Under persistent eutrophication of European water bodies and a changing climate, there is an increasing need to evaluate mitigation measures for reducing nutrient losses from agricultural catchments. In this study, we set up a daily discharge and water quality model in Hydrological Predictions of the Environment for two contrasting agricultural catchments in Sweden to forecast the impacts of future climate trajectories on nutrient loads. The model predicted a slight increase in inorganic nitrogen (IN) and total phosphorus (TP) loads under RCP2.6, likely due to precipitation-driven mobilisation. Under RCP4.5 and RCP8.5, the IN loads were forecasted to decrease from 16%-26% and 21%-50% respectively, most likely due to temperature-driven increases in denitrification and evapotranspiration. No distinct trends in TP loads were observed. A 50% decrease in nutrient loads, as targeted by the European Green Deal, was backcasted using a combination of mitigation scenarios, including i) a 20% reduction in mineral fertiliser, ii) introducing cover crops, and iii) stream mitigation by increasing the size of floodplains and wetlands. Target TP load reductions could only be achieved by stream mitigation, which is likely due to legacy effects and secondary mobilisation within agricultural streams. Target IN load reductions were backcasted with a combination of stream mitigation, fertiliser reduction, and cover crops, wherein the required measures depended on the climate. Overall, the diverging responses of nutrients to climate change and mitigation scenarios indicate that water quality management needs to be tailored to the catchment characteristics, and to the spatial and time specific effects of climate change.

Northern temperate forests are experiencing changes from climate and acidification recovery that influence catchment nitrate-nitrogen (N) flushing behavior. N flushing behavior is characterized by metrics such as: (a) N flushing time—the exponential decrease in stream N concentration during the peak snowmelt episode; and (b) N concentration (C) and discharge (Q) hysteresis metrics—flushing index (FI) and hysteresis index (HI)—representing the slope, direction and amplitude of the C-Q loop. We hypothesize that climate-driven hydrologic intensification results in longer N flushing times, lower FI (less flushing to more diluting), and lower HI (less proximal to more distal N sources). We tested this hypothesis using 38 years of data from two headwater catchments. Hydrologic intensification was estimated by changes in the ratio of potential evapotranspiration to precipitation and the ratio of actual evapotranspiration to precipitation. From 1982 to 2005, a period of hydrologic intensification and a decline in atmospheric acidic deposition was associated with a decrease in C and Q, leading to stable C-Q patterns that reflected flushing (positive FI) of proximal N sources (positive HI). However, from 2006 to 2019, a period of hydrologic de-intensification coupled with an ongoing decline in atmospheric acidic deposition was associated with a continued decrease in C but an increase in Q, leading to unstable C-Q patterns that reflected diluting (negative FI) of distal N sources (negative HI). C-Q instability was buffered in the catchment with a large wetland, indicating the potential of wetlands to buffer against changing climate conditions.