Abstract This paper describes the Brimstone Farm dataset used for the comparison of pesticide leaching model performance. The tasks set to the modellers are then described. The soil of the site is heavy clay, in which cracks have a major effect on the hydrology. Data are presented for the leaching of isoproturon and mecoprop from two contrasting plots. Contrasting soil moisture states, resulting from different cropping, result in higher drainage rates from the wetter plot, which were associated with higher and earlier losses of isoproturon to the drains. No mecoprop was detected in the drain¯ows from this site.
