Comparative study of coupling approaches for surface water and subsurface interactions

In the core of an integrated watershed model there is coupling between surface water and subsurface water flows. Recently, interest in hydrology literature, regarding the fully coupled approach for surface and subsurface water interactions, has increased. For example, the assumption of a gradient-type flux equation, based on Darcy's law and the numerical solution of all governing equations in a single global matrix, has been reported. This paper argues that this "fully coupled approach" is only a special case of all possible coupling combinations and, if not applied with caution, the nonphysics interface parameter becomes a calibration tool. Generally, there are two cases of surface/subsurface coupling based on the physical nature of the interface: continuous or discontinuous assumption; when a sediment layer exists at the interface, the discontinuous assumption may be justified. As for numerical schemes, there are three cases: time lagged, iterative, and simultaneous solutions. Since modelers often resort to the simplest, fastest schemes in practical applications, it is desirable to quantify potential errors and the performance specific to each coupling scheme. This paper evaluates these coupling schemes in a watershed model, WASH123D, with numerical experiments. They are designed to compare the performance of each coupling approach for different types of surface water and subsurface interactions. These experiments are evaluated in terms of surface water and subsurface water solutions, along with exchange flux (e.g. infiltration/seepage rate).