Performance-based weighting of CORDEX projections of hydrological climate change impacts over Europe

Climate change will influence the severity and intensity of floods and droughts throughout Europe. Ensemble projections of future changes in discharge show large variation. Several methods for performance-based weighting exist that have the potential to increase the robustness of the change signal. Here we use future projections of an ensemble of three hydrological models forced with climate datasets from the Coordinated Downscaling Experiment - European Domain (EURO-CORDEX). The experiment is set-up for nine river basins spread over Europe that hold different climate and catchment characteristics. We evaluate the ensemble consistency and apply two weighting approaches; (1) the Climate model Weighting by Independence and Performance (ClimWIP) that focuses on meteorological variables and (2) the Reliability Ensemble Averaging (REA) that is in our study applied to discharge statistics per basin. For droughts we focus on the change in discharge during low flow periods of 7-days.

For basins with a strong climate signal, in Southern and Northern Europe, the consistency in the set of projections is large. For rivers in Central Europe the differences between models become more pronounced. Both weighting methods favor projections from similar GCMs and assign high weights to a single or few best performing GCMs. The ClimWIP method results in ensemble mean weighted changes that differ only slightly from the non-weighted mean. The REA method influences the weighted mean more. The weights assigned to the CORDEX model simulations based on basin specific discharges highly vary from basin to basin.