Basins
The CEMS-flood sub-seasonal and seasonal products include a basin summary of the forecasted river discharge anomaly and uncertainty. On this layer the basin colours represent the forecast signal of the river pixels in the basins.
The basin delineation was done semi-automatically, including some manual corrections and subjective decisions on where to split very large catchments and how to group small river catchments along the coastal areas. The basin borders perfectly align with the 1 arcmin (~1.5 km) LISFLOOD river network in EFAS, while they follow the 3 arcmin (~5 km) river network in GloFAS down to 250 km2 pixel size. Whilst generally basins are smaller over the EFAS domain than outside (GloFAS domain), the basin size was often determined by the complexity of the river network and whether splitting larger basins made practical sense, i.e. not to create too small / too many basins, in case the river network complexity would not really have allowed it.
In total, 942 basins were defined globally (Figure 1a), of which 204 are within the EFAS domain (Figure 1b). The basin size ranges from 2550 km2 to 783761 km2 globally, while 6467 km2 to 295472 km2 in EFAS, with the average size of 54844 km2 in EFAS and 142481 km2 in GloFAS. The same delineation protocol was used in both EFAS and GloFAS.
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Figure 1. Basins and representative points in EFAS (a) and in GloFAS (b).
Representative stations
GloFAS and EFAS have so-called fixed reporting points, which are used to show the forecast information regardless of the predicted conditions (CEMS-Flood diagnostic and web reporting points) and always appear as a rectangles. These points are not defined everywhere and therefore some of the basins in Figure 1a and 1b do not have representation by those fixed points, where the details of the forecast signal could be checked in a pop-up window (after clicking). Such non-represented basins appear mainly in Africa and Asia, with some 'empty' basins also on other continents. For example, even in EFAS there are some non-represented basins, one or two in Italy and more in the eastern areas in Russia or the basins in northern Africa.
To allow at least a basic representation to all of the basins, a representative station list was defined. To allow the users to easily distinguish these from the grey rectangle fixed points, they are plotted as black circles. The basin-representative points include one single point for each basin. These representative points are the largest upstream area points, either the outlet point of the single river system of the basin, or the largest of the river catchments in the basin, if it contains more catchments. This latter case happens in coastal areas, where it was not meaningful to separate out all the larger catchments, as that would have resulted in very small basins.
Figure 2 highlights a smaller area in western Europe, demonstrating how the basins and the related representative points were defined. For example, the Rhine river catchment is divided into 5 basins, the lightest orange one as the upstream part, then the greenish-yellow basin with the middle part of the Rhine, including also the tributaries of Neckar and Main rivers. Then the lower part of Rhine consists of the purple basin, except the two tributaries again of the Meuse and Moselle rivers. Each of these basins get a representative points at the 'outflow' pixel, where the upstream area is the largest. In addition, for example the pink basin, north of the Rhine river, includes the Ijsselmeer and Ems rivers with many smaller coastal rivers and have one representative point on the Ijsselmeer river, which is the larger one of the two main rivers in this basin.
Figure 2. Basins and representative points in EFAS over western Europe.