Generalizing the Seepage Face Boundary Condition

English Abstract:

The seepage face is a non-linear dynamic boundary. The way it is handled in groundwater modeling strongly affects the pressure head distribution, the water table configuration, and the flow field. In addition, under conditions of heterogeneity and surface flow, complex scenarios may arise. In this study we address four specific questions: 1) When are seepage face boundary conditions critical and when, instead, is it acceptable to use a simpler fixed Dirichlet boundary condition? 2) How do strong vs weak constraints on seepage face convergence affect the accuracy and efficiency of the overall subsurface flow solution procedure? 3) How do we resolve seepage face outflow under conditions of layered and random heterogeneity? 4) In integrated surface-subsurface modeling, how does a seepage face boundary interact with the catchment outlet boundary condition used in overland and channel flow models? This last case is especially intriguing as the two conditions are diametrically opposed: a catchment outlet cell causes convergent flow patterns towards the land surface while a seepage face is typically used to represent outflow from the base of a hillslope. The analysis is performed with the CATHY (CATchment HYdrology) model, a physically-based model for the solution of the three-dimensional Richards equation coupled to a diffusion wave equation for surface flow propagation. The simulations will be conducted for synthetic hillslopes under various configurations (rainfall rate, slope angle, degree of heterogeneity, etc) and for the specific conditions represented by the convergent landscapes at the Biosphere 2 facility in Arizona.