1 Dataset description Currently, there is a joint effort between the Climate Limited-area Modelling Community (CLM-Community, www.clm-community.eu) and the German Federal Ministry of Research, Technology and Space (BMFTR) project “Updating the data basis for adaptation to climate change in Germany“(UDAG; Früh, 2023) to downscale an ensemble of selected climate change simulations of the Coupled Model Intercomparison Project Phase 6 (CMIP6; Eyring et al., 2016). Different to previous studies, this regional climate modelling ensemble is conducted with the same model version and setup of the Icosahedral Non-hydrostatic (ICON) model used in climate limited-area mode (ICON-CLM; Pham et al., 2021). ICON-CLM belongs to the common class of regional climate models that represent atmosphere and land processes without considering lateral water flows at the land surface, i.e. usually designated as river runoff or discharge. However, discharge is an important component of the global water cycle. Changes in discharge can have a significant impact on the water resources of the respective catchment area (Haddeland et al., 2013; Hagemann et al., 2013). In order to fill the gap that discharge is not provided by ICON-CLM, we used a state-of-the-art river runoff model, the Hydrological Discharge (HD) model (Hagemann et al., 2020), to generate discharges that are consistent with the ICON-CLM output. 1.1 ICON-CLM historical simulations In the present study, ICON-CLM was used to conduct regional climate simulations over the European domain of the Coordinated Regional Downscaling experiment (EURO-CORDEX; Jacob et al., 2013). The selected EURO-CORDEX domain has a spatial resolution of 0.11° (approx. 12 km) and the ICON-CLM model setup was determined from optimisation exercises through model extensions and a novel parameter tuning strategy (Geyer et al., 2026). Six historical simulations were conducted from 1950-2014 by downscaling six different GCMs (see Table 1 in https://www.wdc-climate.de/ui/entry?acronym=EU-hist_table, which provides characteristics of the six HD model experiments ). The GCMs comprise EC-Earth3-Veg, CNRM-ESM2-1, MPI-ESM-1-2-HR, MIROC6, CMCC-CM2-SR5, NorESM2-MM. 1.2 European historical river runoff based on ICON-CLM surface and subsurface runoff The HD model (Hagemann et al., 2020) is a river-routing model that is well-established and implemented in a range of global and regional model systems. The HD model was forced by 6-hourly time series of surface and subsurface runoff from the six ICON-CLM historical simulations. In the present study, we applied the HD model v5.2.4 (Hagemann et al., 2025) over its European domain (land areas between 11°W to 69°E and 27°N to 72°N) at 1/12° spatial resolution. The resulting daily series of river runoff (see Table 1) cover the full historical period 1950-2014. Acknowledgments This dataset was generated within the project “Updating the data basis for adaptation to climate change in Germany (UDAG)” that was funded by the German Federal Ministry of Research, Technology and Space under grant number 01LP2326D. I am grateful to Evgenii Churiulin (KIT), Michael Jähn (C2SM), Muhammed Muhshif Karadan (BTU), Sergei Petrov (Hereon), Christopher Purr (DWD) and Heimo Truhetz (Wegener Centre), who conducted the ICON-CLM historical simulations.

1 Dataset description Currently, there is a joint effort between the Climate Limited-area Modelling Community (CLM-Community, www.clm-community.eu) and the German Federal Ministry of Research, Technology and Space (BMFTR) project “Updating the data basis for adaptation to climate change in Germany“(UDAG; Früh, 2023) to downscale an ensemble of selected climate change simulations of the Coupled Model Intercomparison Project Phase 6 (CMIP6; Eyring et al., 2016). Different to previous studies, this regional climate modelling ensemble is conducted with the same model version and setup of the Icosahedral Non-hydrostatic (ICON) model used in climate limited-area mode (ICON-CLM; Pham et al., 2021). ICON-CLM belongs to the common class of regional climate models that represent atmosphere and land processes without considering lateral water flows at the land surface, i.e. usually designated as river runoff or discharge. However, discharge is an important component of the global water cycle. Changes in discharge can have a significant impact on the water resources of the respective catchment area (Haddeland et al., 2013; Hagemann et al., 2013). In order to fill the gap that discharge is not provided by ICON-CLM, we used a state-of-the-art river runoff model, the Hydrological Discharge (HD) model (Hagemann et al., 2020), to generate discharges that are consistent with the ICON-CLM output. 1.1 ICON-CLM hindcast In the present study, ICON-CLM was used to conduct regional climate simulations over the European domain of the Coordinated Regional Downscaling experiment (EURO-CORDEX; Jacob et al., 2013). The selected EURO-CORDEX domain has a spatial resolution of 0.11° (approx. 12 km) and the ICON-CLM model setup was determined from optimisation exercises through model extensions and a novel parameter tuning strategy (Geyer et al., 2026). To evaluate the model performance, ICON-CLM was used to generate a hindcast simulation (ICON CLM EVAL) from 1950-2024 by downscaling the ERA5 reanalysis data (Hersbach et al., 2020). 1.2 European hindcast of river runoff based on ICON-CLM surface and subsurface runoff The HD model (Hagemann et al., 2020) is a river-routing model that is well-established and implemented in a range of global and regional model systems. The HD model was forced by 6-hourly time series of surface and subsurface runoff from the ICON-CLM hindcast. In the present study, we applied the HD model v5.2.4 (Hagemann et al., 2025) over its European domain (land areas between 11°W to 69°E and 27°N to 72°N) at 1/12° spatial resolution. The resulting daily series of river runoff (HD5-EVAL) covers the full hindcast period 1950-2024. 1.3 European hindcast of river runoff based on ICON-CLM atmospheric data Analogous to Hagemann and Stacke (2022), the global hydrology model HydroPy (Stacke and Hagemann 2021) and the Hydrological Discharge (HD) model v5.2.4 (Hagemann et al., 2025) were used to simulate daily discharge time series over the European domain at 1/12° horizontal resolution. First, daily atmospheric data of the ICON-CLM hindcast (precipitation and 2 m temperature, downwelling shortwave and longwave radiation, 2m specific humidity, surface pressure, 10m wind) were interpolated to the HD European domain and used to force HydroPy. Here, a restart state of 1950 was taken from a century-long simulation (Hagemann et al., 2024) and used at the start of the simulation in 1950. Then, daily time series of surface and sub-surface runoff from HydroPy (Hpy-EVAL) were used to simulate daily discharges with the HD model. The resulting daily series of river runoff (HD5-Hpy-EVAL) covers the full hindcast period 1950-2024. Acknowledgments This dataset was generated within the project “Updating the data basis for adaptation to climate change in Germany (UDAG)” that was funded by the German Federal Ministry of Research, Technology and Space under grant number 01LP2326D.