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<br />~ <br /> <br />C001G~. <br /> <br />. <br /> <br />Thus, the production of high yielding crops requires a continued supply <br /> <br /> <br />of readily available moisture in the soil root zone so that, even when <br /> <br /> <br />the evaporative demand is high, water deficits in the plant are minimum. <br /> <br /> <br />The principle purpose of irrigation is to maintain the soil-moisture <br /> <br /> <br />level at sufficiently high values to meet the plant uptake requirements <br /> <br /> <br />for maximum transpiration. <br /> <br />. <br /> <br />In addition to transpiration of water through the plants, some water <br /> <br /> <br />is consumed in crop production by direct evaporation from the soil surface. <br /> <br /> <br />To some extent this evaporation reduces transpiration because the relative <br /> <br /> <br />humidity of the air within the crop canopy is increased. Consequently <br /> <br /> <br />it has become customary to refer to the consumptive water use by crops as <br /> <br />the sum of evaporation from the soil and of transpiration from the leaves <br /> <br /> <br />(evapotranspiration). Evapotranspiration (ET) is dependent almost exclu- <br /> <br /> <br />sively on meteorological conditions throughout the crop growing season <br /> <br /> <br />if soil moisture does not become a limiting factor to water uptake by the <br /> <br /> <br />root system. When soil moisture is reduced to such a degree that root <br /> <br /> <br />absorption cannot meet the ET demand, a plant water deficit develops, <br /> <br />leaf stomates partially close, and transpiration is restricted to the rate <br /> <br />of root absorption. Plant growth and subsequent yield of the crop is <br /> <br /> <br />reduced somewhat proportionally to the degree of plant water stress. <br /> <br />The amount of water used in the production of a crop may be expressed <br /> <br /> <br />in many ways. The water actually consumed is simply that evapo~a:ed into <br /> <br />vapor (from plant stomata or the soil) or converted into plant tissue. <br /> <br /> <br />However, it is usually more meaningful to include water having other fates, <br /> <br /> <br />such as runoff from the field or deep percolation through the soil to <br /> <br /> <br />points below the root zone. In irrigation water management it is usually <br /> <br /> <br />of great concern to determine the efficiency with which the water supply <br /> <br /> <br />is used in crop production. <br /> <br />In the following sections various aspects of irrigation-water effi- <br />ciency are discussed. <br /> <br />. <br /> <br />-4- <br />