Lateral partition patterns and controlling factors of soil infiltration at a steep, near-stream, and humid hillslope scale

•Subsurface lateral flow often occurs at 10–40 cm depths along the hillslope.•Wet soil conditions are conducive to triggering hillslope lateral flow.•Variability of infiltration partitioning remains high in the deeper layers.•Dominant factors controlling PWSI are sand (0–10 cm), EL (10–40 cm), and n (40–80 cm). Infiltration is the process of water entering into, and routings through, the subsurface. It has a profound impact on hillslope and catchment runoff. However, because of the hidden and complex subsurface structures, our understanding of rainfall-related infiltration and how it partitions along a topographic gradient remains challenging. In this study, we used two years of field observations of volumetric soil moisture at 25 combinations of topographic positions and soil depths along a steep subtropical forested hillslope. The lateral partition patterns of infiltration and its control factors were investigated based on a new index, percentage of soil water storage increment at each site to all hillslope sites during a rainfall event (PWSI). Our results showed that the active soil layer involved in hillslope lateral flow was distributed within the depths of 10–40 cm. In deep soil depths (i.e., 40–80 cm), lateral flow was more evident under wet initial moisture conditions than under dry initial moisture conditions, whereas rainfall characteristics had a relatively weak effect on it. Unexpectedly, we found the variability of infiltration partitioning remained high in the deeper soil layers. Among the soil properties, sand, bulk density, n, KS, and SOM were conducive to infiltration, whereas clay had the opposite effect. The dominant factors controlling the lateral partition patterns of infiltration across soil depths were sand in the depths of 0–10 cm, topographic gradient in depths of 10–40 cm, and n (parameter of soil retention curve) in the depths of 40–80 cm. Findings of this study reveal the infiltration partitioning indices were useful to quantitatively describe the distribution patterns of infiltration after rainfall events at the hillslope, and provide new insights into the detection of hillslope lateral flow, which is valuable for understanding subsurface hydrological processes and improving water resource management in humid mountain ecosystems.