Introduction
Flood return-period thresholds in GloFAS are derived from annual maximum discharge (AMAX) series extracted from the long-term river discharge reanalysis. The standard approach assumes that the annual maxima are stationary over the reanalysis period. However, long-term changes in the meteorological forcing or other components of the modelling system can introduce systematic trends in the simulated annual maxima. Large trends in GloFAS river discharge have previously been documented, for example by Zsótér et al. (2020), particularly in the Congo basin and the southwestern USA. If such trends are sufficiently large, fitting a stationary extreme-value distribution directly to the complete historical sample may result in flood thresholds that are not representative of the more recent discharge regime and, consequently, of the real-time forecasts.
To address thisTo address non-stationarity, an adjustment is applied to the annual maxima before fitting the extreme-value distribution. The objective is to remove the estimated long-term change in the magnitude of the annual maxima while retaining the complete historical sample and its interannual variability, essentially transferring the AMAX time series to a common reference level representative of the recent discharge regime. For GloFAS v5, this reference level is the last reanalysis year, 2025, of the full 1979–2025 period.
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