Soil moisture response to rainfall and its controls on hillslopes in alpine mountain areas of the Tibetan Plateau

Understanding the response of hillslope soil moisture (SM) to rainfall and associated control mechanisms is crucial for understanding the partitioning of rainfall into runoff, soil water storage, and groundwater recharge in mountainous areas. However, the mechanisms governing hillslope SM response to rainfall are complex and remain unclear, especially in data-scarce alpine regions. Therefore, we established a slope-scale SM observation network consisting of 15 SM monitoring sites on a grassland slope in the Tibetan Plateau. The sites were categorized into semi-sunny (Sem), middle (Mid), and sunny (Sun) slopes based on slope aspect and position. We evaluated SM response patterns using the rainfall threshold (RT), SM increment (SMI), and soil storage increment (STI). Results revealed a distinct RT distribution across the hillslope, ranging from 0.72 to 8.19 mm, with high spatial heterogeneity (coefficient of variation = 51 %), mainly controlled by the slope aspect. Following rainfall events, lateral unsaturated subsurface flow produced a substantially higher SMI at Mid sites compared to the Sun and Sem sites. Moreover, the STI was concentrated in the surface soil layer (10 cm, 82.27 %), with smaller increases in deeper layers (50 cm, 17.17 %; 100 cm, 0.55 %). Rainfall amount was the primary driver of SMI temporal dynamics, while saturated hydraulic conductivity, bulk density, soil organic matter, antecedent SM, and slope gradient collectively governed its spatial distribution. This study provides new insights into hillslope-scale soil hydrological processes in mountainous areas.