Characterizing rapid infiltration processes on complex hillslopes: Insights from soil moisture response to rainfall events

Evaluating the soil moisture response characteristics to rainfall events is valuable for understanding ecohydrological effects in the context of global climate change. However, the effects of soil thickness and rainfall characteristic indicators on soil moisture responses remain unclear, especially in the karst hillslopes characterized by highly complex topography. This study examined the soil moisture response to 35 rainfall events at 5-min intervals by deploying 20 sets of monitoring devices across two hillslope plots (5 x 20 m) with distinct mean soil thicknesses. The response metrics included the soil moisture response time (Tp2p) and wetting front velocity (Vwf), whereas indicator factors considered were soil thickness distribution, slope position, and rainfall characteristics. Overall, the results showed that Tp2p increased from downslope to upslope, as well as from the surface to deep layers. Surprisingly, the mean Vwf (1373 mm h- 1) in this study site was significantly higher than that in non-karst regions (17-610 mm h- 1). This suggests that rainwater can rapidly infiltrate the soil profile and contribute to subsurface runoff generation. Rainfall characteristics are the primary controlling factors influencing the soil moisture response to rainfall, with their contribution priority (44.3-63.9 %) higher than that of antecedent soil moisture conditions (26.1-35.2 %). The variation in soil moisture response metrics in shallow soil cover hillslopes was more affected by rainfall indicators (41-64 % vs. 31-47 %), thereby potentially weakening the potential influence of topography and soil properties. These findings highlight the effect of refined monitoring in characterizing soil moisture heterogeneity in response to rainfall and emphasize the potential impact of rapid responses on karst eco-hydrological processes.