Future changes in runoff over western and central Europe: disentangling the hydrological behavior of CMIP6 models

Abstract Large uncertainties remain in the projected changes in runoff over western and central Europe. In this study, a large ensemble of climate projections from the Coupled Model Intercomparison Project Phase 6 is analyzed to characterize these changes in the late 21st century with a high-end emissions scenario, and to gain a better understanding of the mechanisms responsible for the inter-model spread. A hierarchical classification algorithm is used to group the models based on their hydrological response, and additional sensitivity experiments from two Model Intercomparison Projects are studied. In most models, annual precipitation changes do not emerge from internal variability. Evapotranspiration changes generally do emerge from internal variability, but with no inter-model agreement on the sign of the signal. Finally, no robust multi-model response in annual runoff changes is projected. A quarter of the models still project a significant decrease in annual runoff. Similar annual changes in runoff can be driven by very different changes in annual precipitation and evapotranspiration, as well as contrasted seasonal evolutions. Some models project a decrease in runoff despite an increase in precipitation due to a large increase in evapotranspiration. Due to the large uncertainties in the reference datasets, it is difficult to reject any models based on their accuracy to represent climatological averages and recent trends. Large-scale circulation and the representation of the physiological impact of CO2 are important for the extreme hydrological changes projected by some models. The soil-moisture precipitation feedback is important for the multi-model ensemble mean but not for the inter-model spread.