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36 <br />Selection of Factors <br />• Wischmeier and Smith (1958) analyzed 8250 plot years of erosion data ~~ <br />from 35 widely scattered experiment stations east of the Rocky Mountains. ~, <br />They found the best single variable for predicting soil loss from culti- <br /> voted fallow soil with constant slope and slope length is the product of <br />' <br /> the total kinetic energy of a storm combined with the storm <br />s maximum <br /> 30-minute intensity, or EI30 index (Wischmeier, 1959). <br /> The annual erosivity factor is determined by summing EI30 indexes <br /> for all rainstorms over one year's period, according to the equation, <br /> n Ei I30i <br />R <br /> year i=1 <br /> 100 <br /> where: <br /> / <br /> R year is annual erosivity factor in meter-tonnes per hectare per year <br />-~ <br />• / (foot-tons per acre per year) <br /> I30 is maximum 30-minute intensity of rainstorm in centimeters per hour <br /> (inches per hour) <br /> E is rainfall kinetic energy in meter-tonnes per hectare per centimeter <br /> of rain (foot-tons per acre per inch of rain). <br /> Wischmeier and Smith then used the rainfall data for numerous sta- <br /> tions and developed an iso-erodent map which connects points of equal <br /> values of the average annual rainfall erosivity factor, R, for 37 eastern <br /> states. Clyde et al. (1976) extended the iso-erodent map to the 13 <br /> western states. The R factor values in Table 4 under "Clyde et al." <br /> (metric units converted from English units) were taken from their iso- <br /> erodent map for each of the mine sites sampled. <br /> " <br />" <br /> were taken from a map by <br />SCS <br />The R factor values in Table 4 under <br />• l <br /> ues were <br />the U. 5. Soil Conservation Service, 1477. The SCS R factor va <br /> <br />