There is a widespread consensus among scientists that the climate will change in the future, and that this change has already begun. These climatic changes will undoubtedly challenge the use of pesticides, which has been proposed to increase in the future. Accordingly, the primary aim of this PhD-project was to contribute to the knowledge of how climate change will effect pesticide leaching in the future, which was done by use of mathematical modelling. The agro-ecological model Daisy, was used in all simulations, as well as the 2 model soils: a coarse sand and a subsurface drained sandy loam containing preferential flow pathways. The sensitivity of pesticide leaching towards single high intensity events was tested by use of the artificial Chicago Design Storm (CDS), which were inserted in the driving weather file. Glyphosate showed a strong dependence, as short intense events resulted in relatively high leaching amounts under specific pre and post event weather conditions. This clearly illustrated the importance of including weather variability in pesticide fate modelling.

An ensemble of 11 climate model projections were downscaled by perturbing a weather generator calibrated on local meteorological data, resulting in 3000-year long weather series of statistically stationary climate. Effects of pesticide properties (sorption and degradation), pesticide application dates, and soil properties were included. The synthetic weather series produced in relation to objective (II) were used to simulate future changes in pesticide fate. The simulation results indicate a decreased risk in future leaching, as predicted by the ensemble mean. This accounts for all investigated pesticide properties, application dates, and soil types, and is generally caused by increased pesticide degradation. However, a degree of model uncertainty was found, as some models projected increased leaching of especially autumn applied pesticides of 13-19 % (worst case scenario). Variation in leaching result, as caused by changed weather patterns, only increased for autumn applied pesticides. It can therefore, not be concluded that potential future increases in extreme rainfall, will affect the future leaching of pesticides.