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• zs <br />1-week cumulative precipitation for the same area (Figure 3) illustrates <br />how these various precipitation sources or mechanisms affect precipitation <br />probabilities (Becker and Alyea, 1964). <br />Consumptive use is the amount of precipitation that would be required to <br />maintain soil saturation against evapotranspirative stress (Trelease, et <br />al. 1970 - from Marwitz and Rodgers 1974). This can be calculated to determine <br />how much irrigation will be necessary knowing potential evapotranspiration <br />and precipitation. This has been plotted for grasslands in the Powder <br />River Basin as the broken line on Figure 2. The monthly consumptive use <br />should be considered as an index to evapotranspiration during irrigation, <br />not as an index of moisture lost under natural conditions (Chow, 1964, page <br />11-33). The dotted line in Fig. 2 represents natural precipitation that <br />can effectively be available for soil and plant uptake. It is calculated <br />by modification of actual precipitation by factors accounting for interception <br />loss, sublimation of blowing snow, and thunderstorm runoff (Trelease, et al., <br />1970). <br />To see how evapotranspiration may modify the effectiveness of any given <br />amount of annual precipitation, one mist calculate monthly or short-period <br />evaporative demand as a function of available soil moisture. Since evapo- <br />transpiration varies as a function of soil moisture and plant cover as well <br />as climatic variables, actual evapotranspiration can only be approximated. <br />By assuming that annual precipitation mast be equal to or less than actual <br />evaporation for an open soil unit that is not subirrigated above a depth <br />