Modeling surface and subsurface pesticide transport under three field conditions using PRZM-3 and GLEAMS

Contaminant transport models should be evaluated over a wide range of condi tions to determine their limitations. The models PRZM and GLEAMS have been evaluated many times, but few studies are available in which predicted move ment in runoff and percolate were simultaneously evaluated against field da ta. Studies of this type are essential because pesticide leaching and runof f are mutually dependent processes. For this reason, PRZM-3 and GLEAMS were evaluated for their ability to predict metribuzin concentrations in runoff , sediment, subsurface soil, and pan lysimeters under three field condition s (yard waste compost amended, no-till, and conventional-till) on a Lowell silt loam soil. Sensitive input parameters were either site specific (clima tic, soil, and chemical) or calibrated (K-factor, C-factor, curve number). In general, both models under-predicted metribuzin concentration in runoff water runoff sediment, subplow layer sail (15-75 cm), and pan lysimeter wat er (75 cm). Contrary to field data, bath models predicted that a large perc entage (> 50%) of metribuzin would move below the "mixing zone" (top 1 cm) during the first rainfall event after application. Relatively little metrib uzin was predicted to move beyond the plow layer (top 15 cm) into the pan l ysimeters or subsurface soil throughout the simulation period, possibly due to the lack of a macropore component in the models. High metribuzin concen trations in sediment (field data) indicated that relatively little metribuz in moved below the "mixing zone", possibly because of hysteresis but much o f the metribuzin that did move was quickly transported into the pan lysimet ers, probably due to macropore flow, GLEAMS more accurately predicted pesti cide concentration in sediment and PRZM predicted subsurface soil concentra tion somewhat more accurately than GLEAMS. Little difference in accuracy wa s detected between models on metribuzin concentration in runoff or metribuz in concentration in percolate. Although both models generally under-predict ed metribuzin concentration in runoff runoff transport (mass of metribuzin in runoff) for the study period was over-predicted by both models which emp hasizes the importance of accurately predicting herbicide concentration and runoff volume soon after application when the surface pesticide concentrat ions are highest.