MODELING LINKED WATERSHED AND LAKE PROCESSES FOR WATER-QUALITY MANAGEMENT DECISIONS

A physically based modeling approach is used to link watershed with lake processes and to simulate their responses to land management and weather conditions. Components of the watershed model, AGNPS (agricultural nonpoint-source model), are hydrology, erosion, sediment transport, transport of nitrogen and phosphorus, and chemical oxygen demand. Using a cellular structure, runoff, sediment, and chemical variables from the watershed provide input to a lake model. This one-dimensional model of water bodies simulates temperature stratification, mixing by wind, sedimentation, in-flow density current, and algal growth. Unsteady advection-diffusion equations characterize the dynamics of suspended sediment, soluble and sediment-attached N and P, and chlorophyll. This model, AGNPS-LAKE, is driven by random generation of weather conditions on a daily basis. Resulting impacts of alternative management plans are simulated by changing agricultural practices and land use, thereby modifying inflow characteristics to a lake. Modeling capabilities are being tested on eutrophic lakes in Minnesota for the purpose of simulating long-term trends and impacts of best management practices.