Reporting points have two main goals: they provide an overview of the floods globally over the CEMS-Flood domain over the next 30 days forecast time horizon and also give access to local detailed forecast information. They are determined based on the maximum flood signal in the 30-day forecast horizon.
But how are the location of the dynamic points determined, and how the severity of the signal, which drives to the colour of the reporting points and the river cells in the flood summary layers, is computed?
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Figure 1 highlights how the rivers are represented in GloFASCEMS-Flood. It follows a simplified and discretised way by converting the real rivers into a sequence of cells, where the water flows from one cell to the next downstream.
In the dynamic reporting point generation algorithm, from EFAS v5 and GloFAS v4 we currently only consider cells above 1000 km2km2 for GloFAS, and 500km2 for EFAS, so cells with very small size can not become reporting points. In the following, the reporting point generation is demonstrated on this small idealised river network area.
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Next step is to compute the flood threshold exceedance values. GloFAS CEMS-Flood primarily uses three main flood severity levels, the 2-year, 5-year and 20-year return periods. We determine for each of these
thresholds whether the predicted 30-day maximum river discharge in the ensemble forecast members (the control member and the 50 perturbed members) exceed the threshold value. We assign 1 if they exceed and 0 if they do not. This process is represented in Figure 2 by the coloured river cells, which show that the 30-day maximum river discharge exceeds the flood thresholds. Please note the triviality that if the 20-year threshold is exceeded for any cell, than both the 5-year and 2-year must be also exceeded for those cells, i.e. more and more cells are coloured (indicating exceedance), as we go from 20-year to 2-year level.
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An example of the product that is available on the CEMS-Flood website for GloFAS website ('Flood summary for days 1-30') based on the merged probabilities is shown in Figure 4c.
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The fixed reporting points will always appear in GloFAS CEMS-Flood (Figure 5a). They will be shown by one of the three colours, yellow, red and purple, representing the three flood severity levels. This level comes from the combined probability map, generated in the previous step (i.e. the map shown as 'Flood summary for days 1-30' on the GloFAS website). This means, whichever (highest) of the three severity levels is exceeded by at least 30% will define the colour of the fixed reporting points. In our examples one of the points will remain grey (i.e. below 30% 2-year probability, so no flood) and the other will be shown as purple (20-year flood level).
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