Multi-model hydrological seasonal forecasts for Europe in the C3S climate data store

Abstract. Seasonal hydrological forecasts play a crucial role in water resource management and climate change adaptation, supporting a wide range of planning and decision-making activities. Access to tools, products, data and services is essential in helping societies to better prepare for and adapt to conditions in a changing climate. The demand for forecasts at monthly or longer timescales is increasing, and has resulted in several meteorological prediction systems being available. Notably, hydrological seasonal forecasts have shown high skill for longer lead-times than their meteorological counterparts, which makes them potentially useful for planning purposes for water resources management and risk reduction. Under the umbrella of the Copernicus Climate Change Services (C3S), an operational service has been established to provide operational hydrological seasonal forecasts for Europe. The service provides historical re-forecasts and operational monthly initialized forecasts of daily and monthly mean river discharge, runoff, snow water equivalent and soil wetness index up to a six-month horizon. A total of six hydrological models (E-HYPEcatch00, E-HYPEcatch01, E-HYPEcatch06, E-HYPEcatch09, E-HYPEgrid, and VIC-WUR) are running in the service, with forcing from two meteorological seasonal forecast systems (ECMWF-s51 and CMCC-s35). The output resolution is at catchment delineations for the E-HYPEcatch models, and at the 5 km drainage network of the Lisflood hydrological model used in the European Flood Awareness System (EFAS) for the two gridded models (E-HYPEgrid and VIC-WUR). This paper describes the production chain and the included model components, with evaluations of performance of the separate models, and the skill of the seasonal forecasts calculated for a 23 year long re-forecast period. All data are freely available from the Climate Data Store (CDS), with updated 6-month forecasts issued each month (doi:10.24381/84dbf36e and 10.24381/ebac3954).