Comprehensive Factors Influencing Lateral Soil Water Flow Patterns on Hillslopes: Insights from Experimental and Simulation Studies

This study conducted a rainfall tracer experiment on the slope to investigate water flow patterns. Three distinct water flow patterns were observed within the slope, including lateral upslope and vertical flow during rainfall, followed by lateral downslope flow after rainfall. A series of scenario simulations were conducted to enhance understanding of the comprehensive factors influencing lateral water flow on the slope. These simulations considered various factors, including two rainfall patterns (RP), three soil types (ST), three slope angles (SA), three anisotropy ratios (AR), and two layered slope systems (LS). The Hydrus-2D model was applied based on the two-dimensional Richards equation under given initial soil water and boundary conditions. Spatiotemporal variations in horizontal flux (qx), vertical flux (qz), and deviation of the water flow direction from vertical (DWFFV) in the central slope profile were calculated. The simulations of water flow on slopes under different conditions also confirmed three distinct water flow patterns manifesting within slopes during rainfall. The results indicated complete consistency between the direction of lateral water flow (lateral unsaturated upslope or downslope flow) and soil water potential horizontal gradients (∂φ/∂x). This suggests that the direction of lateral water flow is regulated by ∂φ/∂x rather than the change in water content over time (∂θ/∂t). The rate of movement of the wetting front differed among soil types, with the highest in sand. DWFFV and the corresponding qx value showed positive correlations with SA and AR.