Right on the edge of the world's largest, driest desert-- the Sahara-- there lies a large freshwater lake. Lake Chad borders four countries in West Africa: Nigeria, Niger, Chad, and Cameroon. Lake Chad was once the sixth-largest lake in the world, but persistent drought since the 1960s shrank it to about a tenth its former size.¹

The lakebed 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. You can see these lush wetland areas (bright red in the Landsat images) ringing the lake in 1973, and especially in 1987 as vegetation had replaced open water.²

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 northwest quadrant of the Landsat images, 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.³

As you can see in the map of Africa, the Lake Chad basin is closed, with no outlet to the sea. Lake Chad technically has an outlet to the east, the dry Bahr el Ghazal riverbed-- but the last time the lake was high enough to spill into it was probably 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.⁴

Lake Chad level fluctuations

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).⁵

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, so our satellite images are all near the annual peak, except the 1963 image which probably is slightly pre-peak.

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. The chart shown here shows levels since the 1870s, from actual measurements and from estimates based on Nile River discharge.⁶

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.⁷

Farming in the fluctuations

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.⁸

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.⁹

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.¹⁰

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 1988.¹¹

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 started 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.¹²

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.¹³

Question

For thousands of years, people have transformed land cover by a method more powerful than building, planting, irrigating, grazing, draining, poldering, or anything else mentioned above. Do you see it in these images?

(Hint: Look in the southeast of the 1963 image. See the answer below.)

Footnotes

Thanks to Jean-Claude Olivry of ORSTOM for his assistance with this article and for the ground and aerial photographs.

1. Charles F. Hutchinson and others, 1992, Development in arid lands; lessons from Lake Chad: Environment, vol. 34, no. 6, July/August 1992, p. 16. U.S. Agency for International Development, Famine Early Warning System, 1997, Lake Chad; untapped potential: FEWS Special Report 97-4, 27 May 1997.

2. FEWS 1997. Hutchinson p. 16. Stanley R. Schneider and others, 1985, Monitoring Africa's Lake Chad basin with Landsat and NOAA satellite data: Int. J. of Remote Sensing, vol. 6, no. 1, p. 59.

3. Schneider p. 59. FEWS.

4. Hutchinson p. 17. A. Kolawole, 1987, Environmental change and the South Chad Irrigation Project (Nigeria): Journal of Arid Environments, no. 13, 1987, p. 170.

5. Kolawole p. 170-171, 173. Schneider p. 69. Dieter Jackel, 1984, Rainfall patterns and lake level fluctuations at Lake Chad: in: N.A. Morner and W. Karlen (eds.), Climatic Changes on a Yearly to Millenial Basis, D. Reidel Publishing Co., 1984, p. 196.

6. Schneider p. 61. Hutchinson p. 18, 42. Jackel p. 191. FEWS. A.T. Grove, 1978, Geographical introduction to the Sahel: The Geographical Journal, vol. 144, no. 3, p. 410. The chart of lake levels is after Jean-Claude Olivry and others, 1996, Hydrologie du lac Tchad: Paris, ORSTOM, p. 206.

7. FEWS. Kolawole p. 169.

8. FEWS. Schneider p. 60.

9. FEWS. Kolawole p. 175.

10. Hutchinson p. 18. FEWS.

11. Kolawole p. 172. Hutchinson p. 21.

12. Kolawole p. 169.

13. Hutchinson p. 41. Kolawole p. 172-173.

14. Hutchinson p. 20.

Satellite images

DS09059A073MC034 (Argon low-resolution satellite photograph, 31 October 1963)

DS10251026DF044 (Corona medium-resolution satellite photograph, 7 October 1965)

1973 mosaic: Landsat 1 MSS scenes LM1198051007236090 (25 December 1972), LM1199050007303190 (31 January 1973), and LM1199051007303190 (31 January 1973)

1987 mosaic: Landsat 5 MSS scenes LM5184051008702490 (24 January 1987), LM5185050008703190 (31 January 1987), LM5185051008703190 (31 January 1987), and LM5186050008703890 (7 February 1987)

AL14010297123847 (NOAA 14 AVHRR, 2 January 1997)

Special Projects Images

A comparison image of the southwest corner of Lake Chad for 8 December 1972 and 14 October 1987 is available as Special Projects Image E-1849-810CT from the U.S. Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS) Customer Services.

The 1973 mosaic showing all of Lake Chad is available as Special Projects Image E-2028-99CT from EROS Customer Services.

The 1987 mosaic showing all of Lake Chad is available as Special Projects Image E-2027-99CT from EROS Customer Services.

Maps

Defense Mapping Agency, 1973, Fort Lamy, Africa: scale 1:2,000,000, stock no. 2201X13-004.

Institut Geographique National, 1969, N'Djamena (Fort-Lamy): scale 1:1,000,000, Carte Internationale Du Monde (International Map of the World) ND-33.

Photographs

These photographs appeared with French captions in: Jean-Claude Olivry and others, 1996, Hydrologie du lac Tchad: Paris, ORSTOM. Reproduced with permission:

Plate II, no. 2: The prolonged stay of elephants on an island in the Bol archipelago results in destruction of shrub vegetation. (Christian Leveque, 1968)

Plate VI, no. 2: The town of Bol lies between an arm of the lake and another arm (lower right of aerial photograph, center right of satellite photograph) which has been made into a polder. (Cadou, 1971)

Plate VII, no. 2: Temporary camps shelter herdsmen who move from island to island according to available pasture. (Christian Leveque, 1970)

Plate VIII, no. 3: At seasonal camps, hemen catch fish which women dry and smoke over a fire of aquatic herbs. (Christian Leveque, 1970)

Plate XII, no. 2: With the drought, many farmers turned to fishing. This is probably the case for the inhabitants of this new town in Cameroon on the southern shore of the lake. (Jacques Quensiere, 1989)

Plate XIII, no. 1, 2, and 3: (1:) To monitor the water level in these changing conditions one must adapt, as at Kindjeria station in the center of the northern basin in November 1975 or September 1977, with rejuvenated vegetation before another dry period. (2:) It takes some work to read the limnograph (3:) and water-level scale. (A. Chouret)

One impact on this area's vegetation is people burning rangeland during droughts, to improve fodder. Some of the blackish areas in the 1963, 1987 and 1997 images are burn scars. ¹⁴