Effect of hillslope profile shape on runoff and soil erosion in red soil hilly areas of China using the WEPP model

The effect of hillslope shape on runoff and soil erosion is a critical factor in understanding erosion dynamics and developing effective soil conservation strategies. This research employs the Water Erosion Prediction Project (WEPP) model, which is calibrated using observed runoff and sediment data from a bare fallow runoff plot located in the red soil hilly areas of southern China, to simulate the effects of hillslope shape on annual runoff and soil erosion. The simulations covered a range of concave and convex hillslope shapes with various curvature radius. The results indicated that runoff generation decreases with increasing curvature radius on concave hillslopes, while it increases on convex hillslopes. As slope gradient increases and slope length decreases, the effect of hillslope shape on runoff gradually diminishes, potentially becoming negligible. Additionally, the hillslope shape has a significant impact on soil erosion, with convex hillslopes exhibiting the highest erosion rates, followed by uniform hillslopes, and concave hillslopes showing the least erosion. The soil erosion rate of convex hillslopes can be up to 2.02 times that of concave hillslopes and 1.25 times that of uniform hillslopes. For concave hillslopes, soil erosion modulus increased logarithmically with curvature radius, whereas for convex hillslopes, it decreased following a power function. Increases in slope gradient and reductions in slope length intensify the effect of profile curvature on soil erosion. A strong linear relationship (r2 > 0.94) was observed between profile curvature and the curvature factor, enabling accurate erosion predictions for curved slopes based on uniform hillslope data. These findings enhance soil erosion modeling accuracy and provide a scientific basis for hillslope management in soil conservation.