| A new modeling approach has been developed to evaluate the response of stream water quality to man-induced changes in watersheds, such as deforestation, urbanization, and the implementation of new agricultural practices. The approach is based on the integration of watershed and channel network models for the continuous simulation of transport and fate of nutrients and other pollutants.;The method for model integration combines the use of the terrain analysis model TOPAZ with especially designed algorithms to create a conceptual description of the watershed and its channel network and to define a common data structure that allows the automated transfer of data among all modeling components.;The integrated simulation is controlled by a software module implemented using the ArcView GIS program. Besides providing a graphical interface to the modeling system, the software controls the execution of the other programs and performs the necessary data conversions and transfers. The watershed model AGNPS has been selected to compute nutrient loads originating from upland areas. The CCHE1D one-dimensional channel network model is used to route unsteady flows and compute sediment transport.;A new model has been developed to simulate transport and fate of pollutants in channel networks. The advection-dispersion equation is discretized using the Control Volume Method and the Exponential Scheme. Combined with a fully implicit solution method, the model is well suited for application in complex channel networks. The model simulates the biogeochemical reactions of the nitrogen and phosphorus cycles, as well as the growth of phytoplankton.;The pollutant transport model has been successfully verified for problems with known analytical solutions, and it has been tested for the Chattahoochee River, Georgia. The model integration concept was demonstrated through an application to the Goodwin Creek watershed in Mississippi. A ten-year long integrated simulation was conducted, in which the watershed model AGNPS was used to determine the water, sediment, and nutrient loads that would reach the channels. Results from the watershed model were used as boundary conditions for the channel flow and pollutant transport models, which in turn computed the concentrations of the various nitrogen and phosphorus compounds along the channels of the network. |
