Comparison of the performance of pesticide-leaching models on a cracking clay soil: results using the Brimstone Farm dataset

The leaching of the pesticide isoproturon from the macroporous clay soil at Brimstone Farm was modelled using four alternative models (MACRO, CRACK-NP, SIMULAT and PLM). Model results are presented for two test periods, the whole of one winter for which daily observations are available, and a short subset for which hourly data were presented. The best results are those given by MACRO with an expert user, although satisfactory results were also obtained from CRACK-NP and for the longer test period by PLM. SIMULAT was less successful in modelling the site because it did not include an adequate representation of the site hydrology, it was unable to predict the leaching of pesticide. MACRO was also used by a second modelling group who were less familiar with both the code and the site. Although the initial uncalibrated runs from this group were poor, the final calibrated results were almost as good as those derived by the ‘expert’ user. The simulations showed the difficulty of deriving adequate representations, even where relatively complete soil physical data are available. A shortcoming of the dataset provided was the lack of detailed soil moisture observations, particularly to define the initial conditions. From a well-monitored site, many observations of site hydrology (water table position, drainflow and surface flow) were available, but significantly, fewer pesticide concentrations in either the soil or the discharges were available. Models could thus be evaluated only in terms of their ability to predict the magnitude and timing of major pesticide leaching events.