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CRDSS <br />TASK MEMORANDUM 1.14-8 <br />Consumptive Use Model <br />Consumptive Use Model Terms <br />1.0 ISSUE <br />To reduce the confusion caused by having multiple terms with slightly different meanings, the following <br />definitions for consumptive use terms are recommended for the CRDSS. <br />2.0 DISCUSSION/ANALYSIS <br />The following terms are for agricultural consumptive use: <br />Potential Evapotranspiration (ETp) <br />Potential evapotranspiration is the potential rate for a specific crop at which water can be removed from <br />the soil or plant surfaces, assuming the soil is fully saturated. By applying a time period to the potential <br />evapotranspiration rate, a potential depth of irrigation water can be calculated that is exclusive of <br />precipitation or stored soil moisture required consumptively for crop production. <br />Crop Coefficients <br />A crop coefficient is the ratio of evapotranspiration occurring with a specific crop at a specific stage of <br />growth to reference crop evapotranspiration at that time. This definition is not for the basal crop <br />coefficient K used in the Blaney-Criddle evapotranspiration method. Basal crop coefficients represent <br />conditions when the soil surface is visually dry, so that evaporation is minimal but the availability of soil <br />water does not limit plant growth or transpiration, whereas mean crop coefficients include wet soil <br />effects. <br />Actual Crop Consumptive Use (CCU) <br />Crop consumptive use, often referred to as evapotranspiration, is the total amount of water used for <br />vegetative growth, transpiration, or building of plant tissue, plus the unavoidable evaporation of soil <br />moisture, snow, and intercepted precipitation associated with plant growth for any specified time and <br />given water supply limitations. <br />Effective Precipitation (EP) <br />Effective precipitation is rain and/or snow that meets a portion of the crop's water requirement. Effective <br />precipitation does not include precipitation unused by crops such as surface runoff and deep percolation <br />past the root zone. <br />1 <br />A275 01.09.95 1.14-8 CSU IDS <br />