Discharge observations

Nearly 2500 river gauging stations with daily discharge data were considered for the calibration of GloFAS v4. 

The time interval was 01/01/1980-31/12/2019, thus allowing the tuning and verification of model parameters for a potential time span of 40 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 (December 2021).

River gauging stations were accurately geolocated on the 0.05 degrees resolution drainage network. Model river drainage network is derived from a digital elevation model, which is unavoidably 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 500 km2.
  2. A minimum number of daily discharge observations corresponding to 4 years within the period 01/01/1982-31/12/2019. The optimal calibration period started on 01/01/1982 to allow 3 years of spin-up of the hydrological model.
  3. Stations located close (less 500 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 01/01/1980 were used for 2 calibration points; data from 01/01/1981 were used for 49 calibration points. 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 1995 calibration points. More specifically, 211 calibration points were in Europe, 250 in Asia, 61 in Oceania, 420 in Africa, 617 in Centre-North America, and 436 in South America. The drained area of these stations entailed 47.5 % of the quasi-globa (-180,180,90,-60) domain.

The spatial distribution of the calibration points and of their drained area is shown in Figure 1. The calibration of GloFAS v4 could benefit of a much larger number of observations compared to GloFAS v3, with an increase of 770 caibration points (see Alfieri et al. 20201 for a detailed description of the calibration of GloFAS v3).  This increase is the result of the ongoing and constant effort to expand the global database of discharge data (Share your data with GloFAS - Copernicus Emergency Management Service - CEMS - ECMWF Confluence Wiki) and of the higher spatial resolution. The minimum drainage area of GloFAS v3 was 5000 km2: this threshold was reduced to 500 km2 with GloFAS v4 thus allowing the inclusion of at about 200 additional calibration points.


Figure 1 – Calibration stations: the black points are the calibration stations used in GloFAS v4, as well as in the previous calibration (GloFAS v3, Alfieri et al, 20201); the red points are the calibration stations used for the first time in GloFAS v4.  In light and dark blue the area drained by the calibration stations, in dark blue the area included for the first time in GloFAS v4. In yellow the areas not covered by calibration points.


The temporal extent of the observed time series varies across the global domain. Figure 2 shows the length (total number of 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 used in GloFAS v4. The points in purple were included to increase the spatial coverage of the calibration.


Meteorological forcings

The meteorological forcings used for calibration were provided by C3S ERA5 following the description in CEMS-Flood meteorological observation maps.

LISFLOOD OS then takes as input total precipitation, 2-metre temperature, and evapotranspiration. Consistently with all the previous GloFAS versions, the calibration of GloFAS v4 was completed with daily time steps.


References

1Alfieri L., Lorini V., Hirpa F.A., Harrigan S., Zsoter E., Prudhomme C., Salamon P. A global streamflow reanalysis for 1980–2018. (2020).  Journal of Hydrology X, 6, art. no. 100049. https://doi.org/10.1016/j.hydroa.2019.100049