Modeling Nutrient Dynamics in Watersheds and Streams

Stream water quality is dependent on a number of complex and highly variable pollutant-generating processes that occur in the upland and riparian zones of a watershed. A combination of watershed and channel models is used to determine the response of stream water quality to land use management activities that affect the supply of sediment and nutrients to channels. The models are integrated using terrain analysis and GIS technology, where data processing algorithms define a conceptual description of the drainage network and the corresponding subcatchments to be used by all modeling components. The CCHE1D one-dimensional channel network model is used to compute unsteady flows and the transport and fate of nutrients through the channels of the watershed. The model accounts for the biogeochemical transformation processes that determine nutrient concentrations, in particular the simulation of the aquatic cycles of nitrogen and phosphorus compounds. The watershed model AGNPS is used to simulate the hydrologic processes and to compute the sediment and nutrient loads that reach the channels. The CCHE1D's water quality module is initially validated using data on BOD and nitrogen compounds from primarily point-sources, measured in the Chattahoochee River. The model integration concept is demonstrated through an application to the Goodwin Creek watershed, in north Mississippi. The integrated model is used to evaluate the system's response to hypothetical land use changes, which represent possible management scenarios used to control the supply of nutrients to the streams.