Rainfall and rivers

Africa has virtually the same climatic zones in the northern and southern hemisphere, and these zones 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.

Figure 1: Map of rainfall zones Africa is predominantly a tropical region: only its northern and southern extremes, which are directly influenced by mid-latitude westerly winds, are considered to have temperate climates. Nevertheless, it is, after Australia, the driest region in the world (ECA 2000). The amount, duration and seasonal distribution of rainfall are the most important factors differentiating its climates. Africa has six climatic zones. These are equatorial, savannah, semi-arid, arid, highland and Mediterranean:

  • Equatorial climates occur close to the equator in the west and the central parts of Africa, and in eastern Madagascar. Rainfall is high, typically exceeding 1 500 mm per year and as much as 3 200 mm in some places. Rainfall occurs in every month, and many areas experience two rainy seasons per year.
  • Tropical savannah climates occur north and south of the tropical wet zone, in much of west and southern Africa, and in 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, although limited areas have considerably more: for example, Freetown in Sierra Leone averages 3 800 mm per year. The tropical savannah zone is a transitional zone between tropical wet and semi-arid zones, so there is a progressive decline, moving towards the poles, in total rainfall and the duration of the rainy season. 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.
  • Surrounding the tropical savannah climate zone are areas of semi-arid, and then arid climates in the west, north-central, east-central and southern parts of Africa. The semi-arid zone has a short rainy season of up to three months, with about 250- 500 mm of rainfall per year. Precipitation is unreliable and scarce.
  • Tropical highland climates are common in much of east Africa. 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 south-eastern flank of the Ethiopian plateau receive greater precipitation on their windward slopes.
  • The coastlands of the Cape in 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 and 1 000 mm of rainfall per year.

Most of the African continent receives moisture from air originating over the Atlantic Ocean. In the eastern part, 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.

Average rainfall has decreased since 1968, and has been fluctuating around a notably lower mean (UNEP 1999). In Africa, some 3 988 km³ of renewable water is available annually (ECA 2000). In recent years, the pattern of rainfall has tended to extremes, with the severity and frequency of droughts and floods increasing.

Africa has many rivers, but some, particularly in the mountainous areas, are seasonal and the water flow rates are gradually declining over the years.

The major rivers include the Senqu or Orange, Niger, Zambezi, Limpopo, Senegal, Congo and the Nile. The Nile is the longest river in the world, at about 6 679 km (Pickett 2000). The Nile starts from Lake Victoria and passes through Uganda, Sudan and Egypt to the Mediterranean Sea. The main headstreams, the Blue Nile and the White Nile, join at Khartoum in Sudan to form the Nile proper. The Senqu/Orange River is 2 092 km long and passes through Lesotho, South Africa and Namibia, flowing south-west, north-west, and west to the Atlantic Ocean (Pickett 2000). The Niger River is found in Western Africa, rising in Guinea and flowing about 4 183 km in a wide arc through Mali, Niger and Nigeria to the Gulf of Guinea (Pickett 2000). The Zambezi River is about 2 735 km long, rising in northwest Zambia and flowing south and west to the Mozambique Channel (Pickett 2000). The Limpopo River runs through South Africa, flowing about 1 770 km in a north-east-south-east arc to the Indian Ocean in southern Mozambique (Pickett 2000). The Congo River is 4 666 km long and flows through Congo and the Democratic Republic of the Congo to the Atlantic Ocean (Pickett 2000). The Congo, which alone accounts for some 38 per cent of the continent’s discharge into the ocean, drains an area of more than 4.1 million km², ranking second only to South America’s Amazon River in terms of discharge and size of drainage basin (Microsoft Encarta Online Encyclopaedia 2005). The Congo River basin alone holds almost 30 per cent of Africa’s total fresh surface water reserves and the world’s largest hydropower potential in any one single river basin (ECA 2000). Africa’s rivers provide extensive hydropower potential including for large (> 500 MW), medium (> 10 MW, < 500 MW) and small (< 10 MW) plants. The opportunities and challenges associated with Africa’s rivers and freshwater systems are discussed more fully in Chapter 4: Freshwater.