Modelling the efficacy of catchment remediation measures for reducing sediment & nutrient exports under future climate trajectories

The European Green Deal has the ambition to reduce nutrient losses from agricultural catchments with 50%. In the context of a changing climate, there is an increasing need to evaluate the efficacy of catchment remediation measures and reduction in fertilisation. In this study, we set-up a daily discharge and nutrient load model for two Swedish agricultural headwater catchments using Hydrological Predictions for the Environment (HYPE). The daily model was calibrated and validated using high-frequency sensor data and flow-proportional samples analysed for nutrient and sediment concentrations. Multiple catchment remediation scenarios were run under three downscaled climate models and three Representative Concentration Pathways (RCP 2.6, RCP 4.5, and RCP 8.5). The model predicted that Inorganic Nitrogen loads will decrease in the latter half of the 21^st century under RCP 4.5 and RCP 8.5 driven by increased denitrification under higher temperatures. Moreover, under all RCPs, an increase in Particulate Phosphorous and sediment loads is forecasted due to increased rainfall intensity. Decreasing the amount of mineral fertilisation only resulted in decreased Inorganic Nitrogen loads, but had no effect on Total Phosphorous loads. Catchment remediation measures were most effective for reducing Total Phosphorous loads. However, large portions of agricultural catchments will need to be converted to floodplains or wetlands in order to achieve significant load reductions and offset the predicted increases under future climatic trajectories.