The magnitude and water quality implications of nonpoint source phosphorus loadings to Owasco Lake (New York) are evaluated through the application of a methodology which links geographic characteristics, longterm average runoff loads and a set of critical lakewide water quality response parameters. The approach utilizes the Universal Soil Loss Equation together with empirical loading functions to derive representative phosphorus export coefficients for the local drainage system. Cumulative loadings from individual sub-basins within the watershed serve as input to a simple water quality model of Owasco Lake, showing the expected lake response in terms of average total phosphorus concentration, trophic state, water transparency, and minimum hypolimnetic dissolved oxygen concentration. The methodology facilitates easy and rapid assessment of general watershed management and development scenarios of interest A unique aspect of the approach is its dependence upon descriptive data supplied by a Geographic Information System (GIS) to establish the coincidence of specific land use, soil texture and surface slope attributes within each of the hydrologie sub-basins comprising the overall watershed. The GIS-generated attribute matrices provide a much more accurate depiction of critical geographic characteristics known to impact nonpoint source runoff loadings, thereby improving the reliability of current and projected phosphorus loads to Owasco Lake.

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