Slope Gradient Effects on Soil Loss for Steep Slopes

Data for assessing the effects of slope gradient on soil erosion for the case of steep slopes are limited. Widely used relationships are based primarily on data that were collected on slopes up to approximately 25%. These relationships show a reasonable degree of uniformity in soil loss estimates on slopes within that range, but are quite different when extrapolated beyond the range of the measured data. In this study, soil loss data from natural runoff plots at three locations on the loess plateau in China were used to assess the effect of slope gradient on soil loss for slopes ranging from 9 to 55% steepness. Plot size at each location was 5 m wide by 20 m long, and the soils were silt loams or silty-clay loam. The results indicated that for these plots, soil loss was linearly related to the sine of the slope angle according to the equation: S = 21.91 sinq 0.96, where q is the slope angle and S is the slope steepness factor normalized to 9%. This relationship was assessed in terms of the limited existing experimental data for rainfall erosion on steep gradients and found to be reasonable for data collected on longer plots, but somewhat different than the data from shorter plot studies. The results of this study would indicate a lesser soil loss at high slopes than does the relationship used in the Universal Soil Loss Equation, but a greater soil loss than predicted by the Revised Universal Soil Loss Equation for steep slopes.