A WATERSHED-SCALE, FULLY-DISTRIBUTED MODEL OF PESTICIDE FATE/TRANSPORT

Pesticide contamination is an important concern of land use managers. Hydrologic and water quality modeling of alternative practices (alternative futures) and current strategies gives managers a useful tool in evaluating BMPs and water quality goals. WET_Hydro is a conceptual GIS-based, fully distributed hydrologic model developed for medium-scale (1-1000 km^2) watersheds. This talk introduces the theory and mathematics behind the pesticide model within WET_Hydro and presents some initial validation results. Using fate/transport routines derived from the Pesticide Root Zone Model (PRZM), pesticide mass balances are maintained within each of four compartments (plant, soil surface, vadose and saturated zones) for each pesticide of interest. Volumes are distributed throughout the gridded structure and solved in time using a variable-step Runge-Kutta-Feldberg solution. The combination of conceptual modeling of important hydrologic pathways with spatially distributed volume-based accounting methods and flexible, GIS-based programming yields a highly usable modeling tool for evaluating water quality effects of land-use practices.