Environmental Engineering Reference
In-Depth Information
Figure 3.7
North-South section across West Africa, showing bands of weather associated with the ITCZ
Source:
After Hamilton and Archbold (1945)
Drought and human activity
in general terms, in detail the precipitation
patterns are quite complex. In West Africa, for
example, the existence of four weather zones,
aligned east-west in parallel with the ITCZ, was
recognized by Hamilton and Archbold in 1945,
and these, with some modern modifications (e.g.
Musk 1983), provide the standard approach to
the regional climatology of the area (see Figure
3.7). Each of the zones is characterized by specific
weather conditions, which can be identified in
the precipitation regimes of the various stations
in the area (see Figure 3.8). The precipitation is
caused mainly by convection and convergence,
and reaches the ground in a variety of forms,
ranging from light, intermittent showers, to the
heavy downpours associated with violent
thunderstorms or line squalls. The easterly
tropical jet is also active in the upper atmosphere
at this time, and may well influence the amount,
intensity and distribution of precipitation
(Kamara 1986).
Throughout the Sahelian region, the peak of the
rainy season in July and August is also the time
of year when grass and other forage is most
widely available for the herds of cattle, camels,
goats and sheep belonging to the local
agriculturalists. The original herdsmen lived a
nomadic existence, following the rains north in
the summer and south in the winter, to obtain
the food their animals needed. There was
sufficient moisture available in the southern areas
to allow a more permanent lifestyle, supported
by basic arable agriculture, producing sorghum
and millet. Drawn south by the rains, the
herdsmen eventually encroached upon this
farmed land, but instead of the conflict that might
have been expected in such a situation, the two
societies enjoyed a basic symbiotic relationship.
The nomads exchanged meat and milk for grain;
the cattle grazed the stubble, and provided
fertilizer for the following year's crop.
