A ground-water model was developed for Miami-Dade County Florida, which lies within the Southeast Atlantic Coastal Zone, as a predictive tool that will be used to analyze different water-management scenarios, including regional water-supply plans and the Comprehensive Everglades Restoration Plan (CERP). The model is being developed by the South Florida Water Management District (SFWMD) based on a modified version of the US Geological Survey modular three-dimensional, finite-difference, ground-water-flow model (MODFLOW). This version includes the wet-land and Diversion packages, which are MODFLOW modules that enable the top layer of the grid system to include overland flow through dense vegetation, channel flow through a slough network, and interaction With levees, and thus can closely simulate the natural system. The model domain is discretized into 430 rows by 367 columns with a uniform cell size of 500 ft x 500 ft (152.4 m). Four horizontal layers are used to represent lithologic zones within the surficial aquifer, one of the most transmissive aquifers in the world, with transmissivity values as high as 300,000 ft(2)/d (27,870.9 m(2)/d). Boundary conditions were established by water levels in canals on the northern and southern edges of the modeled region and by water-level measurements in wet-lands along the western edge. The eastern boundary with the Atlantic Ocean was determined by using mean tidal fluctuations to calculate the equivalent fresh-water head. The main advantage of this model, besides the high degree of detail and the number of variables, is that it can simulate hydroperiods within wetland areas using the Wet-land and Diversion packages. These packages allow the model to represent the full hydrologic cycle within the wetland areas, including sources and sinks on a daily basis, starting with total precipitation as a driving force. During calibration (1988-90), a very low sensitivity to conductivity and canal conductances was observed. Therefore, the fit between the model-computed water levels and the observed historical ground-water levels was achieved mainly by adjusting general head boundary conditions and wetland parameters within the active domain. The model is highly sensitive to the operational rules, especially the stages at which the canals are maintained, and is therefore responsive to the way that the system is managed.