Comparison of pesticide root zone model 3.12: leaching predictions with field data.

As part of a process to improve confidence in the results of regulatory modeling, predictions of the pesticide root zone model (PRZM) 3.12 were compared with measured data collected in nine different field leaching studies. Reasonable estimates of leaching were obtained with PRZM 3.12 in homogeneous soils where preferential flow is not significant. The PRZM 3.12 usually did a good job of predicting movement of bromide in soil (soil and soil pore-water concentrations were generally within a factor of two of predicted values). For simulations based on the best choices for input parameters, predictions of soil pore-water concentrations for pesticides were usually within a factor of three and soil pore-water estimates within a factor of 11. When the model input parameters were calibrated to improve the simulation of hydrology, predicted pesticide concentrations in soil pore water were usually within a factor of two of measured concentrations. Because of the sensitivity of leaching to degradation rate, the most accurate predictions were obtained with pesticides with relatively slow degradation rates. When conservative assumptions were used to define input pesticide parameters, predictions of pesticide concentrations were usually a factor of two greater than when using the best estimate of input parameters without any built-in conservatism.