California’s Napa Valley is recognized world-wide for its scenic beauty and premium wines. Balancing water demands for urban areas, agriculture, and ecosystem preservation while maintaining flood protection and water quality is a challenging task. In order to quantify hydrologic conditions in Napa County, officials are employing the use of integrated numerical hydrologic models at varying levels of complexity using a phased modeling approach. Regional models were constructed and calibrated using MIKE SHE/MIKE 11 for the two major watersheds in the county using a linear reservoir module to simulate the groundwater system along with additional modules to simulate the remaining components of the hydrologic cycle. The linear reservoir approach uses a lumped-parameter conceptualization to represent the groundwater system, has the advantage of less stringent computational requirements compared to finite-difference numerical methods, and is ideally suited to addressing regional surface water problems. For areas where groundwater is the primary water-supply, a traditional fully-distributed finite-difference groundwater model was constructed and calibrated to allow for a detailed analysis of groundwater conditions. Outputs from the models include stream flow and water-table hydrographs, water budgets, and nutrient and pathogens concentrations for 186 surface water sub-basins. Additionally, conditions associated with changes in land- and water-use were simulated at regional and local scales.