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 w hole of one winter for which daily observations are available, and a short subset for which hourly data were presented. The best results are those giv en by MACRO with an expert user, although satisfactory results were also ob tained from CRACK-NP and for the longer test period by PLM. SIMULAT was les s successful in modelling the site because it did not include an adequate r epresentation 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 uncalibrate d 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 d ifficulty of deriving adequate representations, even where relatively compl ete soil physical data are available. A shortcoming of the dataset provided was the lack of detailed soil moisture observations, particularly to defin e the initial conditions. From a well-monitored site, many observations of site hydrology (water table position, drainflow and surface flow) were avai lable, but significantly, fewer pesticide concentrations in tither the soil or the discharges were available. Models could thus be evaluated only in t erms of their ability to predict the magnitude and timing of major pesticid e leaching events. (C) 2000 Elsevier Science B.V. All rights reserved.