Use is made in particular of mean annual rainfall figures. A rainfall of 350-400 mm sets the limit, more or less, for dry farming. In both the Sahelian and the Sudano-Sahelian zones varieties of sorghum and millet with short growing periods are predominant. Because of the two-peak rainfall distribution two cultivation seasons and a greater variety of crops are possible. The lower limit for the Sahelian zone could in fact have been put at 100 mm without this making any difference to the Mission's observations.
Rainfall represents only one aspect of the climate.
Other factors such as continentality also play a part and may modify the local characteristics of zones which have the same rainfall. Bearing these reservations in mind, the Mission found that the limits of the zones as given above were satisfactory. They are sufficiently broad, and in addition they agree with the ideas of the local technical experts. The opinion has sometimes been put forward that the Sahelian zone could be defined by the presence of pastoral tribes. Although these people are there in large numbers, we may also in fact note the presence of predominantly agricultural villages such as those of the Soninke, Bambara, Sonhrai, etc., right up to the minimal limits of the zone, while some pastoral tribes, such as the Fulani, are found at the present time in places well outside the zone, in Dahomey, Cameroon and Ivory Coast.