Using Numerical Model Evaluate Subsurface Stormflow Process in Complex Hillslope with V-Shape Convergence

Subsurface stormflow (SSF) is a common way of rainfall runoff in humid mountainous areas. It is so difficult to directly observe in the field that there are still many controversies about SSF runoff processing. SSF process is mainly constrained by underlying conditions, which include topography, bedrock and soil hydraulic properties, depth to groundwater table and vegetation covering and so on. In order to understand the influence of three-dimensional complex convergent terrain of hillslope on the SSF runoff process, it establishes numerical simulation experiments using a three-dimensional fully coupled integrated hydrological model (HydroGeoSphere). The platforms, based on the DEM of Landscape Evolution Observatory (LEO) in Biosphere 2, totally include three different slopes with convergent terrains and a straight slope for comparison. The experimental results show that: (1) Convergence topography is not the only factor controlling the SSF runoff process. It should also consider the spatial distribution of soil depth. The soil depth of the convergence zone directly impacts the hydrological response and hydrography. The hydrological response and dynamics changes of storage in convergence slope and straight slope with constant soil depth are almost the same, but different in their runoff process. With the decrease of soil depth of convergence zone, it leads to the sooner hydrological response, larger total runoff and the higher peak flow; (2) The bedrock with convergent terrain restrains the saturated water movement driven by gravity on the interface of bedrock and soil. The water is concentrated in the middle convergence region to form a variable source saturated area contributing to runoff. Based on the different rain intensity and soil hydraulic properties, concentrating water by bedrock geometry may lead to opposite results, promoting or inhibiting the runoff production; (3) As the rainfall intensity and saturated hydraulic conductivity of soil increase, the driving force and binding force of the convergence terrain on the SSF will be weakened. The difference in response time and flow process would be reduced. The above results provide some references for preparing a hydrological model containing an SSF module.