Evaluation of high-resolution river discharge over Europe based on the ICON-CLM climate ensemble simulations

High-resolution river discharge provides relevant information for evaluating regional climate models and assessing the hydrological impact of climate change. In this study, we generate high-resolution European river discharges by forcing the hydrological discharge (HD) model with runoff fields from the Icosahedral non-hydrostatic climate limited-area mode (ICON-CLM) regional climate model. We then evaluate the quality of the simulated discharge obtained from a reanalysis-driven hindcast by comparing it with observations. Moreover, we assess how well the historical discharge trends are represented across six simulations in which ICON-CLM was driven by different global climate models (GCMs). A comparison of the results with observed river discharge across Europe shows that ICON-CLM-based simulations exhibit widespread dry biases, which are partly attributable to underestimation of precipitation in these regions, and partly to a too weak surface runoff component in the land-surface scheme of ICON-CLM. Using the HydroPy hydrology model as an alternative representation of land-surface hydrology improves the simulated discharge in many regions. ... mehrHowever, biases remain in areas with regulated rivers, as the HD model does not take regulations into account. Analysis of discharge trends from 1960–2014 reveals robust, large-scale patterns in the reference hindcasts, such as an increase in mean and low-flow discharge in Northern Europe and a decrease in Southern Europe. While some historical simulations reproduce these patterns, others show opposing or inconsistent signals, highlighting the strong influence of the driving GCMs. Our results underscore the suitability of ICON-CLM atmospheric forcing for hydrological impact studies and suggest the need for improved model tuning with regard to precipitation, including undercatch corrections in reference observations. They also emphasize the importance of multi-model ensembles to reliably capture climate-induced hydrological changes.