Evaluation of the topographic factor in the universal soil loss equation on irregular slopes

ABSTRACT:

Comparisons of LS values for the universal soil loss equation were made for concave, complex, convex, and uniform slopes while independently varying slope length, gradient, and number of segments used to divide the slope. Slope length varied from 30.5 m to 244 m in increments of 30.5 m. Slope gradients varied from 1 to 20% in increments of 2 units. Each slope was also divided into from two to seven equal length segments. In general, LS values increased with increasing length, at a constant gradient, and with increasing gradient, at a constant length, in the following order: concave<uniform<complex<convex. Analysis of the effect of dividing irregularly shaped slopes into segments prior to LS calculations indicated three equal length segments were sufficient to obtain reasonable LS estimates. Differences between LS calcuations based on uniform slopes and on irregular slopes were used to construct tables that can be used to add the effect of slope shape to estimates of LS based on uniform slopes. On irregular slopes, LS decreases for concave slopes and generally increases for complex and convex slopes. An estimate of the weighted average LS based on uniform slopes was 29% lower than an estimate made after applying corrections for slope shape in a case study in Delaware County, Ohio.