HERA: a high-resolution pan-European hydrological reanalysis (1951–2020)

Abstract. Since 1950, anthropogenic activities have altered the climate, land cover, soil properties, channel morphologies, and water management in the river basins of Europe. This has resulted in significant changes in hydrological conditions. The availability of consistent estimates of river flow at the global and continental levels is a necessity for assessing changes in the hydrological cycle. To overcome limitations posed by observations (incomplete records, inhomogeneous spatial coverage), we simulate river discharge for Europe for the period 1951–2020 using a state-of-the-art hydrological modelling approach. We use the new European set-up of the OS LISFLOOD model, running at 1 arcmin (≈1.8 km) with 6-hourly time steps. The hydrological model is forced by climate reanalysis data (ERA5-Land) that are bias-corrected and downscaled to the model resolution with gridded weather observations. The model also incorporates 72 surface field maps representing catchment morphology, vegetation, soil properties, land use, water demand, lakes, and reservoirs. Inputs related to human activities are evolving through time to emulate societal changes. The resulting Hydrological European ReAnalysis (HERA) provides 6-hourly river discharge for 282 521 river pixels with an upstream area >100 km2. We assess its skill using 2448 river gauging stations distributed across Europe. Overall, HERA delivers satisfying results (median KGE′=0.55), despite a general underestimation of observed mean discharges (mean bias=-13.1 %), and demonstrates a capacity to reproduce statistics of observed extreme flows. The performance of HERA increases through time and with catchment size, and it varies in space depending on reservoir influence and model calibration. The fine spatial and temporal resolution results in an enhanced performance compared to previous hydrological reanalysis based on OS LISFLOOD for small- to medium-scale catchments (100–10 000 km2). HERA is the first publicly available long-term, high-resolution hydrological reanalysis for Europe. Despite its limitations, HERA enables the analysis of hydrological dynamics related to extremes, human influences, and climate change at a continental scale while maintaining local relevance. It also creates the opportunity to study these dynamics in ungauged catchments across Europe. The HERA hydrological reanalysis and its climate and dynamic socio-economic inputs are available via the JRC data catalogue: https://doi.org/10.2905/a605a675-9444-4017-8b34-d66be5b18c95 (Tilloy et al., 2024).