U.S. Geological Survey

Factors Affecting Herbicide Yields in the

Chesapeake Bay Watershed, June 1994

Robert A. Hainly and Joan M. Kahn
Median concentrations and instantaneous yields of alachlor, metolachlor, atrazine, cyanazine and simazine were generally highest at sites in the Lower Susquehanna River Basin and in agricultural subbasins. Instantaneous herbicide yields are related to land use, hydrogeologic setting, streamflow yield, and agricultural row cropping practices. The significance of these relations may be affected by the interdependence of the factors. The percentage of basin area planted in corn is the most influential factor in the prediction of herbicide yield. Instantaneous yields of all five herbicides measured in June 1994 related poorly to averaged 1990-94 herbicide use. Annually averaged herbicide-use data are too generalized to use as a predictor for short-term herbicide yields. An evaluation of factors affecting herbicide yields could be refined with more-current land use and land cover information and a more accurate estimate of the percentage of basin area planted in corn. Factors related to herbicide yields can be used to predict herbicide yields in other basins within the Chesapeake Bay watershed and develop an estimate of herbicide loads to Chesapeake Bay. (KEY TERMS: nonpoint source pollution, water quality, agricultural chemicals, herbicides, triazine, modeling/statistics, pollution load, land use.)

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