Quantifying the Interactions Between Spatially Variable Throughfall and Subsurface Connectivity at the Hillslope Scale

Forested hillslopes exhibit a baffling array of heterogeneity in landscape properties and complexity of their responses to fixed hillslope attributes (e.g. slope, soil depth etc.) and temporally varying throughfall. One widely reported spatio-temporal pattern of hydrological response at the hillslope scale is the threshold nature of subsurface stormflow production. In many upland systems, this is linked to the development of connected patches of subsurface saturation at the soil-bedrock interface - a pre-condition for the initiation of lateral flow at many experimental sites examined around the world. To date however, these findings have been largely based on case studies of individual hillslopes with little synthesis among and across sites. As a result, the hydrology of forested hillslopes still lacks the compact organization of empirical data and observations of hillslope responses that might facilitate extrapolation to and prediction of hillslope behavior in ungauged hillslopes. One pressing issue that is difficult to assess with single-realization hillslope ecohydrological studies is how spatially varying throughfall inputs conspire with the subsurface topographic control on the development of connected patches of subsurface saturation necessary to drive flow. Here we describe a new study where we examine how patterns of measured throughfall, applied to a hillslope with variable soil depth and irregular subsurface topography, influence the spatio-temporal hydrologic response of the hillslope to a storm event. We present a number of virtual experiments using Hydrus-3D, a Richards equation-based finite element model. Topography and hydrologic field observations from an existing study hillslope were used to calibrate and test the model. The analysis of spatio-temporal patterns of subsurface moisture demonstrates how different realizations of throughfall patterns interact with the underlying subsurface topography, controlling the formation of subsurface saturation connectivity at the soil- bedrock interface and the generation of subsurface stormflow.