HERA: a high-resolution pan-European hydrological reanalysis (1950–2020)

Abstract. Since 1950, European rivers have been put under increasing pressure by anthropogenic activities, resulting in changes in climate, land cover, soil properties and channel morphologies. These evolving environmental conditions can translate into changes in hydrological conditions. The availability of consistent estimates of river flow at global and continental level is a necessity to assess and attribute 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 1950–2020 using a state-of-the-art hydrological modelling approach. We use the new European set up of the LISFLOOD model, running at 1 arcminute (≈1.8 km) with six-hourly time steps. The hydrological model is forced by climate reanalysis data (ERA5-land) bias-corrected and downscaled to the model resolution with weather observations. The model also ingests 72 surface fields 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 changes in society. The resulting Hydrological European ReAnalysis (HERA), provides six-hourly river discharge for 282 521 river pixels with upstream area > 100 km2. We assess its skill using 2901 river gauging stations distributed across Europe. Overall, HERA delivers satisfying results, with a general weak underestimation of observed mean discharge and flow variability. We find that the performance of HERA increases through time between 1950 and 2020. The fine spatial and temporal resolution result in an enhanced performance compared to other reanalysis for small-to-medium-scale catchments (100–10 000 km2), with degraded performance remaining for small catchments. HERA is the first long-term, high-resolution hydrological reanalysis for Europe. Despite its limitations, it enables the analysis of hydrological dynamics related to extremes, human influences, and climate change at a continental scale while keeping local relevance. It also creates the opportunity to study these dynamics in ungauged catchments across Europe.