The return of the water - IUCN

The Return of the Water
4.4 Historical rainfall/river flow relationship
River flow records are good indicators of past rainfall conditions. There is a long-term flow gauging
station on the Logone River at Bongor, close to the upstream end of the Waza Logone area (see
Figure 4.5). This has records dating from the 1940s. Another flow gauging station was installed in
the Chari River at N’Djamena, near the downstream end of the Waza Logone area. Its flow records
date from the 1930s. Statistical analysis (Mott MacDonald, 1993) of the flow data at Bongor and
N’Djamena reveals that since their records began, the driest decade at Bongor was the 1980s; and
the wettest decade was the 1960s. At N’Djamena, the driest decade was also the 1980s, the wettest
decade was the 1950s and the second wettest decade was the 1960s. The last 30 years (i.e. since
1970) have been relatively dry compared with the previous 30 years (see graphs in Figure 4.5).
However, the length of record is not sufficiently long to allow detailed long-term runoff trend analysis
to be undertaken. Therefore, projections of future runoff patterns in the Logone catchment area
cannot be made.
The shift to a drier-climatic regime in the 1970s is recognised as a general trend across the Sahel
region of Africa, and is not peculiar to the Logone-Chari catchment area. Studies have indicated
that this shift may be related to sea surface temperature anomalies (Folland et al., 1991; Rowell et
al., 1992). It is not clear whether these changes are cyclical, and a return to a wet period could be
expected, or if they represent a modal shift in operation of the global ocean-climate system, in which
case persistence of drier conditions is likely to continue. Future predictions are further complicated
by the difficulty of predicting the effects of global warming due to increased levels of greenhouse
gases.
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