Lake Chad

Lake Chad in African humid period (blue) and in 20th century (green)

The Chad Basin was formed by the depression of the African Shield. The Chad Lake located in the center of the basin is the remnant of the Quaternary ancient Chad Sea.[5][6][7] Its area experienced four heydays between 39,000 BC and 300 BC, leaving thick diatomaceous earth and lacustrine deposits in the strata.[5] The largest area in history is about 340,400 square kilometers, the volume is about 13,500 cubic kilometers, the maximum depth is about 160 meters, and the lake is about 325 meters above sea level. It flows into the Benue River through the Mayo Kébbi, and finally flows into the Atlantic Ocean through the Niger River.[8]

The Chad Basin contains the earliest evidence of ancient human habitation found so far in West Africa. The Lake Chad area was settled as early as 500 BC, and major archaeological discoveries include the Sao civilization.[5] According to the records of Claudius Ptolemy, the Roman general Septimius Flaccus led an expedition to the Sahara Desert in 50 AD, crossed the Tibesti Mountains, and reached the north of Lake Chad, known as "Hippo and Rhino Lake". The merchant Julius Maternus led an expedition through the same route around 83 AD and brought a rhino back to Rome from Lake Chad.[9][10]

Kingdoms arose around Lake Chad during the Middle Ages, and the Lake District became a refuge for tribes that refused to assimilate into these kingdoms. Lake Chad is recorded in many Arabic writings from the 9th to the 14th centuries due to the expansion of Islam into sub-Saharan Africa and the increased Arab interest in geographical exploration. Following the growing interest in Africa among European academic and business communities, the Lake Chad area was extensively described by Europeans in the 19th century, and three scientific expeditions to Lake Chad were conducted between 1898 and 1909.[5]

Chari river map

Lake Chad is divided into north and south parts by a natural dam, with the bottom of the northern basin at an altitude of 275.3 meters and the bottom of the southern basin at 278.2 meters. When the water level in the south exceeds 279 meters above sea level, it will flow into the north.[11] In the south, there is continuous open water at the mouth of the Shali River, and the western part of the water is covered by reed swamps,[12] and the sand dunes that are not completely submerged in the eastern waters form an archipelago.[7] The average depth of the southern lake basin is between 0.5 and 2 meters, that of the northern lake basin is between 0 and 1.8 meters, and that of the eastern archipelago is between 0 and 2 meters.[13]

The Lake Chad basin covers an area of about 1 million square kilometers, and is injected by the Shari River, Engada River, and Yobe River.[7][5] The water supply of the lake is seasonal. Most of the precipitation comes from the Adamawa Plateau in the south of the basin, which is transported to the lake basin through the Shari River and the Logone River. The two contribute 95% of the total inflow of Lake Chad, while the Yobe River only contributes less than 2.5%. The part is only slightly salty.[14][6]

The average annual precipitation in the Lake Chad area is 330 mm, with an average annual precipitation of 560 mm on the south bank and about 250 mm on the north bank. The highest temperature in the rainy season is 30°C, and the highest temperature rises to more than 32°C when October and November enter the dry season. The temperature difference between day and night is almost twice that of the rainy season, and the lowest nighttime temperature sometimes drops to 8°C in December and January. April is usually the hottest month of the year, with temperatures occasionally reaching 40°C, the lowest water levels appear in June to July, and the highest water levels in November to December, with surface water temperatures ranging from 19°C to 32°C.[5][7]

Lake Chad now

The Lake Chad Basin is one of the major river basins most affected by climate change in the world. Small changes in atmospheric circulation will have a great impact on the rainfall in the Lake Chad Basin. In addition, Lake Chad is an inward shallow lake basin.[15][16] Dry climate due to vegetation loss from overgrazing and deforestation and large-scale irrigation projects that diverted water from the rivers that feed the lake are the main reasons for the shrinkage of Lake Chad.[17]

In 1870, the area of Lake Chad was about 28,000 square kilometers. The lake was able to flow out of the Ghazal River during the rainy season. At the turn of the 20th century the area of Lake Chad shrank briefly, and reached a new high in the middle of the 20th century and overflowed from the Ghazal River again.[5] A major drought started in the Sahel region in the late 1960s and caused severe damage in 1972 and 1984. It was thought to be related to vegetation loss, global warming, and sea surface temperature anomalies.[15] During this period, Lake Chad shrunk considerably and fluctuated in the range of 2,000 to 5,000 km2 thereafter.[11]

From June 1966 to January 1973, the area of Lake Chad shrank from 22,772 square kilometers to 15,400 square kilometers,[17] further shrunk to 4,398 square kilometers in 1975,[11] and only 1,756 square kilometers in February 1994.[17] Since then, the area of Lake Chad has entered a relatively stable stage with a slight increase.[18] From 1995 to 1998, it fluctuated within the range of 1,200 to 4,500 square kilometers. The area once reached 5,075 square kilometers in 2000,[11] and the average area of surface water from 2013 to 2016 was about 1,876 square kilometers, with the largest area being 2,231 square kilometers in July 2015.[19]

Wetland of Lake Chad

Part of the Lake Chad Basin is located within the Chad Basin National Park in Nigeria, and the country and Cameroon have established the Lake Chad Ramsar Wetland with a total area of 8,225 km2.[20] The wetland plants in the south mainly include cyperus papyrus, etc. Reeds mainly grow in the north where the salinity is high, and the floating plant pistia sometimes covers large areas of open water. Plants such as hyparrhenia rufa grow on the shores of lakes with long floods in the south.[20] The area of permanent vegetation has increased from about 3,800 square kilometers in 2000 to about 5,200 square kilometers in 2020 as water levels have dropped and temperatures have increased.[21] The surrounding dense woodland has been converted to open forest with acacias, baobabs, palms and Indian jujube.[5]

Lake Chad is permanently or seasonally inhabited by hundreds of species of birds such as northern shoveler, Egyptian goose and marabou stork.[5] It is an important wintering ground for European anatididae and wading birds. There are raptors such as steppe eagle and booted eagle on the lakeshore,[20] and more than one million ruff can be observed on the lake at one time.[22] The once common large mammals include red-fronted gazelle, dama gazelle, patas monkey, striped hyena, cheetah and caracal, while African elephant, otter, hippopotamus, sitatunga and kob are distributed in the wetlands. At present, most of the large mammals have been hunted to extinction, replaced by a large number of cattle.[20]

The entire Lake Chad Basin has 179 species of fish, of which 127 are the same as the Niger River Basin, 85 are the same as the Nile River Basin, 47 are the same as the Congo River Basin, and 84 fish species are distributed in the lake.[6] The seasonal influx of floods combined with seasonal increases in air temperature leads to decreased salinity, increased turbidity, and increased trophic levels, which catalyzed a surge in the number of phytoplankton and zooplankton, allowing large fish to migrate seasonally within the watershed to feed and breed in the fertile floodplain when floods arrive.[12]

Lake Chad replenishment project

Lake Chad in a 2001 satellite image, with the actual lake in blue. The lake lost more than 90% of its surface area between 1987 and 2005.[23]

There are more than 30 million residents in the Chad Lake Basin. There are more than 70 ethnic groups around the lake, most of whom are distributed on the south bank where the population density exceeds 100 people per square kilometer. They rely on the water source of Chad Lake for irrigation, breeding, animal husbandry and drinking.[14] Since the drought in the 1970s, the soil that can be planted without irrigation and fertilization has been exposed at the bottom of the lake, and it has been reclaimed as a polder for planting corn, cowpea, rice, sorghum and other crops.[24] Farmers have shifted from planting mainly dry crops, such as wheat, to rice with high water demand, resulting in more serious soil salinization and water eutrophication.[14]

At the same time, the conflicts between countries and ethnic groups competing for water and land are also escalating, and the four countries along the lake are all facing the problem of extreme poverty.[14] The dams built on the upper reaches of the rivers entering the lake changed the time and scope of seasonal floods and disrupted the migration of fish, resulting in a sharp reduction in the populations of Alestes baremoze and Nile perch, the main catches of Lake Chad, and a significant reduction in the catch.[22][6] Due to the difficulty in meeting their livelihoods, some local residents have been involved in drug and arms trade, and even spawned terrorist organizations such as Boko Haram.[14]

Cameroon, Niger, Nigeria and Chad established the Lake Chad Basin Commission on 22 May 1964. The Central African Republic joined in 1996, and Libya joined in 2008. The headquarters of the committee is located in Njanna, Chad. The commission's tasks include managing Lake Chad and its water resources, protecting the ecosystem, and promoting regional integration, peace, security, and development in the Lake Chad region.[25] The surrounding countries' water replenishment plan for Lake Chad includes the construction of a 2,400-kilometer canal to transport 100 billion cubic meters of water from the Congo River Basin to the Chari River Basin every year, and use a series of dams along the route to generate electricity.[26]