A significant upgrade of the European Flood Awareness System (EFAS) was launched in 2020, which provides more precise forecasts of the timing and magnitude of flooding events. Enhancements were also made to the user platform, and large amounts of past EFAS forecast data were made available.

The new version of EFAS benefits from a complete recalibration of the hydrological modelling system at six-hourly time steps and it can make better use of sub-daily weather data. The twice-daily forecasts produced with ECMWF ensembles now provide six-hourly, rather than daily averaged, information.

River levels can rise rapidly, over a matter of hours for smaller catchments, and so the new system provides much more precise guidance on when peak river discharge might occur. Six-hourly information also means that peak river flows can be represented more realistically in small to medium catchments where timing is crucial in the decision-making process.

The aim of EFAS is to support preparatory measures ahead of major flood events in the pan-European domain, particularly in Europe’s large trans-national river basins. Managed by the EU Joint Research Centre, EFAS is part of the Copernicus Emergency Management Service (CEMS) and involves many partners across Europe. ECMWF operates the EFAS computational centre, which produces forecasts and hosts the EFAS Information System platform. ECMWF is also responsible for developing and integrating improvements to EFAS.

EFAS uses a hydrological model (LISFLOOD), with weather and hydrological observations and medium-range weather forecasts from a range of centres, to produce an ensemble of flood forecast information.

The new release, EFAS v4.0, follows a complete recalibration of LISFLOOD at six-hourly time steps using data from over 1,100 river stations across the EFAS domain. A dataset containing daily and six-hourly discharge data at river gauges for the period 1990 to 2017 was put together at ECMWF, based on data provided by the EFAS Hydrological Data Collection Centre. The EFAS Meteorological Data Collection Centre produced new datasets with six-hourly gridded meteorological maps of precipitation and other variables.

Overall, EFAS 4.0 shows a marked improvement in the hydrological performance, except in strongly regulated catchments, where the new calibration did not bring much change. Improvements were seen in both the magnitude and timing of simulated flood peaks.

Improvement in forecastsExample of the improvement to simulated hydrographs between (a) EFAS 3 and (b) EFAS 4 for the river Inn at Mühldorf (Germany). The observed discharge is represented by the black line; LISFLOOD outputs are represented by the green and red dots. Flood thresholds bands are shown on the background, expressed as return period levels: 1.5- to 2-year (green), 2- to 5-year (yellow), 5- to 20-year (grey), >20-year (pink). The range of the forecast ensemble is shown in blue.
Image: Copernicus Emergency Management Service

The EFAS Information System platform was also upgraded. A new evaluation tab helps users assess hydrological model performance and forecast skill, and there are new descriptions of the way LISFLOOD represents rivers and locates reservoirs and lakes, for example.

As part of ECMWF’s wider move towards open access and transparency, a huge volume of ‘re-forecast’ or ‘hindcast’ EFAS data were also made available via the Copernicus Climate Change Service’s Climate Data Store.

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This represents one of the biggest changes to EFAS, if not the biggest, since it began running operationally in 2012.

Christel Prudhomme, Manager of the EFAS Computational Centre at ECMWF