Modeling water flow and pesticide transport at lysimeter and field scale

Lysimeters are used in European countries as a final tier for assessing the groundwater contamination potential of pesticides with leaching tendencies . The effect of the artificial lysimeter bottom boundary condition on soil water budget components was investigated with the help of the WAVE model in comparison with data from a lysimeter study after pre-emergence applicatio n of [C-14]methabenzthiazuron (MBT) conducted over 252 days. Simulated drai nage fluxes in the degraded loess soil were larger using a free drainage bo undary condition. Furthermore the effect of measured spatial variability of the soil water retention characteristic on simulated water fluxes was inve stigated with a monte-carlo modelling approach. The predicted variation of leaching volume was considerably larger than observed in five lysimeter rep licates. The effect of spatial variability of the sorption distribution coe fficient on soil residue profiles was negligible for the herbicide methaben zthiazuron (MBT). In contrast variable soil hydraulic properties did affect the centre of mass of the pesticide residues. Considering the statistical uncertainty of the degradation rate parameter estimation from laboratory da ta resulted in huge variability of predicted total MBT residues.