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40 <br />The soil erodibility factor was originally obtained by direct field <br />• measurements on a few agricultural soils in the central and eastern <br />United States. Later, Wischm®ier et al. (1971) extended the method and <br />developed a nomograph for easy solution of the equation, given the nec- <br />essary soil information. Since the nomograph can be used to estimate K <br />factor values for both agricultural soils and dense subsoils exposed on <br />construction sites, the solution of K factor values for surface mined <br />areas is relatively simple (U. S. Soil Conservation Service, 1977). <br />Wischmeier et al. (1971) claim about 95 percent of all K values esti- <br />mated with the nomograph lie within ±0.04 (English units of the true <br />value. <br />The slope length factor, L, and the slope gradient factor, S, are <br />combined into one factor, LS, and defined (Wischmeier, 1977) by: <br />• m 430 sinZe + 30 sine + 0.43 <br />LS = (726) ( 6 ) <br />where: <br />m = 0.5 if slope 4 percent <br />m = 0.4 if slope = 4 percent <br />m = 0.3 if slope 4 percent <br />e is angle of slope <br />a is length of slope (feet). <br />A table (U. S. Soil Conservation Service, 1977) and a graph (U. S. <br />Soil Conservation Service, 1975) are available for easy solution of <br />the equation. At most of the sampling sites the length of slope and <br />glope gradient were in a range where the LS factor would have been de- <br />termined from extrapolation of experimental data. Furthermore, the <br />• length and shape (concave, straight, convex) of the slopes were very <br /> <br />