Discharge observations

GloFASv5 calibration made use of daily discharge data from in-situ gauge stations. Data collection and preparation was largely performed in 2024. The interval of observed data was 01/01/1980-31/12/2023, thus allowing the tuning and verification of model parameters for a potential time span of 43 years.

In 2024, the CEMS Hydrological Data Collection Centre database counted over 15,500 stations with discharge data: detailed information about hydrological data collection, validation, and post-processing is provided in the CEMS Hydrological Data Collection Centre – Annual Report 2024 (Garcia Padilla, M., et al, 2026¹).

River gauge 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). 

GloFASv5 calibration stations were selected based on the following criteria:

1. Drainage area larger than 500 km².
2. Discrepancy between real drainage area (as indicated by the data provider) and drainage area computed according to the GloFAS local drainage network lower than 10%.
3. A minimum number of daily discharge observations corresponding to 4 years within the period 01/01/1980-31/12/2023.
4. Good data quality (where possible, outliers and flatliners were removed).
5. 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.
6. Where stations were available both upstream and downstream of a confluence, stations located on tributaries were preferred to enable parameter calibration close to the runoff generation area.
7. Stations located immediately downstream of reservoirs and highly affected by the regulation were generally excluded: reservoir parameters were calibrated independently, according to the methodology explained here. Where possible, use of stations upstream of reservoirs providing information on reservoir inflow and/or stations further downstream showing a more natural behavior was preferred.
8. Stations affected by physical processes not included in the model set-up (e.g. river diversions) were excluded to avoid spurious values of calibrated parameters.
9. Stations covering areas and years with well-known issues in the meteorological forcings were excluded to avoid spurious values of calibrated parameters. A notable example is provided by stations in the Congo and Chad basins: stations with available discharge data older than 01/01/2000 were excluded to account for the local inaccuracy of the precipitation dataset used in calibration (see Meteorological forcings)

Some of the stations that did not fulfill criteria 1, 2, or 3 were still used if they allowed to improve the spatial coverage of calibrated catchments in data scarce areas.  The use of these stations was validated via careful assessment of the calibration results. Specifically, the following exceptions were allowed:

• Inclusion of stations for which information on the real drainage area was not provided.
• 31 stations had less than 500 km2 drainage area, of which only 1 smaller than 400km², 10 smaller than 450km². 
• 42 stations had less than 365x4 daily data, of which 36 had more than 365x3 daily data. 

This selection process led to the identification of 5379 calibration points, with an increase of over 160% compared to GloFASv4 (1995 stations):

• 646 calibration points were in Africa (+226 stations compared to GloFASv4),,
• 1314 in Asia (+1064);
• 1314 in Central-North America (+697), 
• 1298 in South America (+862), 
• 743 in Europe (+532),
• 64 in Oceania (+3)

The drained area of these stations entailed 51.6% of the quasi-global (-180,180,90,-60) domain. GloFASv4 calibration entailed 47.5% of the global domain: the large increase of stations in GloFASv5 allowed model calibration for an increased number of head-catchments and inter-catchments, thus enabling parameter calibration closer to the areas where runoff generation occurs.

Figure 1 shows the spatial distribution of the calibration points and of the total extent of GloFASv5 area with available gauged data.

Figure 1 – Calibration stations: the yellow points are the calibration stations used in GloFAS v5, the black points are the calibration stations used for GloFAsv4.  In orange and blue the area drained by the calibration stations, in orange the area included for the first time in GloFAS v5. The insets highlight the extension of calibrated area in GloFASv5.  In grey the areas not covered by calibration points.

Figure 2 shows the distribution of inter-catchment area. The size of the inter-catchments was mainly driven by data availability. The largest inter-catchment was located in the Congo basin, with a drained area of just over 3.000.000 km².  The median value was 3.300 km², thus representing a sensible decrease compared to GloFASv4 (where the median was 14.000 km²).

Image Added

The temporal extent of the observed time series varies across the global domain. Figure 3 shows the length (total number of daily measurements in equivalent number of years) of the observation time series for each calibration point; Figure 4 shows the distribution of values: 

Figure 3 – GloFAS v5 calibration stations: length of the observation time series in years, spatial distribution. The points in pink were included to increase the spatial coverage of the calibration.

Figure 4 – GloFASv 5 calibration stations: length of the observation time series in years.

Reservoir observations

References

¹ Garcia Padilla, M., Garcia Sanchez, R., Jiménez Molina, A., Márquez Arroyo, M., Serratosa Márquez, A. et al., CEMS Hydrological Data Collection Centre – Annual Report 2024, Publications Office of the European Union, Luxembourg, 2026, https://data.europa.eu/doi/10.2760/7145824 , JRC145381.