Modelling climate change impacts on pesticide leaching

Climate change projections for Sweden indicate increases in both temperature and precipitation. In a warmer and wetter climate, weed and pest pressures are likely to increase, which might in turn trigger an increased use of pesticides. This thesis analysed potential impacts of climate change on pesticide losses from Swedish arable soils under present (1970-1999) and future (2070-2099) climate conditions. The pesticide fate model MACRO was used to evaluate the direct effects of climate change on pesticide losses to tile-drains at the field scale accounting for uncertainties related to model structure (i.e. the description of temperature dependent processes), parameters and climate input data. At the regional scale, MACRO-SE was used to assess the direct and the indirect effects of climate change (i.e. changes in cropping patterns and herbicide use) on the leaching of herbicides towards groundwater in southern Sweden. At the field scale, the results showed that differences in model structures affected predictions of pesticide losses under climate change, despite large parameter uncertainty. The effect of climate input uncertainty was more important than the effect of parameter uncertainty for predicted changes in pesticide losses between present and future climates, while it was the opposite for simulated absolute pesticide losses. The direction and magnitude of predicted changes in pesticide losses depended on pesticide properties, application season and climate scenario. In the regional scale study, the area at risk of groundwater contamination was only slightly affected by direct effects of climate change, whereas the area at risk doubled due to the indirect effects of climate change that were included in the analysis. The main conclusions are that (1) the relative importance of different sources of uncertainty depends on the pesticide properties, application season and whether the focus is on absolute losses or predicted changes, (2) ensembles of climate scenarios are necessary for robust assessments and (3) indirect effects need to be considered alongside the direct effects as predictions can be significantly affected. Despite large uncertainties, this thesis highlights the need to strengthen policies, to adopt improved mitigation measures and to implement management strategies that will limit pesticide use and minimize the risks of contamination of ground- and surface waters