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• 26 • <br />of 15 cm, agricultural climatologists feel that actual evapotranspiration <br />can be estimated by many techniques and that one can interpolate values for <br />soil water capacity equal to or less than maximum depth of natural seasonal <br />saturation. Table 1, derived by Bureau of Land b1anagement (1974) from a <br />U.S. Department of Commerce - U.S. Department of Agriculture publication <br />(undated) provides estimates of actual and potential evapotranspiration for <br />three Powder River Basin stations. These sites all have seasonal precipita- <br />tion-to-evaporation ratios of about 1:3 which is typical for the entire high <br />northern great plains region This assumes a growing season defined between <br />-2 C <br />days with temperatures of^ 28o F)or lower. It is significant to note that <br />for all months during which there is significant plant growth, actual preci- <br />pitation falls below calculated evapotranspiration. These calculations are <br />made assuming that all moisture that falls becomes effective soil moisture <br />and that all snowfall ultimately melts to be absorbed into the soil. These <br />are therefore rather conservative figures and show that seasonal and annual <br />soil moisture deficits would exist even if all precipitation could be trapped <br />and made effective for plant growth through micmclimate modification. Fig. 4 <br />illustrates this annual moisture deficit graphically for the same stations. <br />What this means to a native plant community is that different species cease <br />growing at various times throughout the potential growing season. This ability <br />of a natural community to contain a mosaic of species that each complete their <br />growth cycles and cease demanding additional water sequentially through the . <br />season as soil moisture declines, gives rise to maximum efficiency of niche <br />utilization and soil protection for limited soil moisture resources. <br />