Africa

B

3

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.

C

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.

---

---

C

1

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.

C1

a

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).

C1

b

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.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain