Comparison of pesticide root zone model predictions with observed concentrations for the tobacco pesticide metalaxyl in unsaturated zone soils

Abstract

AbstractThe pesticide root zone model (PRZM) was developed to evaluate pesticide leaching threats to ground water for different crops under varying climatic conditions, soil characteristics and cropping practices. In this study, PRZM was evaluated to establish its predictive capability by comparing observed metalaxyl concentration profiles with predicted concentration profiles using field‐averaged pesticide data and best estimates for several hydrologic characteristics and pesticide transport properties at field sites in Florida and Maryland. Coefficients of determination for the Florida site at 26, 55 and 85 d after application of 0.33, 0.90 and 0.95, respectively, were obtained when field observations were regressed against model predictions. An overall coefficient of determination for the Maryland site of 0.75 was determined for 14 samplings after metalaxyl application.Pesticide transformation in the Florida soil was best described by a first‐order model. A first‐order degradation rate in soil of 0.014 d−1 was estimated. Pesticide transformation in the Maryland soil was best described by a phased first‐order model. A phase determination point that occurred 30 d after metalaxyl application was used to estimate soil degradation rates. Differences were attributed to the timing of rainfall events and soil infiltration characteristics. Estimated phased first‐order degradation rates in soil for the Maryland site were 0.046 and 0.0045 d−1 before and after the phase determination point.Evaluation of PRZM, using best estimates for transport and transformation properties of metalaxyl and limited calibration for water balance, suggests that the model effectively simulates the important processes operating on the pesticide.