Are maps of nitrate reduction in groundwater altered by climate and land use changes?

Abstract. Nitrate reduction maps have been used routinely in Northern Europe for calculating efficiency of remediation measures and impact on climate change on nitrate leaching and are as such valuable tools for policy analysis and mitigation targeting. Nitrate maps are normally based on output from complex hydrological models, and once generated, are largely assumed constant in time. However, the distribution, magnitude and efficiency of nitrate reduction can not necessarily be considered stationary during changing climate and land use as flow paths, nitrate release timing and their interaction may shift. This study investigates the potential errors when a constant nitrate map is assumed during land use and climate change, both for N-loads and the spatial variation in reduction. For this purpose, a crop and soil model (Daisy) was setup up to provide nitrate input to a distributed hydrological model (MIKE SHE) for an agricultural catchment in Funen, Denmark. Nitrate reduction maps based on an observed dataset of land use and climate were generated and compared to nitrate reduction maps generated for all combinations of four potential land use change scenarios and four future climate model projections. Nitrate reduction maps were found to be more sensitive to changes in climate, leading to reduction map change of up to 10 %; while land use changes effects were minor. The study, however, also showed that the reductions maps are products of a range of complex interactions and that the combination of the choices made for selected scenarios, model formulations and assumptions are critical for the resulting span in reduction capability.