Keywords: Surface water; Ground water; Wetlands; Variable density; Everglades

Coastal wetlands are a difficult hydrologic environment to represent with a numerical model because of the large number of contributing hydrologic processes, shallow hydraulic gradients, and variable-density flow conditions. Existing numerical modeling strategies have been developed for either the freshwater wetland system or the estuary, but simulations rarely span both domains. Recently, distributed-parameter physics-based computer codes have been developed to simulate coupled surface-water and ground-water flow for inland freshwater systems. Examples include: InHM (VanderKwaak, 1999; VanderKwaak and Loague, 2001), MIKE SHE (Graham and Refsgaard, 2001), MODHMS (HydroGeoLogic Inc., 2003; Panday and Huyakorn, 2004), and WASH123 (Yeh and Huang, 2003). To simplify surface and subsurface coupling techniques and to minimize computer runtimes, many integrated models use the diffusive wave approximation to the St Venant equation to represent overland flow. The diffusive wave approximation, in which the convective and local acceleration terms are neglected, is normally a valid approximation for inland systems due to relatively high frictional resistances, small flow velocities, and shallow flow depths. Most integrated models are also based on the assumption of constant fluid density, and thus their applicability to coastal regions is questionable unless it can somehow be shown that model results are insensitive to density variations. Conversely, estuary and oceanic models typically solve the full St Venant equations because the convective and local acceleration terms are significant under tidal and wind-driven conditions. Furthermore, most estuarine and oceanic models contain options for including the effects of density on surface-water flow, and have transport capabilities to simulate salinity. Estuarine and oceanic models, however, normally assume ground-water exchanges are negligible, or that the exchanges can be represented as a simple source term (Wang et al., 2003; Brown et al., 2003). Thus, most of the existing codes are not well suited to represent both the inland and marine systems, and the coastal wetlands that separate them.

This paper describes the development and application of an integrated surface-water/ground-water flow and solute-transport code designed to simulate two-dimensional overland flow and three-dimensional fully saturated ground-water flow. The integrated code was designed specifically for the coastal wetland transition zone between inland freshwater systems and marine systems. Surface-water flow and transport are simulated using the Surface-Water Integrated Flow and Transport in Two Dimensions (SWIFT2D) two-dimensional, finite-difference hydrodynamic code originally designed for estuaries (Leendertse, 1987). The SEAWAT three-dimensional, finite-difference code is used to simulate variable-density ground-water flow (Guo and Langevin, 2002). The two models are explicitly coupled with a one-timestep lag using a variable-density form of Darcy's Law for flow exchange and non-diffusive salt flux between models. The paper first describes the governing equations for flow and transport in both systems and then presents the numerical procedure for implementing the two codes in a coupled framework. Lastly, the integrated code is applied to the southern Everglades of Florida and northeastern Florida Bay to quantify flow and salinity patterns for a 7-yr period (1996-2002) and to examine the effects of selected hydrologic processes.