GloFAS v5 reanalysis hydrological model performance for selected catchments

This page complements  pre-operational - GloFAS hydrological performance verification methodology and GloFAS v5 calibration hydrological model performance. It describes a group of 228 stations where reservoirs upstream were built during the observation record, after the period of observations used to calibrate the model. These stations are spread over all continents: 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

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 represent the reservoirs differently:

• Calibration – reservoirs not yet active: The observations used in the calibration describe the natural flow regime before the reservoirs were built. In the calibration, reservoirs are introduced in the model only from their construction year, so they have no effect during the calibration period (see [GloFAS v5 calibration methodology and parameters]).
• Reanalysis – reservoirs always active: The reanalysis is used to derive the flood thresholds (return periods) for the GloFAS forecasts, so it must represent present-day, regulated conditions. All reservoirs are therefore active over the whole reanalysis period, including the years before they were built.

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. 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, the calibration simulation (with the final calibrated parameters) and the reanalysis are compared at each of the 228 stations, against the same observations and over the same period: the observations and 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.