Effects of topography and soil depth on saturated-zone dynamics in steep hillslopes explored using the three-dimensional Richards’ equation

The generation of subsurface saturation has large implications for hydrological response in a catchment. Although rainfall–runoff responses have been observed worldwide in many catchments, the spatiotemporal patterns of soil–water movement and subsurface saturation processes in hillslopes and catchments are not yet fully understood because of the small number of observations and their limited areal extent. Numerical simulation allows for exploration of the entire subsurface saturation process in a hillslope or basin. In this study, as a virtual experiment we used a three-dimensional Richards’ equation and detailed observational data of topography and soil depth to examine the dynamics of subsurface saturated-zone generation in steep hillslopes in a catchment during a storm event. Such a simulation approach had not been previously applied for a catchment-scale area. In order to identify the effects of topographic factors on saturation development, we excluded the influence of the heterogeneity of hydraulic properties and used three generalized hydraulic parameter sets that were previously proposed for environments similar to our site.