Use of the Universal Soil Loss Equation The USLE computes upland erosion from small watersheds on an average annual basis. It includes the detachment and transport components, but it does not account for the deposition component. Therefore, the USLE cannot be used to compute sediment yield. For example, in a 1000-mi2 drainage basin, only 5 percent of the soil loss computed by the USLE may appear as sediment yield at the basin outlet. The remaining 9S percent is redistributed on uplands or flood plains and does not constitute a net soil loss from the drainage basin. Example 15-3 Assume a 600-ac watershed above a proposed floodwater-retarding structure in Fountain County, Indiana. Compute the average annual soil loss by the universal soil loss equation for the following conditions: (1) cropland, 280 ac, contour strip-cropped, soil is Fayette silt loam, slopes are 8% and 200 ft long; (2) pasture, 170 ac, 50% canopy cover, 80% ground cover with grass, soil is Fayette silt loam, slopes are 8% and 200 ft long; and (3) forest, 150 ac, soil is Marshall silt loam, 30% tree canopy cover, slopes are 12% and 100 ft long.

1. From Fig. 15-2, R = 185. From Table 15-4, K = 0.38. From Fig. 15-3, LS = 1.4. The value of C for cropland is obtained from local sources; assume C = 0.12 for this example. From Table 15-7, P = 0.25. Using Eq. 15-6: A = 185 X 0.38 X 1.4 X 0.12 X 0.25 = 2.95 tons/ac/y.

2. R = 185; K = 0.38: LS = 1.4. From Table 15-5, C = 0.012. No value of P has been established for pasture; therefore, P = 1. Using Eq. 15-6: A = 185 X 0.38 X 1.4 X 0.012 X 1.0 = 1.18 tons/ac/y.