African Unification Front
|
 |
|
|
|
|
 |
 |
|
THE CHARI-CHAD BASIN
A DISAPPEARING WETLAND
Once the second largest African Wetland, satellite image of Chad Basin
Lake Chad was once the sixth-largest lake in the world, but persistent drought since the 1960s shrank the lake from 25,000 kmē in surface area in 1963 to about 2,500 kmē. The catchment area of the lake (the system of rivers, streams, and aquifers that feed into it) is over 2.5 million kmē, covering parts of seven constituent republics of the African Union.
A 30% decrease took place in the lake between 1966 and 1975. Irrigation accounted for five percent of that decrease, with drier conditions accounting for the remainder. Irrigation demands increased four fold between 1983 and 1994, accounting for 50 percent of the additional decrease in the size of the lake. The warming climate and increasing desertification in the surrounding region have dropped water levels far below the average dry season level of 4,000 square miles (10,000 square kilometers) to only 839 square miles (1,350 square kilometers).
The lake bed is flat and shallow, so small changes in depth mean huge changes in area. Even in normal times, Lake Chad was no more than 5-8 meters deep. It may be more accurate to think of it as a deep wetland. Considered this way, Lake Chad was once the second largest wetland in Africa, highly productive, and supporting a diversity of wildlife.
Lake Chad has a large drainage basin (1.5 million sq km), but almost no water flows in from the dry north. The Komadougou-Yobe River, in the north west, now flows only in the rainy season. 90% of Lake Chad's water flows in from the Chari River, at the southeast of the lake. The Chari averaged about 40 billion cubic meters per year from the 1930s to the 1960s, but now averages only about half that.
The Lake Chad basin is closed, with no outlet to the sea. Lake Chad technically has an outlet to the east, the dry ,b>Bahr el Ghazal riverbed, but the last time the lake was high enough to spill into it was in the 1800s. Unlike most closed systems, Lake Chad's water is surprisingly fresh. The Chari River puts few dissolved solids into the lake, as many of its suspended solids drop out onto its wide floodplain. Once in the lake, some dissolved solids precipitate, and some are absorbed by plants. Also, 5-10% of the lake water seeps away through the ground, carrying away dissolved solids with it.
LEVEL FLACTUATIONS
Low-rainfall regions are usually also variable-rainfall regions. On the dry, northeast side of Lake Chad, at the town Bol, rainfall from 1954 to 1972 ranged from 125 to 565 mm (about 5-22 in), averaging 315 mm (about 12.5 in).
The lake is very responsive to changes in rainfall. When rains fail, as in 1972, the lake drops rapidly because annual inflow is 20-85% of the lake's volume. Much of the soil in the Chari basin is clay particles which swell together when wet, so that water runs off as rapid sheetwash rather than slow percolation. There is naturally some delay between upstream rainfall and the resulting rise in lake level. About 90% of the rain falls from June to September, but the lake suddenly rises in November. Highest lake levels are in December, tapering off slowly for several months.
Fluctuations are not new to Lake Chad. About 10,000 years ago Lake Chad almost filled its present drainage basin, and spilled southwest out the Benue River to the Atlantic. In the last 1,000 years, according to fossil evidence, the lake probably dried out a half-dozen times. (Most of its fish are river-adapted species.) Geologic data, climate data, historical accounts and reconstructions all indicate a higher long-term variability than the relatively short period we have actually measured.
Following highs in the 1870s and 1890s, the lake dropped enough by 1908 to separate into north and south pools, with the "Great Barrier" between. In the 1950s the lake rose enough to flood out irrigation systems, peaking this century in 1962. The lake then tapered off until the early 1970s, when it plummeted. The recent low levels are a concern, and have been monitored through satellite and other means by the Lake Chad Basin Commission and others.
FOOD PRODUCTION
People around Lake Chad are among Africa's most chronically vulnerable to food insecurity. They deal with variability through mobility and through diversity of food sources. Lake-related activities include fishing and soda-mining. Some people raise livestock, typically moving closer to the lake for grass in the dry season, then moving away in the rainy, mosquito season; some will graze their animals up to 100 km away. After the 1970s droughts, herders shifted from grazing animals (cattle and camels) to browsing animals (sheep and goats), which affected the area's vegetation by consuming the woody plants.
Crop strategies include farming the lake bottom; the receding lake left behind an estimated .5 million ha of cultivable land, some of which was being cropped. Farming is also done on "recessional lands", where the lake water recedes every year, in the "polder" depressions between dunes. Rice, wheat, maize and vegetables are grown. In a traditional polder, one crop a year is grown as the lakewater recedes. If dams and pumps are used, up to three crops a year can be grown. Besides fewer fish, a low lake also means a shorter shoreline and thus fewer polders. Chad's Lac Prefacture estimated that only 10% of its polder areas were being used.
The Southern Chad Irrigation Project
Human diversion from the lake and from the Chari River may be significant at times of low flow, but rainfall is still the determining factor in lake level- the lake still affects irrigation more than vice-versa. Some lake Chad development schemes have failed due to poor management or civil strife, but many have been frustrated by the lake's rise and fall. One prominent example is SCIP, Nigeria's Southern Chad Irrigation Project, southwest of the lake.
SCIP is the largest irrigation project in Nigeria, with a goal of irrigating 67,000 ha (about 260 sq mi, slightly larger than the Isle of Man) with an average cropping intensity of 130% (200% would be two crops per year). It seeks to resettle 55,000 farming families onto the irrigated land. Nigerians had already practiced resettlement as a drought strategy; the number of villages in the Nigerian portion of Lake Chad rose from 40 to 100 between 1975 and 1988.
SCIP planning started in 1962-1963, the very peak of the wet years. A successful 1966 pilot project irrigated 1,000 ha. The major project start-ed in 1974, and was commissioned at 67,000 ha in 1979. A system of pumps and canals was to carry water from the lakeshore intake point to farmers' inland fields.
But the plans were dependent on the lake's level. When the lake fell below 279.9 m (about 2 m above the baseline of the chart shown here), no irrigation could take place. The system operated only 6 of the first 10 years, with a maximum of 7,000 ha irrigated. Few of the farmers got water, few crops were produced, and water efficiency was low. The canals were unlined, so water seeped into the ground, and only about half reached farmers. When water did come, many farmers overcompensated by breaching or siphoning the canals to get more water, thereby wasting water and waterlogging their fields. Some fields were also poorly prepared for irrigation. Successful irrigation may have to wait for higher water, but as the 1997 image shows, the lake is very low.
END
|
|
|
|
|
|
|
|
printable version
