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Figure 6 shows an example snapshot in the same area as Figure 1a above. One can compare, how the variable signal on the river network averages into the basin signal. The averaging is done from the river pixel information. A Rank-mean and a Rank-std value will be calculated for each basin and with those the exact same method is used (as for the river pixels on the river network map) to define the forecast anomaly category and the uncertainty category and thus the colour of the basin (see the inset figure with the colour legend in Figure 4). The colours used are the same as for the river network map, except there is a slight shift in transparency, in order to allow visibility for all river pixels, even for those with the same colour as the basin colour.
The basin Rank - mean and Rank - std values are determined using all the large enough river pixels in the basin. Currently, only pixels above 50km250km2 are used in EFAS and pixels above 250 1000 km2 are used in GloFAS in the averaging. The basin Rank - mean /and Rank - std values are calculated as an arithmetic average of the Rank - mean and Rank - std values of the individual river pixels, weighted by the square root of the upstream area, as described below:
The weighting by the square root of the catchment size gives more weight to the larger rivers in the area averaging and thus a basin with a proportionally much larger river with a lot of very small rivers will
Figure 6. Example snapshot of the basin summary maps with the lead time navigation indicated and the categories and basin colours explained.
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