Testing PESTLA using two modellers for bentazone and ethoprophos in a sandy soil

Two modellers tested the PESTLA model (version 2.3.1) against results of a field study on bentazone and ethoprophos behaviour in a sandy soil. Both mo dellers achieved an acceptable description of the measured moisture profile s but only after calibration of the soil hydraulic properties. Both could d escribe the bromide-ion concentration profiles measured at the end of the f irst winter reasonably well. However, both predicted that practically all b romide had leached out of the top 50 cm of the soil at the end of the secon d winter, whereas about 10% of the bromide dose remained in this layer. Thi s is attributable to a systematic deviation of bromide transport from the c oncept assumed in the convection/dispersion equation and/or to the release of bromide from dead root remnants. Both modellers derived pesticide transf ormation and sorption parameters from laboratory studies with soil from the field. Both described bentazone movement reasonably well. Modeller 1 descr ibed the concentration profiles reasonably well, whereas Modeller 2 strongl y overestimated the concentrations at the end of the study. This difference was mainly attributable to a difference in interpretation of the temperatu re dependence of the transformation rate of bentazone. Only Modeller 2 simu lated ethoprophos behaviour. He simulated the persistence of ethoprophos in the top 20 cm very well during the first 200 days. However, thereafter the transformation in the field proceeded much faster than simulated. This is probably caused by accelerated transformation resulting from exposure of th e top soil layer to about 1 mg kg(-1) of ethoprophos over 200 days. Simulat ed penetration of ethoprophos was deeper than measured. By including accele rated transformation (admittedly on an ad-hoc basis) within the simulations , good agreement was achieved between measured and simulated penetration of ethoprophos. Calculations showed that the effect of calibrating water flow was substantial for bentazone but small for ethoprophos. However, the effe ct of calibration of water flow for bentazone was much smaller than the eff ect of the difference between the transformation rate parameters derived by the two modellers. We recommend that the guidance for deriving pesticide-s oil input parameters be improved in order to reduce differences between mod ellers because a large influence of the person of the modeller on the outco me of model tests is unacceptable for methodological reasons. (C) 2000 Else vier Science B.V. All rights reserved.