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• 24 • <br />18 to 51 an <br />precipitation onto stripable lands in Wyoming and Montana i~(7 to 20 inches) <br />30.5 an 35.5 <br />with the area between th ~(12-inch)and(14-inch)isohyets encompassing the <br />largest proportion of the stripable reserves (for example, see Figure 3, <br />Appendix C, U.S. Bureau of Land Management, 1974). Precipitation that can <br />potentially be absorbed to become effective soil moisture for the Powder <br />23 to 25 an <br />River Basin and much of the Montana coal area thus ranges fromr(9 to 10 inches). <br />The 25 an index figure suggested by the National Academy was tempered in <br />the final draft of the report by the statement that evapotranspiration <br />ought to be considered in determining the reclaimability of sites with greater <br />than 25 Qn precipitation. Now that individual site cork as suggested by <br />the Academy is being conducted on such variables as effective precipitation, <br />consumptive use, and evapotranspiration, we can begin to assess the applica- <br />bility of the suggested index value. <br />Using the Powder River Basin as an example, one can derive evapotranspiration <br />figures for the 4 regional precipitation stations for which an adequate period <br />of record is available. Figure 2 represents a graphical display of average <br />precipitation for the 4 appropriate stations (Gillete-2E; Rochelle-3E; Douglas, <br />and Dull Center). This curve illustrates that the composite average annual <br />33.3 an <br />precipitation is^(13.1 inches)with the highest monthly values occurring in D1ay <br />and June (due mainly to highly variable rainfall from thunderstorms) and a <br />second much smaller mode in August through October due to late thunderstorms <br />or early snowstorms (from hlarwitz and Rodgers 1974). Probability analysis for <br />