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Rainfall in West Africa

The flooding of the Inner Niger Delta depends on the river flow of the Niger and Bani Rivers, which in its turn depends on the rainfall in the Upper Niger Basin. The rainfall in the Upper Basin in July-August is therefore a good indicator of the flooding of the Inner Niger Delta 3-4 months later.  Today, satellites are used to estimate rainfall across the world. The Famine Early Warning System website (FEWS) gives satellite-derived rainfall estimates for Africa. This information is used here.

Rainfall in the Upper Niger Basin

Using satellite-derived rainfall data (see Recent rainfall), we calculated the accumulated amount of rainfall in the Upper Niger Basin during the current wet season and compared this to rainfall in the past. The graph shows whether the rainfall in the current season so far (yellow line) is relatively high or low.

The flow of the Bani and Niger River, and thus also the flooding of the Inner Niger Delta, is determined by the rainfall in the Upper Niger Basin upstream of the Inner Niger Delta, thus in the eastern part of Guinea-Conakry and the SW part of Mali. The daily estimated rainfall (see Recent rainfall) within this area is used in OPIDIN to improve the prediction of the flood level. In these calculations, the daily rainfall in the Upper Niger Basin is converted to km3 water. The basic information is derived from satellite images and made available through various websites (see Recent rainfall and Rainfall in the current season so far). The graph above shows this information from 1 April onwards in comparison to the rainfall over the same seven months during previous years (2001-2016).

Recent rainfall

Daily rainfall is monitored by satellites and the maps given below are taken from the website of CPC/NCEPO/NOAA  showing the rainfall in West Africa during four recent days:

Satellite-derived rainfall estimates of 1 day ago; source: NOAA/FEWS.

 

Satellite-derived rainfall estimates of 2 days ago; source: NOAA/FEWS.

Satellite-derived rainfall estimates of 3 days ago; source: NOAA/FEWS. Satellite-derived rainfall estimates of 4 days ago; source: NOAA/FEWS.

 

The total rainfall is also given for last week:

 
 
Satellite-derived rainfall estimates (ARC2) for last week; source: NOAA/FEWS  

 

All maps above give the rainfall in mm, but to know whether the rainfall was above or below the long term-average, the results are presented in another way. The recent rainfall can also be presented as deviation from the long-term mean from the years before. The same map of last week is given below as percent deviation (relative difference). For the absolute difference (deviation from the mean), go to the following link: 

http://www.cpc.ncep.noaa.gov/products/international/africa_arc/africa_arc_7day_af_anom.gif

 
Satellite-derived rainfall estimates (ARC2) for 28-07-17 to 3-08-17, not given as mm but as percent deviation from the long term mean; source: NOAA/FEWS.  

Rain forecast

Using the weather satellites, it is also possible to give a global prediction regarding the rainfall in the coming weeks. In the maps below, this prediction is given by FEWS for the coming week, and the week after the coming week. Green=above-average rainfall; yellow=below-average rainfall, and for the week after coming week. More detailed information about the weather forecasts can be found on the cpc.ncep.noaa.gov site.

Satellite-derived rainfall forecast for the next week; source: NOAA/FEWS. Satellite-derived rainfall forecast for the week after next week; source: NOAA/FEWS.

 

Rainfall in the current rainy season so far

The flow of the rivers and the flooding of the Inner Niger Delta is not determined by the rainfall of the last week(s), but by the accumulated, total rainfall in the wet season. Also this information is gathered, based on the daily estimates of rainfall as shown above. The map below shows the total rainfall (mm) for the last 180 days in three ways:

Rainfall during the last 180 days: total amount (mm).
Rainfall during the last 180 days: absolute deviation (in mm above or below the long-term average).
Rainfall during the last 180 days: relative deviation (in per cent above or below the long-term average).