Site-Specific Impacts of Climate Change On Crop Production, Water Use and Pollution Across Regions of Germany Using Different CO2 Response Functions.

Impact of climate change on crop growth, water use and nitrogen leaching in winter wheat production in Germany was assessed using the agro-ecosystem model HERMES with a downscaled climate change scenario A1B from the ECHAM5 global circulation model. Three alternative algorithms describing the impact of atmospheric CO₂ concentration on crop growth in combination with a Penman-Monteith approach which includes a stomata conduction model for transpiration under changing CO₂ concentrations were compared within the framework of the model. The effect of differences in regional climate change, site conditions and different CO₂ algorithms on yield, groundwater recharge and nitrogen leaching was assessed in 22 regional simulation case studies across Germany. Simulated effects of climate change on wheat production varied across Germany due to different regional expressions of climate change projection. Predicted yield changes between the reference period (1961−1990) and a future period (2021−2050) range from −0.3, −1.1 and −0.9 t ha⁻¹ at sites in southern Germany to +0.8, +0.6 and +0.8 t ha⁻¹ at coastal regions for the three CO₂ algorithms, respectively. On average across all regions, a relative yield change of +0.3%, +2.5%, and +6.3%, respectively, was predicted in contrast to a decrease of –11% without any consideration of the CO₂ effect. However, simulated yield changes differed even within regions as site conditions had a strong influence on crop growth. Particularly, groundwater-affected sites showed a lower vulnerability to increasing drought risk. Groundwater recharge was estimated to change correspondingly to changes in precipitation. Simulation of the CO₂ effect on transpiration led to a prediction of higher rates of annual deep percolation (+16 mm on average across all sites) due to higher water-use efficiency of the crops. In contrast to groundwater recharge, simulated nitrogen leaching varied with the choice of the photosynthesis algorithm, predicting a slight reduction in most of the areas.