Effect of rainfall moving direction on surface flow and soil erosion processes on slopes with sealing

Although rainfall characteristics play important roles in runoff generation and soil erosion, our understanding about how rainfall moving direction impacts surface flow and soil erosion on slopes considering spatial process of soil surface sealing is still poor. In this study, a series of laboratory experiments was conducted using a soil flume and a movable rainfall simulator to investigate this impact. Two rainfall moving directions (i.e. upslope-moving and downslope-moving) and two soil surface moisture conditions (i.e. dry runs with no antecedent rainfall and wet runs with antecedent rainfall) were considered in the experiments with different slope gradients and rainfall intensities. The results showed that the upslope-moving rainfalls generated more surface flow from 25% to 700% and less soil erosion than the downslope-moving rainfalls in the dry runs due to the different spatial developing processes of soil surface sealing. The difference between the runoff coefficients of the two moving direction rainfalls decreased remarkably (more than 50%) with slope gradient increasing from 15 degrees to 25 degrees. In the wet runs, surface flow was similar for the upslope-moving and downslope-moving rainfalls, as all of the soil surface had been sealed before the experiments. Soil erosion in the downslope-moving rainfalls was larger than that in the upslope-moving rainfalls due to higher sediment transport capability of the surface flow. For both rainfall moving directions with same rainfall conditions, the total soil erosion in a dry run was mostly larger than that in a wet run because of less erodible soil particles on the sealed slope in the wet run. The result of this study implied that different rainfall moving directions can lead to distinct spatial processes of soil surface sealing and caused large differences in surface flow and soil erosion processes on slopes.