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Le développement du lac Tchad / Development of Lake Chad

 | 
Jacques Lemoalle
, 
Géraud Magrin

I. Analysis

1. Water Resources and Ecosystems

Texte intégral

1The threat of Lake Chad drying out casts a shadow over its future. The variations in the Lake’s surface over the last 50 years or so have in fact been spectacular and are clearly visible in a series of widely distributed satellite images. However, for nearly 40 years, the hydrological data and field observations concerning the Lake have been insufficient and, for the most part, relatively difficult to access. This discrepancy between a plentiful supply of images and a lack of field data is most likely responsible for some of the many inaccurate interpretations published in the scientific literature or intended for the general public. The result is a great deal of misunderstandings about Lake Chad and a measure of pessimism and fear-mongering that does not stand up to scrutiny (see I-1).

2The first section of this document provides an overview of current knowledge about the Lake’s hydrology. It also sets out to answer a number of questions concerning its current state and the tendencies that characterise this Lake’s ecosystem.

The various states of Lake Chad: frequent changes

3There is a major misunderstanding of what is the “Lake’s surface”. It is believed (consciously or otherwise) that a lake only consists of open water. Then, marshlands are often forgotten when estimating the Lake’s surface. This results in different appreciations of a single situation, depending on whether marshlands are included or not. However, marshlands are regularly flooded and so they must be considered as aquatic ecosystems; they contribute to fish production. That’s why, for the residents of the Lake, marshlands belong to the Lake. So this is how we will understand the Lake in this study.

4The most distinctive feature of Lake Chad is its variability. The Lake has experienced a long history of wet and dry periods covering several timescales, ranging from the geological to the annual to the seasonal. Over the last few centuries, Arab geographers and European explorers have described the Lake’s various states. Based on these descriptions, Tilho (1928) proposed the following classification of the Lake’s three main states: “Little Chad”, “Normal (or Medium) Chad”, and “Great Chad”. These different states are all directly dependent on variations in rainfall over the Chari basin.

5At the beginning of the 21st century, Lake Chad was a Little Chad, made up of four main entities: an area of open water area of approximately 1,700 km2 just north of the Chari River Delta, an area of marshland in the southern basin that was flooded most of the time, a belt of marshland across the northern section which was irregularly flooded, and a perimeter zone which dried out in the early 1970s during the transition from Normal Lake Chad to Little Lake Chad (Figure 1). The Lake’s surface water and landscapes changed dramatically during this transition period. But contrary to information that has been published in the press and on the Internet, the Lake is not in the process of shrinking or disappearing. It is simply in its Little Chad state, as it has been several times over the last 150 years, and, notably, as it was represented in 1908 on a map drawn up by the Tilho mission.

Figure 1. Schematic map of Lake Chad’s average situation around 2010.

Figure 1. Schematic map of Lake Chad’s average situation around 2010.

Source: from Lemoalle (Atlas du lac Tchad, forthcoming)

6Lake Chad is made up of two basins—one in the north and one in the south—that are separated by a shallow mid-section, the “Great Barrier”. The Chari River is the primary inflow of the southern basin. The rest of the Lake’s water comes from direct rainfall and a number of small tributaries, including the Komadougou Yobe in the northwest and the El Beïd River to the south.

7During the wet periods, the Lake’s water level is high enough for the Great Barrier to be permanently submerged. The Lake then consists of a huge single body of water bordered by an archipelago on its eastern shores. During the relative dry periods, the Lake’s water level is low and the Great Barrier emerges, separating the two basins—whose levels vary independently—for long periods of time. When the northern basin is no longer fed by water overflow from the southern basin, it can completely dry out. As a whole, the Lake’s landscape transforms from almost an inland sea to a huge area of marshland.

Great (Lake) Chad

8The Great Chad state is characterised by a huge area of open water which extends over 24,000 km2. It is bordered by a relatively undeveloped dune archipelago. To the east, the Lake overflows into the Bahr el Ghazal River, which leads to the Bodélé Depression located 500 km northeast and approximately 120 m lower than the Lake. The threshold beyond which the Lake’s waters flow into the Bahr El Ghazal is at an altitude of 282.3 m.

9The Lake was only in its Great state for a few brief periods in the 20th century, the last time being during the wet years of the mid-1950s. The landscapes that characterise Great Chad are similar to those that characterise Medium Chad, with larger areas of open water.

Medium (Lake) Chad

10When the Lake is in its Medium state, it is comprised of only one body of water throughout the year. This covers an area measuring between 15,000 and 19,000 km2 at an altitude of 280 to 282 m. There are two large basins, south and north, separated by a neck and the shallow waters of the Great Barrier which remains submerged. An archipelago, made up of a long stretch of fossil dunes, is gradually embedding itself into the Lake from the northeast. Medium Chad, sometimes referred to as Normal Chad, is characterised by stretches of open water, by navigable reaches between the islands that make up the archipelago, and by a thin strip of vegetation along the banks.

11When the Lake is in its Medium state, it comprises a well-developed archipelago and stretches of open water covering 4,000 to 6,000 km2 in each of the two basins. The centre of the northern basin is 5.3 m deep, while the centre of the southern basin is 2.7 m deep. The basins’ average depths are 2.1 and 3.5 m, respectively, with water levels of 280 and 282 m. The Lake is in its Medium state when inflow from the Chari River is between 34 and 43 km3/year. Then the inflow compensates water losses, which is mainly due to evaporation (approximately 2.2m/an) of the water –covered surface of 18,000 km2.

12Because of variations in climate, the transitions from Medium to Great Chad are interspersed with phases where the water level is low. There have been three Little Chad phases since the start of the 20th century, the first of which (1904-1915) was described in detail by Tilho. Details of the second phase, which occurred in around 1940, have only been passed down by word-of-mouth. The last Little Chad phase began in 1973 and the Lake has been in this phase ever since.

Little (Lake) Chad

13The whole Lake is more marshland than lake in the conventional sense of the term. It is made up of several bodies of water that are separated by shallows for at least part of the year. The Great Barrier functions as the main threshold, separating the northern and southern basins.

14There is an area of open water of approximately 1,700 km2 in the southern basin just north of the Chari River Delta, the water level of which is between 279 and 281 m high. It is surrounded by huge areas of marshland which are often ignored when the Lake’s surface area is calculated because they are less visible on satellite images. However, these areas are flooded rather regularly. The northern basin is separated from the southern basin by the Great Barrier which is more or less permanently visible and which influences the hydrology in this region of the Lake. The Lake is in its Little Chad state when annual inflow from the Chari River is less than 34 km3/year. The Lake is then covered by between 2,000 and 13,000 km2 of permanent or seasonal marshland.

Dry Little (Lake) Chad

15A new classification has recently been defined to refer to a Little Chad state in which the northern basin is not fed by the southern basin, and so remains dry throughout the year. This state occurs when annual inflow from the Chari River is less than 15 km3/year. This phase differs from the Little Chad state described above mainly in the way it affects the northern basin. It affects the vegetation, which is different from the vegetation in the southern basin, the other natural resources, and consequently the lives of local people. During a Dry Little Chad period, no fishing is possible in the northern basin. There is little in the way of livestock farming and agriculture and it even becomes difficult to access drinking water.

16Between 1957 and 2008, the Lake was in a Little Chad or Dry Little Chad state two-thirds of the time and in a Medium Chad or Great Chad state for the remaining third. The Lake was in its Dry Little Chad state (northern basin dry throughout the year) in 1985, 1987, 1988, and 1991. In 1975, 1977, 1982, 1984, 1990, 1992, 1993, and 1994, the northern basin was dry for some of the year—causing fishermen to temporarily take up farming. The northern basin retained some of its water throughout the year in 1989 and then between 1995 and 2013 (the year this document was written).

Table 1. Characteristics of the various states of Lake Chad.

Table 1. Characteristics of the various states of Lake Chad.

A few features of the Lake in its current state

Why the Little Chad state?

17The Lake’s water level and surface area result from the overall balance of water inflows (from direct rain and from rivers) and losses (evaporation and infiltration into groundwater tables). Main inflows are from the Chari River (85% of the total), which in turn are dependent on rainfall over the basin. The Lake’s other tributaries (Komadougou Yobe, El Beïd, Yedseram) provide less than 10% (the remaining inflow coming from rainfall on the Lake). It has been seen that when rainfall in the Chari basin varies by 10%, the Chari River’s annual flow varies by 30%. This results in a proportional variation in the Lake’s surface area. The Lake therefore amplifies variations in rainfall in its basin (Table 2).

Table 2. Average rainfall over the basin and the Chari River’s flow rate since 1950.

Table 2. Average rainfall over the basin and the Chari River’s flow rate since 1950.

Source: data from the Water Resources and Meteorology Department, Chad and Cambridge research unit (CRU, United Kingdom).

Figure 2. A composite image of Lake Chad close to the peak flooding in 2013.

Figure 2. A composite image of Lake Chad close to the peak flooding in 2013.

Source: images taken by Landsat 8 between 12 April and 23 May 2013, provided by NASA, formatted for the purposes of this study by Pierre Don-Donné Goudoum.

18The Lake’s current Little Chad state has resulted from the transition from a relatively wet period (1950-1970) to a period of drought in the Sahel and the Chari River basin (1970-1990), particularly exacerbated during the very dry periods from 1972-1973 and 1983-1984. Generally speaking, annual rainfall has fallen by approximately 150 mm in the whole basin and the isohyets have shifted south by 150 km. The Chari River’s annual flow rate has fallen below 34 km3/year, resulting in the emergence of the Great Barrier and the transition to the Little Chad state.

19A patchwork of images generated by NASA’s Landsat satellites in April and May 2013 shows the Lake to be near its highest 2013 level. All the blue or black areas represent open water; the green areas represent flooded marshland. The total surface area covered by water—14,800 km2—is the largest it has been since the Lake transitioned into its Little Chad state in 1973 (Figure 2).

What impact do irrigation and other human uses have?

20The shrinking of the Lake and its transition into its Little Chad state have been incorrectly attributed by some authors to water being used for irrigation or even for watering cattle.

21According to a study carried out for the Lake Chad Basin Commission (LCBC, 2011), in 2010, a total of 2.5 km3 of water was drawn from the Lake and its tributaries and their alluvial sheets. Of this total, 0.5 km3 was used for drinking water, 1.8 km3 for irrigation and the remaining 0.2 km3 for watering cattle. This analysis included all of the private areas located along the bodies of water.

22The following conclusion can be drawn from this quantitative information:

  • total water consumption throughout the Lake Chad basin has so far only had a relatively small influence on the Lake’s water levels;
  • variations in the Lake’s water levels can mainly be attributed to variations in rainfall, at least until 2013 (the year this document was drafted);
  • the Lake’s major local hydro-agricultural zones in Nigeria (South Chad Irrigation Project and Baga Polder, a total of nearly 200,000 ha), which could consume more than 2 km3 per year if they were fully operational, do not operate when the Lake is in its Little Chad state and so are not responsible for any fall in water levels.

What impact does sedimentation have on the rivers and the Lake?

23The current widely-held view in N’Djamena is that the Chari River is silting up. People who fish or who travel across the Lake by boat consider that—at equivalent water levels—navigation is becoming difficult in many places because the water is not deep enough. This is sometimes attributed to the silting up of the Lake by stream sediment.

24There is not enough available data to be able to accurately respond to questions about silting in the Chari and Logone Rivers or the filling of the Lake itself. The fact that river levels are lower for longer periods of time during this current period of relative drought may explain the widespread perception of silting. The knowledge that we currently have does not justify dredging the river in the current climate conditions, nor in the event of waters being transferred from the Ubangi to the Chari River.

25The Lake is doubtless filling up with sediment—as is the case with all Lakes throughout the world. But the speed at which this is most likely happening, between 0.5 and 4 mm per year, would suggest that there is no risk of the Lake filling up within any meaningful human timescale (see I-5). If the water is indeed getting shallower, the difference is extremely negligible compared with variations in water levels: even in the most pessimistic of scenarios, it would take 100 years for the depth of the southern basin to decrease by 0.4 m, whereas the water level of the southern basin fell by nearly 3 m between 1964 and 2010.

26On the other hand, the development of vegetation along navigable stretches (between Baga-Sola, Bol, Baga-Kawa, Guitté, and the Chari River delta) is a real hindrance to transport on the Lake. This is due more to an increase in the amount of vegetation than to sedimentation and should be managed by regular maintenance carried out in concert with the affected populations. In certain areas, such as the Bol Archipelago, using the Lake for transport should be seen as a means of opening it up to the transport of heavy products (crops, natron, etc.).

What are the ecosystemic services and biodiversity trends?

27One of the questions which has been raised with respect to managing Lake Chad is which hydrological situation would be most useful to the local people who depend directly on its resources. One possible plan, proposed in the early 1990s, is to transfer water from the Ubangi to the Chari River, revitalising the Lake and restoring it to its Medium Chad state of the 1960s.

28The main advantage of the Little Chad state compared with the Medium Chad state is the greater annual water level differential (the drawdown range due to flooding and flood recession), resulting in larger areas that are especially fertile in each of the two basins and which can be used for livestock and crop farming, mainly in the southern basin (see I-1). It also means higher fish stocks in the northern basin (see below). Both of these processes, associated with the Lake in its Little Chad state, are obviously some of the variables that have resulted in an increase in population and food production in and around the Lake. However, when the Lake is in its Little Chad state, it can transition into a Dry Little Chad state at any time, which is the least favourable state for riparian societies, particularly those in the northern basin.

29The Lake’s plant and animal biodiversity depend on the diversity of its landscapes, which is itself determined by the levels of hydrological variability. The whole Lake is a particularly dynamic ecosystem. It features biotopes that can momentarily favour or hinder the development of certain species, though no irreversible changes having been identified to date. Over a period that comprises one or two transitional phases between the Little Chad and Medium Chad states, the changes undergone by the Lake can be considered intermediary disturbances, as understood by Wilkinson (1999), which in the long term help maintain its levels of biodiversity and biological productivity. A number of transitory phenomena can be seen in the short term, such as the major expansion of marshy vegetation in the southern basin, the development of which was helped by the relative stability of average water levels. An invasive species, Prosopis sp., has also emerged in the northern basin. It should be pointed out that the water hyacinth (Eichhornia crassipes), which has often been mentioned in various documents, has not yet been observed in the Lake.

30The transition from a Medium Chad with a Lake environment to a Little Chad with a marshy environment has significantly reduced the levels of biodiversity of fish stocks in the northern basin. But it is known that these stocks have been built up again from the Chari River in the past. As far as biodiversity is concerned, a few specific examples cannot be used to determine a particular trend or pattern of change. Before a suitable analysis can be carried out, a biodiversity monitoring plan has to be developed and implemented, based on quantitative data about various characteristic or representative species (see I-4).

What impact will climate change have on Lake Chad’s future?

31A number of people blame climate change for the possible disappearance of Lake Chad. Existing global climate change models are not sufficient to make predictions about future climatic changes in West Africa or the Lake Chad Basin by 2050, let alone by 2100.

32We are practically certain that average temperatures will have increased by approximately 2 to 3° C by 2050, which will have complex consequences for ecology and agriculture. However, although extreme rainfall patterns may well become more frequent, it is not possible to determine any mean trend. The Lake’s water and resource management policies must take this uncertainty into account.

What potential is there in groundwater?

33Using the groundwater beneath the Lake and its surrounding regions could help provide high-quality drinking water for people living in rural and urban areas and could better serve livestock and crop farmers than is currently the case (see I-2).

34The most accessible water table for Lake Chad and its surrounding area is the one which dates from the Quaternary period formations. Its depth relative to the ground varies by between 5 m—on the edges of the bodies of water, such as the Chari River and the Lake periphery (which feeds the water table using a process that remains unknown)—and 80 m in the centre of the piezometric cavities. Several piezometric cavities (depressions in the water table) have been identified in Chad (Chari Baguirmi), Cameroon (north of Kousseri and the Limani-Yagoua sand-ridge), Nigeria, and Niger (Kadzell). Research has shown that water table levels have been falling in these areas for a number of decades now; some wells and bore wells have dried out as a result. The water table extends across the region and, due to its accessibility, is the main usable resource for local populations. It generally contains freshwater (1 g/L) with high mineralisation in certain localised areas (up to 7 g/L) (LCBC/BGR, 2012).

35Available quantitative and qualitative data about groundwater is fragmented, and is sometimes limited by borders. This work does not allow their exploitable and renewable potential on the scale of the lake to be assessed. The water table is mainly fed by wet areas, river beds and the lake but quantities at stake are unknown (LCBC/BGR, 2012). The Pliocene water table, located at depths of 250 to 300 m, could be used to provide people with drinking water instead of the Quaternary formation in regions where the water has a very high fluorine content. The use of groundwater in farming is very irregular and not well documented.

A hydrological model to piece together the past and think about the future

36There are too many gaps in hydrological field data about Lake Chad gathered over the last few decades for us to have a precise idea of how its surface and water levels have changed over time. Bol is the only permanent station, but there has been no data about water levels in the northern basin since 1976. A scientific programme has therefore been developed to piece together information about how the Lake has changed using the small amount of available data and a hydrological model (Figures 3, 4, and 5). Furthermore, the model can be used to assess the impact of various climate change scenarios, the transfer of water from the Ubangi to the Chari River (see I-3), or changes in the structure of the Great Barrier.

Figure 3. Lake function and conceptual hydrological model.

Figure 3. Lake function and conceptual hydrological model.

Source: from Lemoalle et al., 2012.

Figure 4. Reconstructed changes in water levels (altitude expressed in metres) in the southern (Hs) and northern (Hn) basins of Lake Chad.

Figure 4. Reconstructed changes in water levels (altitude expressed in metres) in the southern (Hs) and northern (Hn) basins of Lake Chad.

Source: from Lemoalle et al., 2012.

Figure 5. Reconstructed changes in water surface areas in the southern and northern basins and in the whole of Lake Chad.

Figure 5. Reconstructed changes in water surface areas in the southern and northern basins and in the whole of Lake Chad.

Source: from Lemoalle et al., 2012.

37It can be seen that water levels in the two basins have evolved separately, signifying that one figure on its own is not enough to describe the Lake’s water levels. The average water level in the southern basin varied relatively little during the observed period of the Little Chad state, with seasonal variations that reached 2 m, while interannual variations in the northern basin were higher, including frequent periods when it dried out completely from 1980-1990.

38As far as the flooded areas are concerned, the average surface area of the southern basin varies relatively little. Much of the variation in the Lake’s total surface area—going from under 2,000 km2 during the low-water level of 1985 up to 14,000 km2 during the floods of 2000 and 2013—can be attributed to the northern basin.

39As well as providing a means for reconstructing water levels, this model has also been used to study the implications of various inter-basin transfer scenarios and other hydraulic works projects.

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