GloFAS v5 verification of stations with reservoirs built during the observation record

This page provides additional information about GloFAS v5 pre-operational - GloFAS hydrological performance verification methodology when it comes to a subset of station with reservoirs built during the observation period.

IN particular, at 228 stations, the reservoirs upstream of the station were built after the period covered by the observations used for the calibration of the model. These stations are spread all over the globe: 61 in Asia, 47 in Africa, 39 in North America, 38 in South America, 24 in Europe, 11 in Central America and 8 in Oceania.

At these stations, the calibration and the reanalysis are set up differently:

• Calibration – reservoirs off: The observations available for the calibration describe the natural flow regime before the reservoirs existed. The model parameters are calibrated against these observations, so the reservoirs were switched off in the calibration to match them.
• Reanalysis – reservoirs on: The reanalysis is used to derive the flood thresholds (return periods) for the GloFAS forecasts. It must represent present-day, regulated conditions, so all reservoirs are always switched on.

As a result, the reanalysis at these stations is partly compared with observations of a flow regime that no longer exists, and its scores can be lower than those of the calibration run. This is expected. It does not mean that the reanalysis, or the flood thresholds derived from it, are less reliable for present-day conditions.

To show the effect, both simulations are compared at each of the 228 stations against the same observations over the same period: the observations and the period used in the calibration. The difference in skill is measured as ΔKGE = KGE′(reanalysis) − KGE′(calibration):

At most of these stations (178 of 228), both simulations perform about the same, which means the reservoirs have little effect at the station. At 45 stations, the reanalysis scores lower, at 31 of them by more than 0.10. The median KGE′ is 0.70 for the calibration run and 0.64 for the reanalysis. Where the reservoirs have a large effect, the reanalysis typically shows damped flood peaks and higher low flows than the observations from before the reservoirs were built. This lowers the variability ratio γ and, as a result, KGE′.

An interactive viewer shows each of these stations with its scores for both simulations and its hydrograph over the calibration period: https://dev.globalfloods.eu/reservoir-stations-viewer/.

The scores of these stations in the evaluation layer cover the full verification period, so they mix observations from before and after the reservoirs were built. They should be interpreted with the same care.