A fully coupled physically-based spatially-distributed model for evaluating surface/subsurface flow

A physically-based, spatially-distributed model is presented for simulation of surface/subsurface flow and the interactions between these domains. The model is designed for practical application to a wide variety of hydrologic evaluations, at various scales of simulation. The system is represented by the three-dimensional saturated–unsaturated flow equation for the subsurface, coupled with the diffusion wave equation for areal overland flow, both of which are coupled with the diffusion wave equation for flow through a network of streams and channels, including hydraulic structures. Ground surface unevenness at the grid scale is incorporated via the concept of detention storage, and thick vegetation or urban features are included via an obstruction storage exclusion term. Evapotranspiration from the surface and subsurface are modeled using land cover and climatic factors to define the complete water budget using a physically-based formulation. The system of equations is discretized using a fully implicit procedure, with the Newton–Raphson method to handle non-linearities efficiently. Robustness, stability and accuracy of solution are obtained for a wide variety of cases including dry systems and large surface/subsurface interaction fluxes. Adaptive time-stepping schemes and under-relaxation formulas further alleviate the computational burden. Verification and application examples demonstrate the need for a rigorous, fully-coupled solution to the set of equations, for complete hydrologic-cycle analysis.