9 The Land Resources of North East Nigeria A/blume1 The ...

Use of these terms has fallen into some disrepute, generally due to the looseness with which
they have been applied (see comments under 'Vegetation') and to the view that they had proved
so useful as a "blanket term' that investigations of the fundamental parameters involved had
long been ignored. As originally conceived (Chevalier, 1900; Keay, 1953), the zones were
established on the basis of an interrelated interpretation of rainfall records and vegetation/
land use observations. In the project area the Sahel, Sudan and Guinea Zones correspond respectively
with the m.a.r. areas shown on Text Map 3: < 508 mm (20 in), 508-1 016 mm
(20- 40 in), and > 1 016 (40 in). Figure 5 attempts to characterise the zones in terms of
some of the major parameters involved. It can be seen that the characteristics of the zones
are so dominated by the rainfall pattern that a classification based on this alone has considerable
validity. The terms as defined in Figure 5 will therefore be used as climatic descriptions
throughout the report.
The Sub-Sudan Zone
This term (Clayton, 1957) has been the cause of much discussion. It occupies an approximate
rectangle bounded on the west by the Mandara range, on the east by 10° longitude, by the
Benue Valley on the south and by the 890 mm (35 in) isohyet on the north- In this area the
marked displacement of the 1 015 mm (40 in) isohyet is attributable to the westerly and
northwesterly orientation of the rain-carrying winds resulting in a large rain shadow behind
the Jos highlands and giving rise to an atypical broadening of the inter-isohyet belt
between 1 015 m and 890 mm (40 and 35 in). The major significance lies in the fact that the
rainfall change between these isohyets is highly sensitive in its effect on other environmental
factors, a tension belt' in the sense of Fairbairn (1945). There is therefore an
extremely marked orographic reaction to the westerly winds; open areas tend to be truly
transitional, westerly faces tend to Northern Guinea conditions and easterly faces towards
Sudan conditions. A good example is the belt of 1 015 mm (40 in) rainfall between the Biu
Plateau and the sandstone hills to the south where the wind is forced upwards by the Mandara
and Zummo Mountains. This effect dies out on the watershed, which forms the frontier with
the Cameroon Republic.
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