Comparison of Przm and Gleams computer model predictions with field data for alachlor, metribuzin and norflurazon leaching

The ability of the pesticide root zone model (PRZM) and groundwater loading effects of agricultural management systems (GLEAMS) model to simulate movement of three herbicides was evaluated with site-specific soil, environmental, and pesticide data. Field studies examined norflurazon movement in a Dothan loamy sand in which cotton (Gossypium hirsutum L.) was grown, and alachlor and metribuzin movement in an Appling sandy loam. Actual herbicide concentrations were always greatest near the soil surface. The PRZM and GLEAMS computer simulation models accurately predicted the total herbicide present in each profile less than 20 d after application. The models overestimated herbicide movement into the soil profile in later simulations. The model predictions generally agreed with the relative location of alachlor and metribuzin in simulations less than 7 d after herbicide applications, but simulations beyond 7 d after herbicide application deviated from actual concentrations. PRZM accurately predicted that most of the applied norflurazon would remain close to the soil surface 84 d after herbicide application, whereas GLEAMS inaccurately predicted that norflurazon would be located throughout the soil profile, although the predicted depth to the limit of detection by both models was accurate.