State-Forcing and Slope-Position Controls on Near-Surface Hillslope Wetting and Flood-Peak Variability in a Steep Mountain Catchment

Abstract. Rainfall-runoff response in steep, vegetated mountain catchments often varies strongly among events because rainfall input interacts with antecedent wetness, soil water redistribution, and transient hillslope-channel connectivity. Yet it remains unclear whether near-surface soil moisture profiles can indicate when hillslope wetting departs from a simple top-down sequence and whether such signals help explain flood-peak variability at the catchment outlet. We examined 30 rainfall-runoff events in the Ganwuli River basin using 15-minute rainfall records, outlet discharge, and soil moisture observations from three topographic positions with sensors at depths of 10–60 cm. Within this monitored near-surface profile, the 10–30 cm sensors were treated as upper monitored layers and the 40–60 cm sensors as lower monitored layers. Event responses were described using response occurrence, wetting magnitude, response lag, non-sequential wetting, and lower-layer early response. Across events, peak discharge increased primarily with total rainfall, whereas the maximum hydrograph rise rate was more sensitive to the intensity of short-duration rainfall. Soil moisture responses were frequent in the upper monitored layers at the foot-slope position. By contrast, lower monitored layers at the mid-slope and upper-slope positions more often showed early or near-synchronous responses relative to the upper layers, indicating that wetting within the monitored profile was not always governed by sequential vertical propagation. Segmented logistic models and two-dimensional probability surfaces further indicated that these lower-layer responses were most likely to occur when wet antecedent conditions coincided with high short-duration rainfall intensity. Events with lower-layer early responses at the upper-slope position tended to have larger flood peaks, but this relationship is interpreted as an indicator of changing hillslope connectivity rather than direct evidence of deep groundwater flow paths. These findings suggest that high-frequency near-surface soil moisture profiles can add useful process information for interpreting event-scale flood variability, while deeper storage and groundwater mechanisms require independent observations.