Modeller subjectivity in estimating pesticide parameters for leaching models using the same laboratory data set

User-dependent subjectivity in the process of testing pesticide leaching models is relevant because it may result in wrong interpretation of model tests. About 20 modellers used the same data set to test pesticide leaching models (one or two models per modeller). The data set included laboratory studies on transformation and sorption of ethoprophos and bentazone in soil from the top 25 cm, at two or three temperatures. All modellers received the raw data from these studies without guidance for deriving the model input parameters. The modellers were asked to provide the values of the half-lives and sorption coefficients which the model considered would use for this soil layer at 10°C (and at field capacity for the half-lives). The half-life of ethoprophos ranged from 92 to 346 days with an average of 191 days and a coefficient of variation of 29%. The half-life of bentazone ranged from 33 to 204 days with an average of 83 days and a coefficient of variation of 46%. The linear and Freundlich sorption coefficients of ethoprophos ranged from 1.7 to 4.3 dm3 kg−1 with an average of 3.4 dm3 kg−1 and a coefficient of variation of 21%. The linear and Freundlich sorption coefficients of bentazone ranged from 0.08 to 0.14 dm3 kg−1 with an average of 0.11 dm3 kg−1 and a coefficient of variation of 13%. This variability caused by the interpretation of the modeller is so large that it overrules conceptual differences between models in many cases. The most important cause of the variability in the half-lives was the expert judgement involved in establishing the relationship between transformation rate and soil temperature. Differences in fitting procedures played only a minor role for the half-lives but they were an important cause of the variability in the linear sorption coefficient. Some recommendations are proposed to reduce the effect of user-subjectivity on modelling results in future model tests.