Comparing and evaluating pesticide leaching models Results of simulations with PELMO Michael Klein a,* , Ju Èrg Hosang b , Helmut Scha Èfer c , Beate Erzgra Èber d , Herbert Resseler e a Fraunhofer-Institut fu Èr Umweltchemie und O È kotoxikologie, PO Box 1260, D-57377 Schmallenberg, Germany b Novartis, R-1094.2.80, CH-4002 Basel, Switzerland c Bayer AG, PF-E/MR Geb. 6600, D-51368 Leverkusen-Bayerwerk, Germany d Hoechst Schering AgrEvo GmbH, D-65926 Frankfurt, Germany e Novartis Agro GmbH, Liebigstr. 51-53; D-60323 Frankfurt, Germany Abstract The PELMO model was used independently by ®ve modellers to reproduce the results of a lysimeter study performed at Tor Mancina in Italy and a ®eld study performed at Vredepeel in the Netherlands. For the comparisons of the Tor Mancina data set the main features of the measured ¯uxes of water and bromide were well reproduced by the simulations. The deviations between simulated and experimental cumulative amounts of water leached were generally less than 50%. The measured leaching of metolachlor was small (typical concentrations considerably below 0.1 mg/l). These trace amounts were not reproduced by any of the simulations, not even by those calibrated for bromide leaching in the re-packed lysimeters. For Vredepeel, the agreement between the measured and simulated water tables were generally poor, even on a qualitative level. This was mainly due to PELMOs inability to deal with shallow, ¯uctuating groundwater tables. Concentrations of both the tracer and the pesticides were generally satisfactorily reproduced in the initial phases of the experiment but not at later stages. In most cases, the penetration depth of the centre of mass was over-estimated by the model and the dispersion of the pesticide under-estimated. The correct determination of the parameters to simulate the degradation (and adsorption) of pesticide in the ®eld seemed to be of much greater importance for accurately modelling the transport of such chemicals in soils than improvements in the water balance. The degradation data from long-term laboratory studies clearly did not re¯ect ®eld conditions. Additional sampling dates to determine more concentration pro®les and to measure DT50 values from the ®eld would have helped reducing the differences in picking different input data by the Agricultural Water Management 44 (2000) 263±281 * Corresponding author. Tel.: 49-2972-302-317; fax: 49-2972-302-319. E-mail address: klein@iuct.fhg.de (M. Klein). 0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0378-3774(99)00095-5