RAINFALL AND FLOODING IN THE SAHEL
Rain is the lifeline for humans and animals in the Sahel region. It falls during only a few months of the year, and its patterns shape the food security and livelihoods of millions of people in the Sahel. In the north, rainfall drives the annual growth of vegetation, providing green pastures for millions of goats, sheep and cattle. Further south, sufficient rain is needed for the growth and production of crops such as millet and sorghum. Meanwhile, the Sahel’s wetlands and their natural resources rely on rain for annual flooding. Information on annual rainfall therefore helps illuminate daily life, community livelihoods and biodiversity throughout the region.
But rain is not a certainty in the Sahel. Due to erratic rainfall patterns, there is large variation in rainfall from place to place and between seasons, regions and years. This makes calculating average annual rainfall a challenge, with information from many weather stations needed to arrive at reliable estimates. The Sahel Rainfall Index, as presented on this website, helps solve this issue by annually summarising data from numerous Sahel weather stations into a single index.
Along with rainfall, the extent of annual flooding in the Sahel’s main floodplains is also important information. It is presented here as the Maximal Flood Extent of the two main floodplains in West Africa: the Senegal River (including valley and delta) and the Inner Niger Delta. These areas have flood-dependent economies on which several million people—fishermen, pastoralists, rice farmers—rely, and they are hotspots of biodiversity.
Publicly available information on annual rainfall and flooding in the Sahel is useful from a number of perspectives. The Sahel Rainfall Index and Maximal Flood Extent can be used as indicators for annual food production, including rainfed and flood-dependent crops and grazing pastures, as well as informing related issues. Both parameters are also highly informative in terms of biodiversity, for both resident species and migratory birds. For example, population fluctuations of Eurasian bird species that winter in the Sahel are strongly related to the region’s rainfall and flooding.
The website is updated annually. (Most recent update: November 2024).
Sahel Rainfall Index
The Sahel Rainfall Index presents annual rainfall from 1900 to the most recent rainy season in terms of deviation from the long-term mean in the Sahel, from 1900 to the most recent rainy season. See ‘Applications and Downloads’ below to download corresponding data as csv. Background information on Weather & Climate in West Africa and an explanation of the Sahel Rainfall Index is given on this site.
Annual rainfall index for the Sahel region, shown as deviation from the long-term mean (calculated from 1900). For information on the annual number of meteorologic stations used and the standard error of the index, see Methods and Data.
Maximal Flood Extent
The figures below present data from 1922 to the most recent flood season to show the Maximal Flood Extent (× 1000 km2) of the two major floodplains in the Sahel: the Inner Niger Delta in Mali (upper panel), and the valley and delta of the Senegal River between Matam and Podor (lower panel). The 9-year running mean is shown as a smooth curve. Red bars indicate the completion of a large reservoir in each of the two rivers, with a reduced annual flood extent in subsequent years.
See Section ‘Applications and downloads’ below for downloading corresponding data as csv. Background information, including the historical development of the Senegal Delta and the loss of flooding, is provided in the section on Hydrology & Floodplains in West Africa. An explanation of how flood extent is determined is given in this site’s Methods & Data section.
Maximal annual flood extent (× 1000 km2) of the two major floodplains in West Africa, from 1920. Upper panel: Inner Niger Delta. Lower panel: valley and delta of the Senegal River. Smooth curves show the 9-year running mean. See Methods & Data for explanation and sources.
Applications and downloads
Both the Sahel Rainfall Index and Maximal Flood Extent can be used as annual indicators of life in the Sahel, as driven by rainfall and river discharge. For example, the Sahel Rainfall Index is strongly correlated with both natural production and crops such as millet and sorghum, while the Maximal Flood Extent is an important indicator of rice and fish production in the Inner Niger Delta (for references see Methods and Data). Finally, both are important parameters for biodiversity. For example, the dynamics in population size of European migratory birds is strongly related to either rainfall in the Sahel (e.g. Common Whitethroat) or flood extent (Sand Martin, Sedge Warbler, Purple Heron, Black-crowned Night Heron). For references and more information, see Methods & Data.
Do you have another idea for how to apply the Sahel Rainfall Index or Maximal Flood Extent? Please share it with us! Email info@altwym.nl, and we will incorporate your suggested application into this site.
This data is available to be used, provided that you include a citation (https://sahelrainindex.info/).
Data can be downloaded for free, but we encourage you to consider a voluntary donation, which will help us to maintain and update the site annually. Donations can be made by bank transfer to account number NL43 RABO 0116940638 (BIC RABONL2u) of Altenburg & Wymenga; please include ‘donation Sahel Rainfall Index’ as a note.