Impacts of rainfall and inflow on rill formation and erosion processes on steep hillslopes

Limited information has isolated the impacts of rainfall on rill formation and erosion on steep hillslopes where upslope inflow simultaneously exists. Field simulation experiments were conducted on steep hillslopes (26°) under rainfall (60 mm h−1), inflow (6, 12, 18, 24, 30, 36 L min−1 m−1), and combination of rainfall and inflow to explore the impacts of rainfall on rill formation, and the interaction between rainfall and inflow on soil erosion. Rainfall decreased soil infiltration rate (10%–26%) mainly due to soil crust by raindrop impact. Rainfall strengthened rill formation, which behaved in the increment in rill width (5%–26%), length (4%–22%), and depth (3%–22%), but this increment decreased as inflow rates increased. Additionally, the contribution of rainfall on rill formation was most significant at the initial stage, followed by the final stage and active period of rill development. Rainfall increased rill erosion (8%–80%) and interrill erosion (36%–64%), but it played a dominant role in increasing interrill erosion under relatively high inflow rates. The most sensitive hydrodynamic parameter to soil erosion was shear stress and stream power under inflow and 'inflow + rainfall' conditions, respectively. For the lowest inflow rate, the reduction in soil loss by interaction between rainfall and inflow accounted for 20% of total soil loss, indicating a negative interaction. However, such interaction became positive with increasing inflow rates. The contribution rate to rill erosion by the interaction was greater than that of interrill erosion under relatively low inflow rates. Our results provide a better understanding of hillslope soil erosion mechanism.