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 Printable version |
Signification and use of the rainfall index in the Sahel |
Science et changements planétaires / Sécheresse. Volume 19, Number 4, 227-35, octobre-novembre-décembre 2008, Article de recherche
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 Résumé
Article gratuit
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Author(s) : Abdou Ali, Thierry Lebel, Abou Amani |
Summary : The Standardized Precipitation Index (SPI) is commonly used to determine whether the Sahelian region can be considered as wet or dry for a given year. There are however several methodological points worth examining to assess the pertinence of using such a global index for characterising the rainy season in a region where the spatial variability of rainfall is large. Three such major points are addressed in this study: i) do the operational networks make it possible to compute interannual rainfall anomalies with reasonable accuracy? The answer is yes. ii) The Sahel cannot be considered as a homogeneous region in terms of interannual rainfall variability. As a consequence, is it pertinent to use a unique SPI for the whole Sahel? The answer is no: three sub-regions (West, Central, East) are identified and it is shown that, especially since 1990, the West and East SPI are not strongly correlated, meaning that computing a global SPI for the whole Sahel does not make sense every year. iii) Given this result, what usage can be made of the SPI? By studying the spatial variability of the SPI computed at the local scale of a 0.5°x 0.5° grid mesh, a probabilistic model is proposed making it possible to compute the expected proportion of the Sahel where the SPI is below or above any threshold for a given value of the regional index. |
Keywords : drought, probabilistic approach, rainfall index, Sahel, spatial variability |
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