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />A~~P " <br />"," , '! ',' )., l-,) <br />UJ4~~' <br /> <br />The organization of basic concepts and methods for dealing with both <br />nonrecoverable and recoverable water losses in irrigated agriculture are <br />grouped into the following broad categories: <br /> <br />o Evaporation and Transpiration Reduction <br />o Surface Runoff and Deep Percolation Reduction <br />o Irrigation Systems and Irrigation Management <br /> <br />EVAPORATION AND TRANSPIRATION REDUCTION <br /> <br />The physical processes of evaporation (E) and transpiration (T) are <br />generally well understood, with extensive research results available to docu- <br />ment E and T rates as affected by climatic variables, specific crops and <br />their stages of growth, soil-moisture-plant relationships, and other deter- <br />ministic factors. Promising research is underway in genetic adaptations <br />of crops and varieties (cultivars) for reducing plant transpiration <br />requirements; in the development of chemical antitranspirants and growth <br />regulators; in agricultural uses for evaporation suppressants; and similar <br />technological advances. <br /> <br />Crop water requirements are usually expressed as units of evapotrans- <br />piration (ET) per day (or other unit of time). Units of ET are used because <br />of the difficulty and expense of measuring evaporation and transpiration <br />separately. Although both components of ET are important, and offer distinc- <br />tive opportunities for reducing nonrecoverable losses, the T (transpiration) <br />component is the more significant, making up 70 to 95 percent of seasonal ET <br />for most field crops. In situations where frequent wetting of soil and plant <br />surfaces by either precipitation or irrigation occurs, the evaporation com- <br />ponent becomes more significant and may even exceed T rates at times, par- <br />tially during early growth stages. <br /> <br />The magnitude and rates of both E and T are variable. Ambient tempera- <br />tures, wind velocities, humidity, soil conditions, kinds of crops, stage of <br />growth, crop spacing, row or planting direction, and other factors all <br />influence the variability in ET. There are no accurate and dependable <br /> <br />5 <br />