Application of VARLEACH and LEACHM models to experimental data on leaching of a non-reactive tracer and three sulfonylurea herbicides

© CSIRO 2001A chromatographic model (VARLEACH) and a process-based model (LEACHM) were evaluated under low and high water input regimes (‘rainfall’ and ‘rainfall + irrigation’, respectively) using experimental data for a non-reactive tracer (KBr), and 3 sulfonylurea herbicides: triasulfuron, metsulfuron-methyl, and chlorsulfuron. Both qualitative and quantitative comparisons of predicted and measured water contents revealed that both models performed well under ‘rainfall’, but prediction was less satisfactory under the treatment of ‘rainfall + irrigation’. Both models failed to simulate the bimodal distribution of Br⁻ and the herbicides observed under ‘rainfall + irrigation’ treatment. Distribution of Br - and the 3 herbicides in the profile was over- or under-estimated at certain depths and times, probably due to the limitations inherent to the models (e.g. inability to deal with the macropore flow), and to simplifications associated with some input data. Simulations for a dry (1 decile), median (5 decile), and wet year (9 decile) with LEACHM and VARLEACH models using long-term weather data for the site showed that leaching and persistence of the herbicides are highly dependent on the amount of rainfall the site receives. The results indicate that the risk of injury to herbicide-sensitive crops in the following year of application of herbicides is likely to be greater when the herbicides are applied in a drier rather than a wetter year.Ajit K. Sarmah, Rai S. Kookana, and A. M. Alsto