Rainfall and flooding in three river basins, 1998 to 2025

Basins and zones

Rainfall is measured over the whole basin (coloured). FloodScan flooded area is measured in the zones along the rivers (hatched): Shabelle for Wabi Shebele, Afder, Liban and Daawa for Genale Dawa, and South Omo for Omo Gibe. Zones are drawn whole, so some reach beyond the basin outline.

Rainfall and FloodScan flooded area by season

For each season, the top bar is the wettest 7 days over the basin and the bottom bar is the largest share of the river zones that FloodScan shows as flooded on any one day. The five highest seasons are in a darker colour, and years in bold are in the top five for both.

Top seasons in each record, 2003 to 2025

Possible thresholds

This section sets out two possible thresholds, one for river flow in GloFAS and one for rainfall over the basin, and sets the seasons they were reached against recorded floods. The table shows how often each was reached in any of the three basins between 2003 and 2025. "How often" is (23 + 1) divided by the number of seasons. This is for monitoring only. It is not an official trigger and nothing is released.

The days are counted to the FloodScan peak in the river zones (the day with the largest flooded share), from the day GloFAS reached the threshold or from the day the rainfall data came in, which is about two days after the rain. Rank is out of the 23 seasons, 1 being the largest FloodScan flooded area.

• Both thresholds were set using the same 23 seasons they are checked against.
• The flood records are not split by basin. EM-DAT events are matched on the place names they list, and CERF allocations are national. FloodScan's 1-in-5 record covers only the Somali region, which does not include Omo Gibe.
• The river flow check uses GloFAS river flow rebuilt from past weather, not the forecasts issued at the time.

Data and method

• Rainfall is IMERG Late Run daily data from NASA, averaged over each administrative zone, for 1 October to 31 December. Each zone counts towards the basin in proportion to how much of it lies inside the basin. A 7-day total is one day's rainfall plus the six days before it.
• Flooded area is FloodScan's standard flood extent from AER, a satellite estimate: the share of the river zones it shows as flooded each day, from 1 October to 31 January, with each zone weighted by its area.
• Recorded floods are EM-DAT flood events whose locations name the watch's riverine areas and whose dates fall in October to December, and CERF flood allocations to Ethiopia (national record), as on the methods page.
• River flow is the GloFAS version 4 reanalysis (river flow rebuilt from past weather) at the watch page's stations, shown as a share of each river's threshold, for 2003 to 2025.
• ERA5 monthly rainfall from Copernicus is used only to check 2000 (see below).
• Basin outlines:

Limitations

• In IMERG, 2000 has the highest 7-day rainfall of any season in Wabi Shebele and Genale Dawa. For the October to December total, ERA5 ranks 2000 11th and 16th of 28, where IMERG ranks it 5th and 4th, and FloodScan ranks its flooded area 8th and 17th.
• FloodScan in South Omo sits higher from 2022: the median flooded share from October to January was 0.75 to 1.17% in 2022 to 2025, against 0.02 to 0.63% in 1998 to 2021. Two of Omo Gibe's three seasons with the largest FloodScan flooded area, 2025 and 2023, fall in this period.
• In Genale Dawa, 2021 has the fifth largest FloodScan flooded area and the lowest rainfall of all 28 seasons.
• FloodScan is a satellite estimate of flooded area. It is not a record of floods or their impacts.
• IMERG is missing 14 to 16 November 2024 and 27 September 2025, and FloodScan is missing 25 November 2024.
• Zones that lie partly outside a basin bring in some rain from outside it. IMERG is a satellite estimate.

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