Modeling pesticide dynamics of four different sites using the model systemSIMULAT

This study aimed to assess the accuracy of SIMULAT, a computer model primar ily designed for predicting the fate of pesticides in soil. The evaluation was carried out by comparing simulated results on herbicide degradation wit h results obtained from held and lysimeters experiments. For model validati on four different data sets were available. The data sets included field, l ysimeter, and laboratory experiments from Germany (Weiherbach), The Netherl ands (Vredepeel), Great Britain (Brimstone), and Italy (Tor Mancina). The a pplied herbicides and the determined soil and water parameters varied subst antially among the four empirical data sets used for model evaluation. In a first step simulations were run with the model still being uncalibrate d. Afterwards a calibration of hydraulic parameters was performed using mea sured water and bromide contents in soil(Weiherbach, Vredepeel and Brimston e) or leachate (Tor Mancina). In contrast to the hydraulic parameters, the sorption and degradation parameters were not calibrated. Simulations were r un with the calibrated model and the results compared with those obtained f rom field measurements. The wide range of implemented boundary conditions such as lysimeter, free d rainage or fluctuating groundwater table enabled applying SIMULAT to all fo ur data sets. However, usage of parameters obtained in laboratory experimen ts gave no satisfiable simulation of the degradation of the herbicides. In contrast to the macroporous loam soil at Tor Mancina, water and bromide tra nsport were accurately simulated in the loess (Weiherbach) and in the sandy soil (Vredepeel). Severe problems occur while simulating the fluctuating g roundwater and drain flow in the clay soil at Brimstone. (C) 2000 Elsevier Science B.V. All rights reserved.