Bart Schultz
The Logone is a tributary of the Chari that flows into the Chari south of Lake Chad. The Chari provides approximately 90% of the water to the lake. In addition, the annual precipitation on the lake is 200–300 millimetres, which, depending on the size of the lake, contributes further to the water.
Six thousand years ago, Lake Chad was estimated to have had a surface area of approximately 300,000 square kilometres. Over the centuries, the lake has become increasingly smaller. Because it is a shallow lake, with a maximum depth of seven metres, it is very sensitive to small changes in the water level. For example, in the 1960s it had a surface area of over 26,000 square kilometres. The surface area has since then decreased to approximately 1,500 square kilometres.
Two causes are mentioned for the decrease in surface area. On the one hand, over the years there has been less and less rainfall in the river basins of the rivers that flow into the lake. On the other hand, more and more water is being withdrawn from the lake and the rivers that flow into it for irrigation. For example, the Maga Reservoir was constructed along the Logone in Cameroon to supply irrigation water to the SEMRY rice polders, as described in my earlier contribution to Flevolands Geheugen.
In order to compensate for the decrease in the volume and surface area of the lake, an Italian engineering firm had a plan at the end of the 1980s to divert water from the Congo River to the lake through a 2,500 kilometres long canal. Nothing was done with it at the time. The plan was recently re-proposed on the basis of implementation by a consortium of French, Chinese and Italian companies. It is not yet clear whether this plan will be realised.
Data on the water level of Lake Chad are available from 1875. In the period 1875-1900 the lake level fluctuated between 282 and 283 metres above mean sea level. The lake level then dropped in a few years and then fluctuated more or less between 281 and 282 metres above mean sea level until 1970. Since then the lake level has fluctuated mainly between 279 and 280 metres above mean sea level.
The report The Lake Chad conventional basin: a diagnostic study of environmental degradation, commissioned in 1990 by the United Nations Environment Programme (UNEP) and the United Nations Sudano-Sahelian Office (UNSO), notes that small valleys between sand dunes exist along the northeastern shore of Lake Chad. These valleys flooded seasonally as Lake Chad rose. Others were moistened by rising groundwater, corresponding to the rise in the lake level. As Lake Chad retreated or infiltrated and evaporated, these polders became arable land for growing wheat, maize, cotton and potatoes. The authors of the report argued that a complex series of events could occur in polder development. On the one hand, the lake's surface area would decrease and the areas where salt deposits would be reduced. On the other hand, local increases in salt concentrations would reduce crop production and soil fertility over time.
Italian authors G. Visentini and A. Linoli wrote in 1990 that traditional polders are located along the coastline in Chad, particularly between the villages of Baga Sola and Konlodin. Here, a series of islands and sand dunes extend into Lake Chad. During the driest months, the local population created polders by building sand dams between the peninsulas. Visentini and Linoli also describe that every two to four years, when the water level in the lake was high, the farmers would pierce the sand dams to let in water from the lake and flush the accumulated salts through the soil.
Visentini and Linoli distinguished three types of polders along the coast of Lake Chad. In the lowest polders, the groundwater level either appears at the ground surface or the water rises to the surface by capillary forces. In slightly higher polders, the capillary rise reaches the root zone of the crops, but never reaches the surface. In the highest polders, the surface level is so high that the roots of the crops are not reached by capillary rise.
In 2014, the French authors J. Lemoalle and G. Magrin described the situation around Lake Chad. They also showed the irrigation systems in the lake basin and that some of these predominantly small irrigation systems in the flood-prone areas around Lake Chad are polders. In Chad, they show several small polders along the eastern coast of Lake Chad and that in the traditional polders, one crop is grown per year. If dams and pumps are used, up to three crops per year can be grown. In Cameroon they show the SEMRY rice polders along the Logone River, in Niger, the Bultungur polder located near the lake, and in Nigeria, the Bagapolder and the Kirinowa polder.
