The CEMS Hydrological Data Collection Centre (HDCC) is a fully operational and automated hydrological data collection service of the Copernicus Emergency Management Service (CEMS). HDCC collects real-time and historical hydrological observations from data providers across Europe and supports in particular the European Flood Awareness System (EFAS) with the provision of quality-controlled and standardised hydrological observations. These hydrological observations are used mainly for the calibration of the hydrological model as well as for the real-time monitoring of the hydrological conditions across Europe.

During 2021, the CEMS Hydrological Data Collection Centre welcomed three new hydrological data providers: the “Republic HydroMeteorological Service of the Republic of Srpska”, the "Water management Agency of Luxembourg", and “Agenzia Regionale di Protezione Civile (Protezione Civile Lazio)”.

By the end of 2021, 49 data providers contributed with near real-time hydrological data at 2,386 stations across Europe to the CEMS Hydrological Data Collection, covering 34 countries and 51% of all the European water basins.

This report presents the analysis of the hydrological data which were collected from 1,963 stations across Europe. These 1,963 were selected because they guaranteed a stable data provision to HDCC since 01st February of 2021 and throughout the year. Out of those 1,963 stations, 490 deliver exclusively discharge data, 390 only water level data and 1,083 stations provide discharge and water level data.

More specifically, this report includes the following topics:

• data collection and data providers,
• hydrological conditions of EFAS gauging stations,
• protocol to quality check the hydrological data,
• protocol to analyse the gaps in the collected hydrological time series,
• analysis of exceedance events.

The EFAS team, and particularly HDCC, would like to thank the EFAS Partners and Data Providers that contributed to the CEMS hydrological data collection. We would like to acknowledge their dedication to the EFAS project, their commitment and the sharing of their hydrological data. We thank them for their cooperation with the HDCC, both in the provision of data and for their proactive role in responding to the questions and solving issues. Without their collaboration, the delivery of this report would not be possible.