Discharge observations

Nearly 4000 river gauging stations with daily and/or sub-daily discharge data were considered for the calibration of EFAS v5. 

The time interval was 01/01/1990-31/12/2021, thus allowing the tuning and verification of model parameters for a potential time span of 31 years. Moreover, this time period is consistent with the availability of both discharge data and the meteorological forcings (see "Meteorological forcings" below) at the time in which the calibration was started in 2022.

River gauging stations were accurately geo-located on the 1 arcmin (0.01667 deg) resolution drainage network. Model river drainage network is derived from a digital elevation model, which is inevitably affected by approximations. Consistency between real and model drained area for each gauge station is essential to allow reliable comparisons between observed and modelled discharge values. A detailed explanation of the relevance of this step is provided here. Observed discharge data time series were then manually quality checked to exclude stations with obvious data issues (e.g., outliers). 

The calibration stations were selected based on the following criteria (with some exceptions):

  1. Drainage area larger than 150 km2
  2. A minimum number of daily or 6-hourly discharge observations corresponding to 4 years within the period 01/01/1990-31/12/2021. The optimal calibration period started on 01/01/1993 to allow 3 years of spin-up of the hydrological model.
  3. Stations located close (less 200 km2 inter-catchment drainage area) to another station and having the same data quality, but shorter time series were excluded.

Some of the stations that did not fulfil the criteria were still selected for calibration if they allowed to improve the spatial coverage of calibrated catchments. Specifically, the following exceptions were allowed:

  • Stations with nearly 4 years of data.
  • Stations with only old observations: data from 01/01/1991 were used for some of the stations allowing for shorter spin-up periods: only 1 year of spin-up for 2 stations and only 2 years of spin-up for 7 stations. The adequate model spin-up was verified through numerical experiments prior to acceptance of these stations in the calibration data set.
  • Stations for which information on the real drained area was not provided. Inclusion of these stations required careful verification through numerical experiments.

This selection process led to the identification of 1903 calibration points. The drained area of these stations entailed 43.4% of the EFAS v5 pan-European domain.

The spatial distribution of the calibration points and their drained area is shown in Figure 1. The calibration of EFAS v5 could benefit of a much larger number of observations compared to EFAS v4, with an increase of 860 calibration points.  This increase is the result of the ongoing and constant effort to expand the EFAS database of discharge data (Share your data with EFAS - Copernicus Emergency Management Services - CEMS - ECMWF Confluence Wiki) and of the higher spatial resolution. The minimum drainage area of EFAS v4 was 500 km2: this threshold was reduced to 150 km2 with EFAS v5 thus allowing the inclusion of about 473 additional calibration points with a drainage area smaller than 500 km2.

Figure 1 – Calibration stations: the black points are the calibration stations used in EFAS v5, as well as in the previous calibration (EFAS v4); the red points are the calibration stations used for the first time in EFAS v5.  In light and dark blue the area drained by the calibration stations, in dark blue the area included for the first time in EFAS v5. In yellow the areas not covered by calibration points. The area in black is not included in EFAS v5 modelling domain.

The temporal extent and coverage of the observed time series varies across the pan-European domain. Figure 2 shows the length (total number of daily or sub-daily measurements in equivalent number of years) of the observation time series for each calibration point.

Figure 2 – Calibration stations: length of the observation time series in years. The points in purple were included to increase the spatial coverage of the calibration in data scarce areas.

Meteorological forcings

The meteorological forcings used for calibration were provided by CEMS Meteorological Data Collection Centre following the description in CEMS-Flood meteorological observation maps.

LISFLOOD OS then takes as input total precipitation, 2-metre temperature, and evapo-transpiration. Consistently with EFAS v4, the calibration of EFAS v5 was completed with 6-hourly time steps.