Update of Watershed Regressions for Pesticides (WARP) for predicting atrazine concentration in streams

AGRO 259

Wesley W. Stone, wwstone@usgs.gov and Robert J. Gilliom, rgilliom@usgs.gov. U.S. Geological Survey, 5957 Lakeside Blvd, Indianapolis, IN 46278
Watershed Regressions for Pesticides (WARP) models for predicting atrazine concentrations in streams were updated by incorporating annual atrazine use and annual precipitation data as explanatory variables. The original WARP models were based on atrazine use estimates from either 1992 or 1997. Separate WARP regression models were derived for selected percentiles, annual mean, annual maximum, and annual maximum 21-day, 60-day, and 90-day moving-average concentration statistics. Development of the regression models used the same model development data, model validation data, and regression methods as those used in the original development of WARP. The updated models accounted for more variability in the atrazine concentration statistics among the development sites than the previous WARP models. The explanatory variables used in the updated WARP models include atrazine use intensity, K-factor, R-factor, watershed area, percent contribution from Dunne overland flow, and total precipitation during May and June of the year of sampling.