WATER RESOURCES AND CLIMATE OF AFRICA

Rivers, Lakes, and Wetlands

The water systems of Africa are extremely diverse, a reflection of the continent’s great range of climate and physical geography. These systems vary from region to region, and from season to season and year to year as well. 

Major River Systems

Africa has several of the world’s greatest rivers. The Congo, which alone accounts for some 38 percent of the continent’s discharge into the ocean, drains an area of more than 4.1 million sq km (1.6 million sq mi), ranking second only to South America’s Amazon River in terms of discharge and size of drainage basin. The Nile, which extends for 6,695 km (4,160 mi), is the world’s longest river; it occupies the fourth largest drainage basin. Other important rivers include the Niger in West Africa and the Zambezi in southern Africa.

The seasonal occurrence of rainfall in most of Africa often results in highly variable river flows. A river may have virtually no discharge in the late dry season followed by severe flooding in the rainy season or early dry season. Few of Africa’s rivers have the relatively constant, year-round discharge of the Congo. Great seasonal fluctuations in discharge create challenges, both in limiting flood damage and in using rivers for irrigation, hydroelectric power generation, and navigation. Several major dams and reservoirs have been constructed, especially during the 1960s and 1970s, to address these problems. Today, dams regulate a greater proportion of total river discharge in Africa than on any other continent.

Africa’s river systems reflect the continent’s unique physical geography. One-third of its area consists of inland basins, such as the Lake Chad and Kalahari (Okavango) basins, where rivers and streams never reach the ocean. Other major river systems, notably the Nile, Niger, and Congo rivers, have large inland deltas in midcourse, indicating that the upper portions of these rivers’ drainage basins were also landlocked at some point. Several major rivers, including the Congo, Zambezi, and Orange, pass through narrow valleys and drop sharply as they cross escarpments fringing the continent. In its lower course, the Congo drops 270 m (886 ft) through a series of 30 rapids and waterfalls. River courses such as this provide ideal conditions for hydroelectric power generation. Africa has about 40 percent of the world’s hydroelectric potential, but only a small proportion has been developed.

Lakes

Africa’s many lakes have diverse characteristics. They include deep lakes of tectonic origin (such as Lakes Malawi and Tanganyika in East Africa) and shallow lakes located at the center of drainage basins (including Lake Chad in West Africa). Lake Victoria, in East Africa, is the world’s third largest lake by area, while Lake Tanganyika is the world’s second deepest lake and third largest by volume of water. Africa’s natural lakes have quite distinct ecologies: Lakes located close to each other often vary significantly in both abundance and types of fish and plant species. Most lakes contribute significantly to their regional economies, particularly as a source of fish.

In drier regions, several lakes that lack natural outlets have high concentrations of mineral salts, or are actually saltpans that only contain water for part of the year. Some of these salt deposits are mined, among them the commercial soda works of Lake Magadi in Kenya and the centuries-old salt and natron extraction sites in pans in the Sahara.

In addition to natural lakes, Africa has several large artificial lakes that were created by damming major rivers. These reservoirs include Lake Volta on the Volta River, Kainji Lake on the Niger, Lake Kariba on the Zambezi, and Lake Nasser on the Nile. Most of these projects were designed to generate hydroelectricity and, in some cases, to promote irrigated agriculture. A significant fishing industry has developed around some of the artificial lakes, especially Lake Volta. The new lakes flooded settlements and farmland—as well as vital floodplain ecosystems—in valley bottoms. The loss of annual floods due to damming has had a variety of ecological and economic impacts. Annual summer flooding of the Nile once deposited rich sediment along its banks, creating fertile farmland in Egypt for millennia. With the construction of the Aswān High Dam in the 1960s, most silt was deposited in the newly created Lake Nasser, drastically reducing sedimentation and soil fertility downstream.

Wetlands

Africa’s freshwater wetlands come in several forms. They are often located in floodplains, low-lying areas adjacent to rivers that are subject to annual or periodic flooding. Freshwater marshes, such as the huge As Sudd in Sudan, are usually associated with riverside locations, and are dominated by herbaceous species such as papyrus. Swamp forests occur in floodplains or lakeside environments, most notably in the Middle Congo River Basin. This area contains about 8,000 sq km (3,000 sq mi) of permanently or seasonally flooded swamp forest.

Wetland areas play a vital hydrological and ecological role in Africa. They trap and slow seasonal floods, dampening the magnitude of floods downstream and spreading out peak flows over several weeks or months. The delay and extension of flood peaks can facilitate downstream fishing and irrigation, especially in areas with an extended dry season. Wetlands also provide habitat for numerous species of animals and plants, many of them unique to these ecosystems. Wetlands near the edge of the Sahara provide vital staging grounds for migratory birds preparing to cross the desert. Wetlands also trap and hold silt carried by rivers, creating fertile alluvial soils that may be used to grow crops such as rice, cotton, and vegetables. However, agricultural development schemes, taking advantage of the presence of both fertile soil and water, pose a threat to many wetlands.

Botswana’s Okavango Delta is one of Africa’s largest and most unique wetlands. The Okavango River, with its source in the highlands of Angola, forms a huge, swampy inland delta as it approaches the Kalahari Desert. During the annual floods, the swamp doubles in size. Although it has several outlets, virtually all of the water entering the Okavango Delta evaporates or is absorbed into the sandy subsurface. The Okavango supports a rich indigenous flora and fauna, and attracts huge numbers of migratory wildlife during the dry season. Like many others, this vital and sensitive wetland ecosystem is threatened by the growth of ranching and tourism, and by proposals to divert water for irrigation and other uses.

Climate

Lying between latitudes 37° north and 35° south, Africa has virtually the same climatic zones in the Northern Hemisphere as in the Southern Hemisphere, and they are arranged symmetrically on either side of the equator. The zones are determined mainly by latitude, except in the east where highlands greatly modify the climate. Africa is the most tropical of the continents: Only its northern and southern extremes are directly influenced by mid-latitude westerly winds and are considered to have temperate climates.

Climatic Zones

Most of Africa lies between the Tropic of Cancer (in the north) and the Tropic of Capricorn (in the south) and has high temperatures throughout the year. The amount, duration, and seasonal distribution of rainfall is therefore the most important factor differentiating its climates. Africa has six types of climatic zones: tropical wet, tropical summer rainfall, semiarid, arid, highland, and Mediterranean.

Tropical Wet

Tropical wet climates, also called equatorial climates, occur close to the equator in West and Central Africa, and in eastern Madagascar. Rainfall is high, typically exceeding 1,500 mm (60 in) per year and 3,200 mm (130 in) in some places. Rainfall occurs in every month, and many areas experience especially rainy periods in the spring and in the fall. Temperatures remain high throughout the year, averaging more than 27°C (81°F) annually, and rarely falling below 21°C (70°F).

Tropical Summer Rainfall

Tropical summer rainfall climates, also known as tropical savanna climates, occur north and south of the tropical wet zone, in much of West Africa and southern Africa and most of Madagascar. This climatic zone is marked by a well-defined dry season of three to eight months. Annual rainfall is usually between 500 and 1,500 mm (20 and 60 in), although limited areas have considerably more—for example, Freetown, Sierra Leone, averages 3,800 mm (150 in) per year. The tropical summer rainfall zone is a transitional zone between tropical wet and semiarid zones, so there is a progressive decline, moving poleward, in total rainfall and the duration of rainfall. Areas with a longer rainy season tend to have two rainy periods separated by a short dry spell, while areas with a shorter rainy season have a single rainy period. Temperature ranges in the tropical summer rainfall zone are slightly higher than in the tropical wet zone, and increase with distance from the equator. In the northern section of this zone, daily high temperatures average more than 30°C (90°F) over the course of the year. Temperatures in the southern and eastern sections of this zone tend to be cooler because of higher altitudes.

Semiarid and Arid

Surrounding the tropical summer rainfall zone are areas of semiarid and then arid climates in north central Africa, east central Africa, and southern Africa. The semiarid, or hot steppe, zone has a short rainy season of up to three months with about 250 to 500 mm (10 to 20 in) of rain per year. Precipitation is unreliable and scarce, creating difficult conditions for plant growth. Temperatures vary in the semiarid zone, with average daily highs ranging from 25° to 36°C (77° to 97°F). Africa’s arid desert regions receive little rainfall. Although classified as hot deserts, these regions have significant annual variations in temperature, and extreme fluctuations in temperature over the course of a day. In the Sahara, daytime summer temperatures can exceed 50°C (120°F), and winter nighttime temperatures can drop below freezing.

Highland and Mediterranean

Tropical highland climates are common in much of East Africa. Temperatures in the highlands of Ethiopia and Kenya average 16° to 21°C (60° to 70°F), on average about 5 Celsius degrees (9 Fahrenheit degrees) cooler than the lower plateau areas of Kenya, Uganda, and Tanzania. In most parts of the world, higher elevations receive higher levels of precipitation, but the highlands of East Africa are an exception, experiencing rather low levels of rainfall. However, the highest mountains and the southeastern flank of the Ethiopian plateau receive greater precipitation on their windward slopes.

The coastlands of the Cape region of South Africa and the North African coast from Morocco to Tunisia have Mediterranean climates. These areas have mild, rainy winters followed by a prolonged summer when conditions are warm and dry. They receive between 250 to 1,000 mm (10 to 40 in) of rainfall per year.

Major Climate Controls

Several factors influence the climate of Africa, determining the continent’s climatic regions, creating seasonal variations, and altering day-to-day weather. The most important climate control is related to atmospheric conditions and wind patterns; other major controls include topography, ocean currents, and clouds and other airborne material.

Seasonal Shifts of Winds, Atmospheric Pressure, and Air Masses

Air masses moving over Africa have different characteristics depending on their areas of origin. Air originating over the tropical oceans, known as maritime tropical air, is warm, moist, and unstable. When this moisture-laden air is forced to rise and become cooler, condensation and precipitation often occur. Conversely, the hot, dry air that originates over the African continent (continental tropical air) is incapable of producing rain, even if it is forced to rise.

Over most of Africa, air moves toward the equator. Northern Hemisphere winds from the northeast and Southern Hemisphere winds from the southeast converge at the intertropical convergence zone (ITCZ), a low-pressure zone centered on the equator. Air rises at the ITCZ, spreads out, and descends in the north and south, in subtropical high-pressure zones centered on the Tropic of Cancer and Tropic of Capricorn. After descending, the air flows outward, either poleward into temperate regions or back toward the equator.

The ITCZ constantly shifts its position, moving north of the equator during the Northern Hemisphere’s summer, and south of the equator during the Northern Hemisphere’s winter. This shift brings about seasonal changes, notably in the tropical summer rainfall zone. During the summer season (July to September in the Northern Hemisphere and December to February in the Southern Hemisphere), moist maritime air is drawn in toward the ITCZ and produces rainfall when it is forced to rise. During the winter season (December to February in the Northern Hemisphere and July to September in the Southern Hemisphere), when the ITCZ is centered in the opposite hemisphere, dry conditions prevail due to the dominance of hot, dry winds coming from the subtropical high-pressure zone. These global, seasonal shifts in wind and pressure zones also bring about the seasonal changes that characterize the Mediterranean climates of Africa’s northern and southern margins.

These seasonal shifts have a comparatively minor impact on rainfall patterns in equatorial and desert regions. The mid-Sahara receives little rain, even in summer, because the ITCZ seldom advances that far north. On the other hand, equatorial regions lack a well-defined dry season because they continue to benefit from moist, maritime air throughout the year. Their position near the equator also ensures that temperatures remain uniformly high year-round. In contrast, Africa’s desert regions experience significant temperature shifts seasonally, a reflection of their higher latitudes.

Most of the continent receives moisture from air originating over the Atlantic Ocean. In eastern Africa, however, rainfall south of the equator comes from large tropical cyclones originating over the Indian Ocean during the Southern Hemisphere summer. Rainfall from these cyclones is particularly high in eastern Madagascar and the coastal mainland between South Africa and southern Tanzania. North of the ITCZ—centered during January over Tanzania—East Africa receives little rain because its air originates in dry, mainland Asia.

Altitude and Relief

Africa, unlike other continents, has no great mountain ranges to impede or channel atmospheric circulation. However, altitude is an important influence on temperatures, notably in the high plateaus and highlands of southern and East Africa. Highland regions have lower temperatures throughout the year, compared to lower-elevation areas at the same latitude. Highlands also influence rainfall patterns: Moisture-laden air forced to rise over a mountain barrier produces more rainfall on the windward side than on the leeward side.

Ocean Currents

Ocean currents affect the climate of the Atlantic Coast from South Africa to southern Angola, and from southern Morocco to Mauritania. The former is affected by the Benguela Current, while the latter is influenced by the Canaries Current. These cold ocean currents result in somewhat cooler temperatures, very low rainfall, and the frequent occurrence of fog near the coast. Elsewhere, an area of surprisingly low rainfall along the coast near Accra, Ghana, has been attributed to localized upwelling of cold water.

Clouds and Atmospheric Dust

Clouds help to reduce daily temperature fluctuations by reflecting solar radiation away during the day, and by slowing the loss of heat at night. The lack of cloud cover in Africa’s desert regions—and during the dry season in other climate regions—results in substantial day-to-night fluctuations of temperature.

The Sahara is estimated to generate 300 million metric tons of airborne dust each year, 60 percent of the worldwide total. During the dry season wide areas south of the Sahara are affected by the harmattan, dust-laden winds originating in the desert. Typical episodes last for three to five days, with a dusty haze obliterating the Sun, lowering temperatures, and sometimes reducing visibility to a kilometer or less. The frequency and intensity of the harmattan varies; regions near the desert margins are often affected for 20 to 30 days per year. Dust originating over the Sahara also affects North Africa, southern Europe, and the Arabian Peninsula. The hot, dust-laden winds that occur in North Africa between February and June are known collectively as sirocco, and locally by a variety of names (for example, khamsin in Egypt).

Climate Change

The Sahel—the semidesert transition zone between the Sahara and the wetter tropical areas to the south—suffered from a severe drought from the late 1960s to the early 1980s. Scientists initially interpreted the drought as a new phenomenon in which reduced rainfall and inappropriate human use of the delicate environment were causing the desert to expand relentlessly. After greater research, however, many scientists now believe that the Sahelian drought is part of a long-standing cycle of change.

The study of environmental change in the Sahel and in adjacent regions shows that the margins of the Sahara have shifted during the last 20,000 years. Some 18,000 years ago, glaciers covered much of northern Europe, and global climate zones shifted to the south. The boundary of the Sahara was far to the south of its present location, passing through southern Senegal and central Nigeria. Large areas of sand dunes in the Sahel demonstrate that the climate was formerly much drier than today. With the melting of the glaciers in Europe, the Sahara’s southern margins shifted to the north as the climate became warmer and moister. About 9,000 years ago the desert had shifted far north of its current margin. Evidence of this shift includes sedimentary deposits indicating that Lake Chad was much larger than today, and that other large lakes existed in now-dry Saharan basins. Many vivid rock paintings in the central Sahara show savanna animals and people herding livestock more than 3,000 years ago. Since then, the climate has become progressively drier, the desert margin has moved south, and most of the lakes and wildlife have disappeared.

Other climate changes in the Sahel have occurred over the past 500 years or so. These changes, especially in the amount of rainfall, have tended to be cyclical: a decade or two of poor rains, followed by moister conditions, followed in turn by drought. Evidence of these fluctuations comes from written and oral accounts of droughts and famines, archaeological evidence, and the analysis of fossils.

In addition, meteorological records show cycles of increasing and decreasing rainfall just since the early 20th century. For example, the Sahelian drought was preceded by two decades of far-above average rainfall, and was followed by several years of average rainfall. Records also suggest an overall downward trend in average rainfall since 1900. Scientists have suggested that the clearing of vegetation and global warming may account for this apparent trend. They argue that the removal of vegetation causes higher surface temperatures, increased evaporation, and reduced rainfall.