Flood return-period thresholds in GloFAS are derived from annual maximum discharge (AMAX) series extracted from the long-term river discharge reanalysis. The generation of the GloFAS flood thresholds for GloFAS v4 and earlier versions (such as the v1, v2 and v3 families) follows the below workflow:

• Extract the annual maxima time series in the long reanalysis period
• Fit a Gumbel extreme value distribution using the first two L-moments (λ1, λ2)
• The Gumbel parameters are:

and 

• For return period T, the related flood threshold X is calculated as:

The plot below demonstrate the process for an example station on the Ganges river (Harding Bridge). It displays the empirical distribution of the extracted annual maxima and the flood threshold curve (out to return period 100 years) based on the Gumbel distribution. In this example for GloFAS v4, the considered reanalysis period was 1979-2022.

Adjusting for non-stationarity

The flood thresholds are generated from the unaltered annual maxima until GloFAS version 4, including the v1, v2 and v3 families. From the introduction of v5 in 2026, an annual maxima adjustment methodology was introduced that addressed the negative impact non-stationarity, i.e. large river discharge trends, in the GloFAS reanalysis that is present is some areas in the world, like the Congo basin or Southwestern US.

The adjustment methodology, used form GloFAS version 5, is described in more detail on this page: Handling of non-stationarity in the flood thresholds.