Lake Chad, Africa (Chad, Cameroon, Niger, Nigeria)
Figure 1. Shrinking Lake Chad comparing 1973 to 2017 (images courtesy of NSAS)
Lake Chad (French: Lac Tchad) is a historically large, shallow, freshwater endorheic lake in Africa, which has varied in size over the centuries (Figures 1 and 2). The lake is traditionally ranked as one of the largest lakes in Africa with a surface area that varies by season as well as from year to year. Lake Chad is mainly in the far west of Chad, bordering on northeastern Nigeria. According to the Global Resource Information Database of the United Nations Environment Programme, its water -covered area shrank by as much as 95% from about 1963 to 1998, with the lowest level in 1986, with an area of 279 sq. km, Since 1992 the lake level has been higher and stable, with seasonal fluctuations of around a metre (Pham-Duc et al. 2020).
Today, Lake Chad is a shallow lake (<3 m), subdivided most of the time into three areas, the northern and the southern pools separated by an east-west vegetation-covered sand barrier named "The Great Barrier" (Figure 1). The third region makes up the eastern part of the lake, called "The Archipelagos, corresponds to an area of flooded dunes with interdunal corridors that are variably inundated, according to the seasonal water levels in the lake (Figure 3).
The Lake Chad drainage basin covers ~2,500,000 sq. km, making up some ~8% of the African continent. It is a hydrologically closed (endorheic) drainage system in the Central Sahel region of northern Africa, characterized by a south-to-north climatic gradient as a consequence of latitudinally decreasing rainfall. Runoff and river discharge are generated predominantly in the southern portion of the drainage basin and transported into the lake sump mostly via the Chari/Logone river system, which supplies more than 90% of the surface inflow. The remaining water supply comes from the Komadugu-Yobe River and precipitation on the lake surface. Inputs are balanced by evaporation, estimated at >2000 mm/year, and by seepage to groundwater (Pham Duc et al., 2020).
Some 6000 years ago a Mega-lake occupied the Chad basin, with a water level that stood at 320 m. Mega-lake Chad occupied an area of 350,000 sq. km, about five times the size of Lake Victoria. This area included the Bodelé depression, located 500 km NE of Chad, with its lowest point at 166 m and connected to Chad through the Bahr el Ghazal (Figure 2). Prior to this high stage, the Chad sump was near desiccated. This is evident from the broad belt of entirely or partially flooded dunes, trending NW-SE, which make up the complex NE shore of the lake (see Figure 1). The dune field continues to the NE of the shoreline into the Kanem region, where remnant dunes isolate hundreds of interdunal depressions some 1-2 km in size.
Figure 2. Lake Chad sump at the southwest end of the Bodlele depression, which is the site of former Chad mega-lake (base image from NASA).
Figure 3. Interdunal water-filled corridors of the "Archipelagos” region of Lake Chad
The lake hydrology connects to an unconfined aquifer with an area of 500,000 sq. km and known as the Quaternary Phreatic Aquifer. Under present-day conditions, the aquifer is recharged by rainfall and seepage from the lake. This extensive subsurface reservoir ensures the chemical regulation of the lake and allows freshwater to persist in the sump despite the strong evaporative conditions. The Quaternary Phreatic Aquifer is separated from the underlying sedimentary aquifers of the Continental Terminal and Pliocene by a thick clay layer. These two continental sediment layers are separate aquifers in the Nigerian part of the basin and merge below Lake Chad to form a single aquifer with total thickness exceeding 275 m. They are confined and of artesian type. Deep aquifers groundwater are mainly exploited in northern Nigeria and eastern Niger.
A shrinking lake?
In the past decades, Lake Chad became a potent symbol to the environmental movement, offering a prime example of the effects of climate change, evidenced by dramatic shrinkage in the 1980s (Figure 1). Whilst Lake Chad shrank dramatically in the 1970s and 1980s, since 1992 lake water levels have been stable and fluctuate seasonally around 1 metre. That is, Lake Chad is not currently shrinking and has not done so for more than 30 years (Figure 4; Pham-Duc et al., 2020). Overall, taking account of the surface water extent of the northern and southern pool together, plus the total water storage, groundwater and soil moisture, the lake is actually in a period of slight expansion. It has been for the past two decades. Using detailed satellite imagery (as illustrated in Figure 4), Pham-Duc et al. (2020), found that Lake Chad's surface water extent slightly reduced over the last two decades; mostly in the northern pool due to the increase of evaporation and vegetation cover, as well as the decrease of the Komadugu-Yobe discharge. However, the southern pool extent is stable and even, slightly increasing, as a consequence of regular local rainfall and the increase of the Chari-Logone river discharge.
Figure 4 Changes in water level (1992-2020) replotted from downloaded satellite data (www.hydroweb.theia-land.fr)
Despite the slight decrease in the northern pool over the past few yers, Lake Chad's surface water storage shows an increasing trend (Pham-Duc et al., 2020). The subsurface contributes around 70% of Lake Chad's total water storage, meaning that most of its water is stored in soil moisture, and especially in groundwater, with the main Pliocene aquifer depth around 70 m below the lake sump. As a consequence, despite of the uncertainties in predicting the future climate, for now Lake Chad is not disappearing. When considering the amount of water stored in the groundwater reservoir accessible by pumping, with the recharge rate in the active basin, it represents today one of the best opportunity for buffering the huge interannual variability in rainfall that characterizes the current climate changes in Sahel (Pham-Duc et al., 2020).
Figure 5. Typical interdunal capillary sump underlain by a shallow water table ( Lake Chad edge)
Figure 5. Excavating natrun from an interdunal sump in the Mayala region (Lake Chad)
Evaporite variety in interdunal sumps
Interdunal depressions in and about the lake edged are floored by several meters of clays (kaolinite, montmorillonite, illite), diatomites, and calcite laid down presumably during the higher stages of the lake. They are now either dry, contain evaporites formed by capillary evaporation of shallow groundwater or they may contain a small, perennial lake (Figures 3, 5; Maglione, 1974; ). Water levels of the interdunal lakes close to Chad fluctuate with the level of the main lake but isotopic evidence shows that they are only partially fed by infiltration from Chad.
On the piezometric surface, Chad occupies a high, but the nearby Kanem is dominated by a even higher water table which reaches reaches above 300 m. This is presumably a remnant of the high stage of the lake. Isotopic data show that the interdunal depressions also receive a considerable amount of recharge from recent meteoric waters (Roche, 1973). Evaporation is intense and Lake Chad is losing yearly some 2.3 m of water, 90% of which is by surface evaporation.
Three saline environments were investigated in detail by Maglione (1974): (a) the interdunal lakes Djikare, Bodou, Moïla, Mombolo, and Rombou, (b) the salt pan of Liwa, which is an interdunal depression without standing water, but with a shallow water table, and (c) the groundwater and lake brines of two small lakes (Napal and Kangallom) located on an archipelago island.
Capillary groundwaters in small interdunal lakes close to Lake Chad are presumably controlled by the composition of the water of the main lake, while the groundwater in more distal interdunal sumps has distinctly higher Na/Cl and HCO/Cl ratios, presumably inherited from Paleochad. Servant (1973) also cites evidence for dissolution of evaporite lenses, as indicated by locally high groundwater salinities not connected to surface evaporation. Such deposits could have formed in interdunal depressions during the low stages preceding the 6000 year B.P. expansion. They may be responsible for some of the compositional variability of the groundwater in the Chad region. The small interdunal lakes further removed from Chad, such as lakes Bombou, Moïla, Mombolo, and others, have essentially the bicarbonate composition of the local groundwater.
In the Liwa sump, waters at the periphery have less than 1000 ppm dissolved solids, while at the center, 250 m away, the solutes rise to 300,000 ppm and the pH to 10.3 (Na-CO3-Cl brines; Maglione, 1974). A similar gradient exists vertically. Trona is the principal mineral of the natrun surface crusts which are harvested annually (Figure 5). Halite and nahcolite are also present and show a zonal distribution. Occasionally encountered are thermonatrite, gypsum, and natron . Within the unconsolidated muds underneath, a number of authigenic minerals formed through capillary evaporation. Most common are Mg-calcite, gaylussite, magadiite, and nahcolite (Figure 6). Tardy et al. (1974) reported the formation of Mg-montomorillonite in these same muds, and Maglione (1974) found mordenite. Most of the saline muds in this region muds are in a reducing environment and so sulfate is low.
A very different brine and mineral evolution was encountered by Maglione (1974) in saline flats on the island of Napal (Figure 1). This island, representing the crest of one of the many flooded dunes, has very gentle topographic depressions, 2-3 m deep, near its center, and two of these depressions are occupied by the small lakes Napal and Kangallom, which normally dry up yearly. The water table, obviously fed by Lake Chad, is 1.3-4.2 m below the surface and is recharged through a dune-sand aquifer. Evaporation in this well-aerated environment leads to loss of CO2 and calcite precipitation, sulfate is not reduced, and hence the residual brines are Na-Mg-Cl-SO4 brines. Precipitation of thenardite, halite, northupite, and bloediteresults. A surface crust forms, 15-20 cm thick, consisting mainly of finely laminated thenardite and some halite. Thus, this environment is in sharp contrast with the conditions at Liwa.
Figure 6. Magadiite from an interdunal sump in the Lake Chad region
Politics, religion and a “shrinking” lake
The Lake Chad region is the site of an ongoing humanitarian crisis and religious violence. It is the home ground of the Islamist fundamentalist group Boko Haram. Many say it is an obvious example of what happens in response to climate change. A Google® search of "Lake Chad and shrinking" will pull up multiple instances of such climate doomsday discussion, attributing the region's humanitarian problems to anthropogenic climate change. Many political groups and individuals, both local and international rush to chute blame to anthropogenic climate change. But climate change is an easy political scapegoat not requiring the addressing of the root of the societal dilemma, namely poverty and overpopulation that go hand-in-hand with tribalism and religious zealotry.
While lake Chad contracted significantly due to drought in the 1970s and 1980s, the lake is currently not shrinking. Its water storage volume is improving and has been doing so since 1992. The realities driving the region's humanitarian crisis are more complex.
In the 1970s and 80s, people followed the lake as its shores withdrew from their villages, going to new settlements to farm, fish and graze their livestock, while sending food back to relatives who stayed behind. People also moved from other parts of the Lake Chad countries to the lake, particularly parts of Nigeria, Cameroon and Chad bordering the southern pool which offered more reliable livelihoods. Overcrowding with finite resources leads to population stresses and conflicts.
As population centres became concentrated in areas of relatively high soil fertility, the local population around the lake grew fast as a result of this drought-related immigration and lower levels of child mortality. Between 1976 and today, the number of people living around the lake rose from 700,000 people to approximately 2.2 million people. This number is projected to reach 3 million in 2025, with some 49 million people likely to depend on the lake's resources (Vivekananda et al., 2019).
Ongoing political and religious conflict means the adaptive capacity of communities is being undermined by the large-scale displacement of people, restrictions to people's movement as a result of the conflict and weaker social cohesion after years of violence. Add to this the ongoing challenge of recruitment by armed opposition groups such as Boko Haram. This recruitment takes place in the context of stark social and economic inequality, perceived lack of state legitimacy, increasingly vulnerable livelihoods and the lure of financial incentives offered to potential recruits.
Then there is the heavy-handed military response to the violence that can further undermine the communities' resilience and their ability to adapt to climate change. Military measures taken by the region's governments in response to the violence have not addressed the root causes of the crisis. The military has at times had the opposite effect, undermining livelihoods and economic adaptation potential through blanket restrictions of access to certain areas, as well as damaging the social contract through human rights abuses and perceived impunity.
Solving the humanitarian crisis in the region first and foremost requires adequate funding of quality peacekeeping efforts, not politicians attributing blame to a "shrinking lake". Any longer-term climate variability will compound the population-induced stresses, as it undermines already fragile rural economies and livelihoods. Still, the root causes of the region's conflicts are overpopulation, lack of education, especially for women, tribalism and religious superstition.
As we see globally in all semi-arid areas with saline terminal endorheic sumps, water levels oscillate, and climate always fluctuates (Warren, 2016; Chapter 2). In any ecosystem containing a species that builds to the limits of its ecological niche, climate variability will exacerbate, not cause, cycles of "feast or famine."
References
International Crisis Group, February 2017: Fighting Boko Haram in Chad: Beyond Military Measures. Retrieved- 29 Nov. 2020, from <https://www.crisisgroup.org/africa/west-africa/niger/245-niger-and-boko-haram-beyond-counter-insurgency>, and Magrin, Géraud; Jacques Lemoalle and Roland Pourtier (eds.) 2015: Atlas du lac Tchad. Paris: IRD Éditions/Passages.
Maglione, G., 1974. Géochimie des évaporites et silicates néoformés en milieu continentale confiné. Doctoral Thesis, University of Paris, Paris, France, 331 pp.
Pham-Duc, B., Sylvestre, F., Papa, F., Frappart, F., Bouchez, C. and Crétaux, J.-F., 2020. The Lake Chad hydrology under current climate change. Scientific Reports, 10(1): 5498.
Roche, M.A., 1973. Traçage naturel salin et isotopique des eaux du système hydrologique du lac Tchad. Doctoral Thesis, University of Paris, Paris, France, 385 pp.
Tardy, Y., Cheverry, C. and Fritz, B., 1974. Néoformation d'une argile magnésienne dans les dépressions interdunaires du lac Tchad: Application aux domaines et stabilité de phyllosilicates alumineux, magnésiens et ferrifères. C.R. Acad. Sci., Sér. D, 278: 1999-2002.
Vivekananda, J., Wall, M., Sylvestre, F. and Nagarajan, C., 2019. Shoring up stability, addressing climate and fragility risks in the Lake Chad region. Report published by Adelphi Research Gemeinnützige GmbH, Alt-Moabit 91, 10559 Berlin, Germany.
Warren, J.K., 2016. Evaporites: A compendium (ISBN 978-3-319-13511-3). Springer, Berlin, 1854 pp.