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<br />l 2470 <br /> <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 />1.3 BASIC CONSUMPTIVE USE METHODOLOGY <br />1.3.1 General Approach <br />Consumptive use estimates can be made by directly measuring the water <br />balance components of a crop through the equation: <br />ET = P + I - D+ 6 S (7) <br />in which ET is crop evapotranspiration, P is effective <br />precipitation, I is irrigation, D is deep percolation, and 6S is <br />change 'in soil moisture. Water balance measurements using <br />lysimeters can yield very accurate estimates of evapotranspiration <br />at a given location, but are generally not suited to making project <br />level estimates over a wide area. <br /> <br />Several indirect methods of estimating evapotranspiration have been <br /> <br />developed by correlating meterological measurements with known <br /> <br />rates of evapotranspiration. Such methods provide a "potential" or <br />"reference crop" evapotranspiration rat~ which will be defined as <br /> <br />ETp or ETo. For most purposes, an alfalfa based reference crop ET <br /> <br />can be considered equivalent to ETp. <br /> <br />Crop coefficients (Kc) are used to relate potential or reference crop <br /> <br />ET to crop ET (ET crop). <br />ET crop = Kc x ETo (8) <br /> <br />or <br /> <br />ET crop = Kc x ETp (9) <br /> <br /> <br />Crop coefficients reflect the evapotranspirational demand of the <br /> <br /> <br />individual crop as affected by planting or growing date, growing <br /> <br />1- 7 <br />