Mean Annual Rainfall and Rainfall Variability in the Drylands of Sub-Saharan North Africa

Mean annual rainfall, the coefficient of variation of annual rainfall, and rainfall regions (source: Nicholson et al., 1988, whose rainfall regions are shown by the yellow numbers.) The Sahel is the zone with a mean annual rainfall range of 100 mm to 300 mm, with the Sahelo-Saharan zone at the lower end of this amount. The Sahelo-Sudanian zone corresponds with a rainfall range of 300 mm to 600 mm, while the Sudanian zone itself has a southern boundary defined by the 1200 mm mean annual isohyet (see, e.g., Malo and Nicholson, 1990). The Guinean zone has rainfall greater than 1200 mm per year.


The Sahelo-Saharan zone fluctuates between the barrenness of the Sahara and sparse grass cover, depending on the presence or absence and the timing of rainfall in any particular year. The north of the Sahel proper has grasscover, with trees found only along surface or underground watercourses. Central and southern Sahelian grasslands have increasing tree density toward the south. The Sudanian zone itself comprises a wooded grassland in the north and segues to a subtropical dry forest further south, while the Guinean zone proper is rainforest.

Isohyet Migration in West Africa over 25 Years

Migration of isohyets, western Sahelo-Sudano-Guinean zones, 1961 - 1987. Spatially-averaged isohyet positions are shown for 1961 - 1970 (solid lines), 1971 - 1980 (dashed lines), and 1987 (dotted lines). The figure shows a temporal drying trend across all zones: “la récession climatique est persistante; elle affecte tout le Sahel.” Source: Rochette, 1989.


Malo, A.R. and S.N. Nicholson, 1990. A study of rainfall and vegetation dynamics in the African Sahel using normalized difference vegetation index. Journal of Arid Environments 19, 1-24.

Nicholson, S.E., J. Kim, and J. Hoopingarner, 1988. Atlas of African Rainfall and Its Interannual Variability. Department of Meteorology, Florida State University, Tallahassee, FL. 237 pp.

Rochette, R. M., 1989 (ed.). La sahel en lutte contre la desertification. Eschborn, Germany: Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ).

The correct citation for this page is:
Milich, L., 1997. Mean Annual Rainfall and Rainfall Variability in the Drylands of Sub-Saharan North Africa.

The Table of Contents of my work on desertification and food security is available.

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This site last updated July 19 1997.