Effects of mid-century climate change and of reduction measures on nutrient emissions and loads in the Elbe River Basin

Changes in temperature, precipitation or runoff cause changes in nutrient emissions and loads as well as changes in the instream transformation processes.Thus, climate change assumably has a direct effect on water quality.This impact was simulated for the Elbe river basin using a model chain (consisting of models that simulate climate, water balance and nutrient emissions). Validation tests for the period 2001-2005 showed that simulations using the model chain based on measured climate variables, are in good agreement with loads measured at two gauges of the Elbe river and at gauges of three major tributaries. We have compared the impact of two climate scenarios on the nutrient emissions. The first scenario comprises time series of temperature, radiation, precipitation and humidity, created by the climate generator STARS. In the second case, the time series were simulated using a model ensemble of five earth system models from the "Inter-Sectoral Impact Model Intercomparison Project" (ISI-MIP) and afterwards combined into one scenario. In both cases (STARS, ISI-MIP), the simulated climate variables constitute the boundary conditions of the ecohydrological model SWIM. SWIM simulates the river basin discharge used for modelling the nutrient emissions by means of the model MONERIS. In general, the emissions and loads decrease underproportionally with the river basin discharge in both scenarios, with loads declining more sharply than the emissions. The concrete effects of the change in river basin discharge are small, reaching only single-digit percentages. The same holds true for the difference between the scenarios. The effect of selected measures for the reduction of nutrient emissions and loads also differs only slightly between varying runoff conditions, represented by different climate scenarios with dry, mean and wet specification.