The effect of hillslope geometry on Hortonian rainfall-infiltration-runoff processes

Topography is one of the main factors in hillslope rainfall-runoff processes. The effect of hillslope geometry, representing an important aspect of topography, on Hortonian rainfall-runoff has not been fully understood. In order to investigate this effect, hillslope geometry was abstracted as the combination of longitudinal profile curvature and plan shape, and a set of representative hillslope surfaces were generated using a variety of values of the profile curvature and plan shape factor. The infiltration and Hortonian runoff processes, mainly characterized by ponding time, infiltration rate, and runoff rate and depth, were computed using Richards’ equation for variably-saturated flow with given initial soil moisture, rainfall, and soil texture. The results show that different profile curvature and plan shape can cause more than 10% difference in cumulative runoff and runoff rate and more than 20% difference in ponding time. The curvature of hillslope longitudinal profile affects infiltration and runoff nonmonotonically and plays a more important role than the plan shape, whose effect is monotonic. The profile curvature and plan shape can complicate each other’s effect, but the effect of profile curvature also varies with rainfall duration. For a fixed plan shape, the longest time to ponding occurs on the straight profile hillslope. Hillslopes with opposite values of the plan shape factor and curvature have similar infiltration and runoff processes because of the similar slope gradient distributions. The widely used time compression approximation (TCA) for rainfall infiltration prediction after ponding can lead to an error of more than 37% when applied to hillslopes with a curved profile and variable width. This is because of the inaccurate estimation of the ponding time, especially for low intensity rainfalls, suggesting the presence of partial area runoff on these hillslopes. In addition, the results show that the effect of subsurface lateral flow near surface is generally negligible, and the run-on effect causes more infiltration on convex hillslope topography. The soil property and rainfall temporal variability do not change the trends but can alter the magnitudes of the hillslope geometry effect. The study provides insights into the rainfall runoff processes on natural hillslopes that could benefit studies related to hillslope hydrology and geomorphology.