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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?
River network
Figure 1 highlights how the rivers are represented in CEMS-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.
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Figure 1. River network schematic.
Threshold exceedances
Next step is to compute the flood threshold exceedance values. 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 maximum river discharge over time exceeds the threshold value for each individual forecast member (and deterministic forecasts for EFAS). 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 maximum river discharge over time 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.
Figure 2. Schematic to represent the flood threshold exceedance for one ensemble member for each of the 2-, 5- and 20-year severity levels.
Exceedance probabilities
We then compute the threshold exceedance probabilities (0-100%) for all the three thresholds of 2-year, 5-year and 20-year. Figure 3 demonstrates the process of the exceedance probability computation for 5-year return period (RP). Depending on how many of the 51 members exceed the thresholds, the probability is computed and represented by the different shades of red, as the darker the red the higher the probability.
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For EFAS, in which the system is composed of multiple forecasting system, the exceedance probabilities of the forecasts are combined into a total exceedance probability, using a weighted average over the forecasts.
Merged probabilities
In the following step we combine the probabilities of the three severity levels and create one probability map that hierarchically combines the three probabilities.
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Figure 4. Representation of how the three probabilities are combined into one map with three subcategories (30-50%, 50-75% and 75-100%) in each of the 2-, 5- and 20-year severity levels.
Fixed reporting points
The fixed reporting points will always appear in 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 (if none of the severity levels are exceeded the points are shown in grey). 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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Figure 5. Schematic representation of the fixed reporting points and how the flood severity with the colour is determined.
Dynamic reporting point generation
Motivation: The dynamic points are generated on river sections that do not have any fixed reporting points and show some expected flood conditions (the 2-year exceedance probability is at least 30%). These points help the users to monitor the expected evolution of the hydrological state with detailed information (i.e. hydrographs) at these river sections (Figure 6a).
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