Sensitivity of atrazine partition coefficient on pesticide root zone model prediction of soil's atrazine residue distribution within the crop root zone

Previous applications of the Pesticide Root Zone Model (PRZM) comparing predicted distributions of pesticides in the soil profile to the measured values suggest that PRZM prediction capability may be improved by selecting proper representation of the distribution coefficient values that directly influence atrazine-soil interactions. The objective of this study was to observe the variation in the predicted distribution of atrazine in the soil profile as the coefficient of distribution (K-d) was varied in one or more soil horizons using the PRZM-2 model. The goal was to identify if more attention should be devoted to understanding the atrazine-soil sorption issues better, or whether discrepancies in field data and model output are explained by the difficulties in assessing the preferential transport of atrazine. The PRZM-2 predicted atrazine distribution was compared with measured values for no-till (NT) and conventional-till (CT) corn plots. Results indicate that reducing K-d by 70% in NT plots reduced the predicted atrazine recovery in Horizon 1 (0-10 cm) by 31%, 34%, and 87% for simulations performed 2, 4, and 8 weeks after application (WAA). Reducing Kd in only the upper soil horizon improved predictions for the upper horizon but overpredicted atrazine concentrations in the second horizon. Reducing Kd throughout the profile reduced predicted atrazine concentrations in the upper horizon and increased predicted concentrations in lower horizons. Improvement in the correlation between predicted and measured atrazine concentrations was greatest for NT plots; however, discrepancies between predicted and measured data still exist and may be due to improper characterization of the atrazine degradation or dissipation parameters. These results suggest that although preferential flow appears to influence atrazine distribution, it may not be the only process influencing our ability to model atrazine fate and transport. Future research on atrazine degradation and dissipation under field conditions seems to be necessary to simulate atrazine fate and transport under preferential flow conditions.