Choices of scale and process complexity in hillslope models

Water flow in the subsurface is a significant process participating in the response of watersheds to a variety of events, such as rainfall or irrigation. Accurately representing flow in a well-chosen fundamental geological unit of the watershed's subsurface environment is, therefore, a precondition for accurately representing the watershed response as a whole. In this work we focus on hillslopes as the fundamental geological units of the watershed's subsurface processes, and investigate the effect of modeling decisions about scale and process complexity on the representation of hillslope hydrological response. As porous medium continuum scale models can include a high degree of process complexity and provide resolution at scales much smaller than hillslopes we first formulate a model of the hillslope unit based on the theory of porous medium continuum dynamics that incorporates the irregular three dimensional geometry of the hillslope, nonlinear and hysteretic submodels of multiphase flow processes, and the heterogeneity and anisotropy of soil parameters. We then compare the behavior of our complex, high-resolution hillslope model to a variety of simplifications in order to understand the degree to which the hillslope representation can be simplified while retaining sufficient resolution for watershed modeling applications.