The return of the water - IUCN

3
Consequences of the Dam and the
Drought
By Paul Loth
3.1 Reduced flooding as a consequence of climatic change?
The high productivity of the Waza Logone region depends to a large extent on the over bank flooding
of the Logone River each year between September and December. After 1979 the annual inundation
of the Waza Logone floodplain was reduced significantly, due to hydro-agricultural works and low
rainfall. The construction of the dam to create a reservoir (Lake Maga) for the SEMRY irrigation
scheme at the southern boundary of the flood plain has blocked water supplies coming from the
Mandara Mountains to the floodplain. In all, the dam reduced the flooded area by almost 1,000 km 2
(Drijver and Marchand, 1985).
Deviation of Mean Max Flow (m3/s)
25,000
20,000
15,000
10,000
5,000
0
wet (dry) wet dry (wet) dry average
1930 1940 1950 1960 1970 1980 1990 2000
Year
Figure 3.1 Cumulative deviation of mean annual rainfall (black dots) and maximum flow discharges (m 3 per second; open
symbols) for two flow gauge stations in the Logone river. Rainfall data is averaged for rainfall stations in the entire Logone
floodplain. Squares represent flow gauge measurements at N'Djamena, triangles for flow gauge data at Bongor. The
indication of wet and dry periods is based on rainfall data. Dotted line represents zero deviation rainfall.
The years following the closure of the dam were coincidentally characterised by below average
rainfall. In Figure 3.1 the cumulative deviation of the mean annual rainfall for rainfall stations in
the Logone floodplain is presented 1 . For the period that rainfall data are available (1930 – 1994),
several wet and dry periods can be distinguished. The series begins with a succession of wetterthan-normal
years until the late 1930s. The following period is almost normal in rainfall, perhaps
with a slight tendency to be drier than the mean. The next 15 years or so are characterised by
1 A cumulative deviation of the mean graph adds the difference of the total yearly rainfall with the mean rainfall
for each year. In this case the mean rainfall over 65 years was 954 mm. A year with less rainfall than average
results in a negative value added to the cumulative sum, a rainfall amount higher than the mean value on the
other hand adds to the total value of the summed deviations. See also Prins and Loth (1988).
2,000
1,500
1,000
500
0
-500
Deviation of Mean Rainfall (mm)
19