Desert Environments of Republic of Chad - Combating Desertification in Asia, Africa and the Middle East - page 165

of 40 billion cubic meters, 95 % of which is lost to evaporation. The size of the
lake is determined by rains in the southern highlands bordering the basin and by
temperatures in the Sahel. Fluctuations in both cause the lake to change dramatically
in size, from 9,800 km 2 in the dry season to 25,500 km 2 at the end of the rainy
season. Lake Chad also changes greatly in size from one year to another. In 1870
its maximum area was 28,000 km 2 . The measurement dropped to 12,700 in 1908.
In the 1940s and 1950s, the lake remained small, but it grew again to 26,000 km 2 in
1963. The droughts of the late 1960s, early 1970s, and mid-1980s caused Lake Chad
to shrink once again (Coe and Foley 2001 ). The only other lakes of importance in
Chad are Lake Fitri, in Batha Prefecture, and Lake Iro, in the marshy southeast.

2.3 Climate

The Lake Chad Basin embraces a great range of tropical climates from north to
south, although most of these climates tend to be dry. Apart from the far north, most
regions are characterized by a cycle of alternating rainy and dry seasons. In any
given year, the duration of each season is determined largely by the positions of two
great air masses-a maritime mass over the Atlantic Ocean to the southwest and a
much drier continental mass. During the rainy season, winds from the southwest
push the moister maritime system north over the African continent where it meets
and slips under the continental mass along a front called the ITCZ (intertropical
convergence zone). At the height of the rainy season, the front may reach as far as
Kanem Prefecture. By the middle of the dry season, the ITCZ moves south of Chad,
taking the rain with it. This weather system contributes to the formation of three
major regions of climate and vegetation.

3 The Saharan Desert System

3.1 Sahara Region

The Saharan region covers roughly the northern third of the country, including
Borkou-Ennedi-Tibesti Prefecture along with the northern parts of Kanem, Batha,
and Biltine prefectures. Much of this area receives only traces of rain during the
entire year; at Faya Largeau, for example, annual rainfall averages less than three
centimeters. Scattered small oases and occasional wells provide water for a few date
palms or small plots of millet and garden crops. In much of the north, the average
daily maximum temperature is about 32 ı C during January, the coolest month of the
year, and about 45 ı C during May, the hottest month. On occasion, strong winds
from the northeast produce violent sandstorms. In northern Biltine Prefecture, a
region called the Mortcha plays a major role in animal husbandry. Dry for 9 months