SWAT - A SEMIEMPIRICAL MODEL TO PREDICT CONCENTRATIONS OF PESTICIDES ENTERING SURFACE WATERS FROM AGRICULTURAL LAND

A semi-empirical model called SWAT has been developed to predict conce ntrations of agriculturally applied pesticides moving to surface water s, an aspect which is not well described by current models for pestici de fate. The model is based upon a direct hydrological link establishe d between soil type and the amount of water moving rapidly to streams in response to rainfall. Attenuation factors describe the decrease in concentrations of pesticide between field application and loss in wate r moving from the site into surface waters. Evaluation of model predic tions against available field data from three sites and four soil type s in England shows that SWAT is capable of predicting the transient pe ak concentrations of a wide range of pesticides during rapid water mov ement to streams in response to rainfall. Predicted concentrations wer e too great when rainfall initiated water movement to streams very soo n after pesticide application, particularly for the more mobile pestic ides, and some predictions for pesticides sorbed very strongly to soil were relatively poor. Almost all of the predicted concentrations were within one order of magnitude of measured values.