Flood return-period thresholds in GloFAS are derived from annual maximum discharge (AMAX) series extracted from the long-term river discharge reanalysis. The same procedure is applied for every GloFAS river pixel. The generation of the GloFAS flood thresholds follows the below workflow:
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Up to GloFAS version 4, including the v1, v2 and v3 model families, flood thresholds were generated directly from the unadjusted annual maxima.With With the introduction of GloFAS v5 in 2026, an annual maxima adjustment methodology was introduced to address the impact of non-stationarity, particularly large long-term trends in river discharge that are present in some regions of the GloFAS reanalysis. The annual maxima adjustment is applied globally on all river pixels, with the adjustment's impact only noticeable over areas with pronounced river discharge trends.
The adjustment methodology, used from GloFAS version 5 onwards, is described here: Handling of non-stationarity in the flood thresholds.