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GIS, watershed modeling, TMDL, nutrients, nonpoint source pollution


A comprehensive, GIS-based modeling approach was developed to enable accurate prediction of nutrient loads in watersheds throughout the state of Pennsylvania; particularly those watersheds for which stream monitoring data do not exist. This approach relies on the use of statewide GIS data sets for deriving reasonably good estimates for various critical model parameters that exhibit considerable spatial variability within the state. Data manipulation and subsequent simulation modeling is managed via an interface (called AVGWLF) between a popular desktop GIS software package (ArcView) and the Generalized Watershed Loading Function (GWLF) model. The modeling approach was tested in thirty-two (32) watersheds throughout Pennsylvania, and a statistical evaluation of the accuracy of the load predictions was made. Nash-Sutcliffe coefficients of correlation derived for the calibration and verification watersheds ranged in value from 0.92 to 0.97 for both nitrogen and phosphorus when considering mean annual loads. The median N-S values for nitrogen varied between 0.64 to 0.70 for monthly, seasonal, and year-to-year load estimates; and for phosphorus they varied between 0.61 and 0.72.