The impact of hydrological alteration on vegetation and of vegetation on water quality can be greatly facilitated by linking existing water engines with general ecosystem models designed to make long-term projections of ecosystem dynamics. This development effort investigated the linkage of soil moisture between the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model and the Ecological Dynamics Simulation (EDYS) model. Conceptually, the EDYS and GSSHA models are well-suited for linkage given that they are both designed to simulate physical or ecological processes at multiple spatial and temporal scales. In particular, EDYS computes small-scale flows (precipitation, interception, evaporation, infiltration, transpiration, and nutrient and contaminant uptake) on a daily basis, and can thereby provide much more accurate estimates of evapotranspiration and water, nutrient, and contaminant uptake by vegetation than would ordinarily be available for calibration of hydrologic models. GSSHA and associated groundwater codes can then provide more accurate estimates of large-scale hydrological and transport processes back to EDYS to effect a system-wide assessment or projection. The long-term objective of this linkage between EDYS and GSSHA is to collaborate with other System-Wide Water Resources Program product lines and provide a dynamic eco-hydro modeling capability for regional applications (i.e., the Upper Mississippi, the Everglades, or the Nueces Basin).