Modeling of agroecosystems has been attempted at both field and watershed scales.  At the watershed scale, ecohydrology is a more comprehensive, integrative science that studies the relationships between hydrological, biogeochemical, and ecological processes in soils, agricultural systems, rivers and lakes.  It proposes a “dual regulation” of an ecosystem by simultaneously studying ecological and hydrological processes to enhance aquatic ecosystems in response to land management and global change.  An ecohydrological river basin model includes a hydrological submodel as a basic component.  Other components describing biogeochemical cycles (carbon, nitrogen, and phosphorus) and vegetation are coupled with the hydrological component, to include interactions and feedbacks between the processes, such as water and nutrient drivers for plant growth and yield, water transpiration by plants, and nutrient transport with water and sediment.  Climate and land use parameters are treated as external drivers.  The Soil and Water Assessment Tool (SWAT) is a continuous time, semi-distributed, process based, ecohydrological river basin model.  It was developed to evaluate the effects of land management decisions on water resources and diffuse pollution for large river basins.  There are numerous SWAT applications reported in the literature for hydrological and water resource assessment, in water quantity (water discharge, groundwater dynamics, soil water, snow dynamics, and irrigation management), water quality assessment (best management practices, land use and land management change), climate change, and crop yield and bioenergy assessment.  We will review case studies related to the impact of agricultural management and climate on water supply, sediment and nutrient loadings to rivers and lakes, and yield and biomass production.  We will also show potential linkage of these indicators to wildlife and stream health.