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To address non-stationarity, an adjustment is applied globally on all 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 areas/pixels, where no long-term change (i.e. trend) is present, there will be no change in the annual maxima. While for area/pixels with well-defined changes/trends, the adjustment of the annual maxima values will be potentially large. For GloFAS v5, this reference level is the last reanalysis year, 2025, of the full 1979–2025 period, which is the closest year in the reanalysis to the real-time forecasts.
Annual maxima adjustment methodology
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