Comparison of the Effects of the Different Methods for Computing the Slope Length Factor at a Watershed Scale

The slope length factor is one of the parameters of the Universal Soil Loss Equation (USLE) and the Revised Universal Soil Loss Equation (RUSLE) and is sometimes calculated based on a digital elevation model (DEM). The methods for calculating the slope length factor are important because the values obtained may depend on the methods used for calculation. The purpose of this study was to compare the difference in spatial distribution of the slope length factor between the different methods at a watershed scale. One method used the uniform slope length factor equation (USLFE) where the effects of slope irregularities (such as slope gradient, etc.) on soil erosion by water were not considered. The other method used segmented slope length factor equation(SSLFE) which considered the effects of slope irregularities on soil erosion by water. The Arc Macro Language (AML) Version 4 program for the revised universal soil loss equation(RUSLE).which uses the USLFE, was chosen to calculate the slope length factor. In a parallel analysis, the AML code of RUSLE Version 4 was modified according to the SSLFE to calculate the slope length factor. Two watersheds with different slope and gully densities were chosen. The results show that the slope length factor and soil loss using the USLFE method were lower than those using the SSLFE method, especially on downslopes watershed with more frequent steep slopes and higher gully densities. In addition, the slope length factor and soil loss calculated by the USLFE showed less spatial variation.