Numerical analyses of subsurface flow in a steep hillslope under rainfall: The role of the spatial heterogeneity of the formation hydraulic properties

The paper aims at analyzing subsurface flow and streamflow generation in steep hillslopes after rainfall. The principal scope of the work is to assess the role played by the spatial variability of the hydraulic properties on the subsurface flow patterns and hillslope discharge. We try to address this task through a three-dimensional numerical model which solves the Richards equation under prescribed initial and boundary conditions. Along an established practice, the relevant hydraulic parameters are modeled as random space functions with given statistical properties. Realistic features of the spatially variable formation parameters and the precipitation data are incorporated into the simulation model. It is found that spatial heterogeneity has a significant impact on the movement of water in the hillslope and the streamflow generation. In particular, the heterogeneous systems are more sensitive to rain input and display an increase of both the water table and saturation which is faster than the homogeneous case. The same is for water velocity, which is everywhere larger when heterogeneity is considered. As a consequence, the dynamics of subsurface stormflow (recognized as the leading mechanism for streamflow generation in the simulations) and the outflow discharge produced by the hillslope after a rain event are enhanced by heterogeneity. Despite of the possible lack of generality of the numerical experiments, they provide useful insight into the complex dynamics occurring in hillslope systems. The simulations show the importance of including realistic spatial heterogeneity in hillslope hydrology studies.