Characterizing Rainfall-Driven Soil Water Dynamics in Soil Profiles and Quantifying the Influencing Factors Over Mainland China

Understanding rainfall-driven soil water dynamics (SWD) in soil profiles is crucial for improving hydrometeorological modeling. This study characterized the spatiotemporal patterns of rainfall-driven SWD in soil profiles and quantified the impacts of environmental factors using the random forest method at hourly interval over mainland China. Results showed that the spatial patterns of rainfall-driven SWD over mainland China at different layers could be classified into three groups: layer 1 (0–10 cm), layers 2–5 (10–50 cm), and layers 6–8 (50–100 cm). The SWC responses at layers 2–5 were weaker in northwest China because of low rainfall amounts in the semi-arid and arid areas, whereas the SWC responses at layers 6–8 were weaker than at the other layers, and were generally concentrated in southeast China. With decreased SWC responses in deeper soils, rainfall-driven SWD showed rapid declines at 0–30 cm and slight declines at 30–100 cm over mainland China. Soil-wetting duration times were high at the surface layer and homogenous at the 10–30 cm and 30–80 cm layers, and were low at the 80–100 cm layers over mainland China. Thus, the analysis of soil water residence time in the plant root zone based on surface SWC should be investigated for their suitability. The preferential flow ratios were higher in deeper soils because SWC at deeper layers responded to heavier rainfalls with higher antecedent SWC, which tended to cause preferential flow. The impacts of rainfall decreased in deeper soils, whereas the impacts of antecedent SWC increased until 30 cm and then decreased in deeper soils, which was influenced by saturation degrees of antecedent SWC for SWC response at different layers. The threshold for SWC increment in the soil profile was a rainfall of 3 mm over mainland China