West Africa lies between latitudes 4°N and 28°N and longitudes 15°E and 16°W. The Gulf of Guinea is the southern boundary, while that to the north is the northern boundary of Mauritania, Mali and Niger; the Mount Cameroon/Adamawa Highlands and the Atlantic Ocean form the eastern and western limits. West Africa includes 16 countries: Benin, Cape Verde, Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Mauritania, Niger, Nigeria, Senegal, Sierra Leone, Togo and Upper Volta (Figure 1), which, with an area of 6 million km2, cover one fifth of Africa. Its physical features, climate, ecology and relevant characteristics are briefly described, but for more detailed information reference should be made to White and Gleave (1971), Brouillette (1974), Grove (1978), Hance (1975), Jarrett (1980), Journaux et al. (1976), Morgan and Pugh (1969), Pritchard (1969) and Thomas (1973).
Most of West Africa consists of an undulating low plateau below 500m asl, fringed on the west and south by a coastal plain; this plain is widest in Senegal, the southern Ivory Coast, the Niger delta and the lower valleys of the Volta and Niger rivers. There are some isolated highland areas above 500m and some peaks exceed 1000m. The most important are Fouta Djallon (1537m), Guinea Highlands (1656m), Sierra Leone Mountains (1948m), Nimba Mountains (1752m), Jos Plateau (1690m), Mandara Mountains (1142m), Adamawa Highlands (2042m), the Saharan Uplands of Air (1850m) and the Plateau of Djado (1120m) in northern Niger.
Rivers flowing west, inland and south dissect the plateau and form five major river systems. The first is a group of short rivers which rise in the Fouta Djallon/Nimba Mountains and flow westwards into the Atlantic. The second is formed by longer rivers (the Bandama, the Oueme and several Nigerian rivers) that flow southwards into the Gulf of Guinea, while the third comprises the Senegal and Volta rivers. The fourth system is formed by rivers which flow inland into areas of internal drainage like Lake Chad, while the fifth is the Niger, which rises in Guinea 240 km from the sea, flows north-east between Segou and Timbuktu in Mali, then southeast past rapids at Bussa in Nigeria and, after receiving a number of tributaries, enters the sea 4 160 km from its source. Rapids and a large variation in volume between dry and wet seasons limit the value of these rivers for irrigation and navigation. Some dams have been built to produce hydro-electric power; in Guinea (Kindia and Konkoure), Ivory Coast (Ayame), Liberia (Mount Coffee and Buchanan), Ghana (Akosombo) and Nigeria (Kainji).
Fig. 1 REGIONS OF TROPICAL AFRICA
(Adapted from Jahnke, 1982)
Although these are designed to supply irrigation water, this potential has only been partly realized.
West Africa has wet and dry seasons resulting from the interaction of two migrating air masses. The first, is the hot, dry tropical continental air mass of the northern high pressure system, which gives rise to the dry, dusty, Harmattan winds which blow from the Sahara over most of West Africa from November to February; the maximum southern extension of this air mass occurs in January between latitudes 5° and 7°N. The second, is the moisture-laden, tropical maritime or equatorial air mass which produces southwest winds. The maximum northern penetration of this wet air mass is in July between latitudes 18° and 21°N. Where these two air masses meet is a belt of variable width and stability called the Intertropical Convergence Zone (ITCZ). The north and south migration of this ITCZ, which follows the apparent movement of the sun, controls the climate of the region.
The lowland climates of West Africa are characterized by uniformly high sunshine and high temperatures throughout the year; mean annual temperatures are usually above 18°C. Areas within 10° of the equator have a mean annual temperature of about 26°C with a range of 1.7 – 2.8°C; the diurnal range is 5.6 – 8.3°C. Between latitudes 10°N and the southern part of the Sahara mean monthly temperatures can rise to 30°C, but the annual range is 9°C and diurnal range 14° to 17°C. In the central Sahara temperatures in the shade in July may be as high as 58°C during the day and as low as 4°C at night; mean annual temperature ranges from 10° – 35°C. The skies in the higher latitudes in inland areas are usually cloudless as compared to areas close to the coast where skies are cloudier and night temperatures are higher. Temperatures are greatly modified by altitude, although this is not as pronounced as in the high plateau of East Africa. Freetown (8° .30'N) and Abidjan (5°N) have annual rainfalls of 3434mm and 1830mm respectively which start in February/March and end in November or December followed by four months of lower rainfall when precipitation is less than evapotranspiration. Inland, Ouagadougou (12°N) receives 890mm of rain in 5 months (June - September) while on the edge of the desert Agades (18°N) has only 175mm in 2-1/2 months (July - September). The general rainfall pattern is modified by ocean currents and physiographic features. A coastal savannah with about 730 mm annual rainfall extends from the middle of the Ghana coast to the extreme southwestern corner of Nigeria. This is because the coastline runs parallel to the direction of the rain-bearing southeasterly winds and the cold Benguella current which impinges on the coast and reduces the rainfall in amount and duration. The cold Canary current has the same effect on coastal Senegal; Dakar has only 585mm annual rainfall. Altitude also affects rainfall with the result that highland areas have more rain than the surrounding lowlands. Dabundja, south of the Adamawa ranges in southern Cameroon, has over 10 000mm of annual rainfall. Other characteristics of the rainfall of West Africa include: (i) frequent heavy rainstorms of short duration which cause severe soil erosion, particularly on cleared, bare cultivated land; (ii) the occurrence of a belt of bimodal rainfall some distance inland from the coast eastwards from Sierra Leone to Nigeria, and (iii) the variability in amount, time of onset, duration and cessation which increases from the wetter areas to the drier areas. In the coastal areas the percentage annual variability ranges from 10 to 20 percent, while close to the Sahara in the Sahel it may exceed 40 percent.
Despite the high insolation over West Africa there is considerable variation in the intensity, effectiveness and duration of sunlight in different areas at different times during the year. Mean annual radiation as a result of cloud cover during the rainy season is about 110 kilo cal/m2/yr near the coast and the equator as compared to 200 kilo cal/m2/yr on the edge of the desert. On the other hand net annual primary productivity ranges from about 4000 g/m2 in the coastal rain forest to 500 g/m2 in inland savannah areas near the desert. There is little variation in day length which ranges from 11.5 to 13 hours; Njoku (1958), however, has shown that as little as 15 minutes variation can initiate the flowering phase in okra and vegetable jute.
Vegetation under the uniformly high temperatures in the tropics is determined to a large extent by rainfall. Consequently, vegetation zones run parallel to each other from north to south and are related to rainfall quantity. Adejuwon (1976) noted that there is a climatic climax vegetation in equilibrium with each regional climate, except to the extent that it may be modified by physiographic conditions. If this situation prevailed, the climatic climax vegetation of West Africa moving from southwest to northeast should be: (i) tropical rain forest; (ii) tropical deciduous forest, and (iii) tropical xerophytic woodland. Various studies, however, indicate that the present vegetation zones are of anthropic and edaphic origin and that man, through clearing and farming, grazing, burning, etc., has had a dominant influence on the vegetation. The tropical rain forest has become modified into: (i) mangrove tidal swamps and marshlands next to the coast, followed by freshwater swamps which extend along river valleys and streams and are dominated by Raphia Palms (Raphia spp) and Screw Pine (Pandanus candelabrum; (ii) rain or moist forest regrowth which retains some characteristics of the multistoried structure and species diversity of tropical rain forest where it has not been drastically modified, and (iii) the derived savannah or forest/savannah consisting of open bushland, isolated trees and oil palms. The moist forest extends 260 km inland and only covers half of its original area, but it extends further inland along the river valleys in central Ghana, Ivory Coast, Liberia, southern Sierra Leone and southern Nigeria. It is the source of valuable timber species including Obeche (Triplochiton scleroxylon) and Mahoganies (Khaya and Entandrophragma spp). Next to the derived savannah, the deciduous forest has become modified into southern Guinea and northern Guinea savannahs consisting of woodland and tall grasses which decrease in size with increasing distances inland. These savannahs consist of Danieila-Detarium woodland with some Parkia clappertoniana, while elements of the moist forest form narrow strips of vegetation (gallery or fringing forest) along the river valleys and streams. North of the Guinea savannahs are found the Sudan savannah, the Sahel savannah and then tropical steppe and desert. The vegetation of the Sudan savannah is dominated by increasing numbers of fire and drought resistant species; of economic importance are Parkia spp, Baobab (Adansonia digitata) and Butyrospermum paradoxum (Shea Butter). The number of dwarf thorny shrubs, acacias and drought resistant species increases with proximity to the desert in areas with less than 200mm of annual rainfall. Montane vegetation occurs in highland areas about 900mm. The tropical moist forest, derived savannah and Guinea savannah areas support tsetse flies and trypanosomiasis makes cattle rearing difficult.
Le Houerou and Popov (1979) presented a biological or ecoclassification of intertropical Africa, which is useful in defining crop and livestock production systems based on criteria such as: (i) rainfall distribution patterns of mono- or bi-modal type; (ii) total precipitation during the year; (iii) annual distribution of rainfall in number of dry and rainy months; (iv) temperature including occurrence of frost, (v) land use for pastures, crops or forest, and (vi) livestock species and breeds and distribution of tsetse flies. Based on temperature and availability of rainfall, both in amount and distribution, which determine the kind of plant and animal life, climates are classified into (i) mono-modal rainfall areas which are tropical and bi-modal rainfall areas which are equatorial. Since parts of both equatorial and tropical areas have rains throughout the year with no dry season and other parts have varying severity of drought, the various acological, climatic and length of growing period zones are shown in Figure 3.
Much of West Africa is composed of ancient crystalline rocks of the Basement Complex which, being resistant to erosion and weathering, form the highlands of Guinea, Adamawa and Atakora. Equally resistant are older sedimentary rocks, mainly sandstones, which also form uplands and scarps which include the Fouta Djallon highlands in Guinea, the Banfora and Hombori mountains in Mali and Gamkaza and Manpong in Ghana. Younger sedimentary rocks cover extensive areas of southern Nigeria, Togo, Benin and Senegambia.
With the exception of soils formed from younger volcanic rocks, and those in flood plains and valley bottoms, the soils of West Africa are highly weathered and of low inherent fertility. The main soil groups consist of Alfisols, Ultisols, and Oxisols, with smaller areas of Entisols, Vertisols and Inceptisols; there is a large area of Aridisols in the Sahara. The Alfisols, according to Juo (1980), are coarse to medium textured soils overlying clayey subsoils with a base saturation over 50 percent; they are formed from Precambrian Basement Complex rocks in well-drained upland areas. The Ultisols are coarse to medium-textured acid soils with clayey subsoils, similar to Alfisols, but the base saturation is less than 50 percent. Oxisols are strongly weathered fine or coarse textured soils exhibiting little variation in texture with depth; they cover large areas in southern Ghana, Benin and Nigeria. Vertisols, or black cotton soils, are dark clay soils formed under drier climatic conditions, rich in swelling clay minerals like montomorillonite; they are difficult to work when wet and form large cracks when dry. Vertisols are found in the Accra plains and the Chad basin. The Entisols are derived from alluvial or colluvial material in valley bottoms and hydromorphic situations. Sulfaquents are acid sulphate soils which represent special kinds of Entisols in tidal swamps and present special problems if cultivated for rice. Inceptisols are young soils with limited profile development while the Aridisols are soils of arid and desert areas, some of which are saline.
Land use systems involving these soils vary in different ecological conditions.
A review of the environmental setting of West Africa is incomplete without a brief consideration of the socio-economic and historical conditions that have influenced agricultural and economic development; statistics and other information are summarized in Table 1. All West African countries with the exception of Liberia have been under colonial rule and all, except Liberia, gained independence between 1957 and 1975.
There are two zones of population concentration, one in the rain forest zone near the coast and the other in the Sudan/Sahel savannah zone within which Kano and Bamako are located (Figure 2). Between these, the socalled ‘Middle Belt’ is relatively sparsely populated, largely as a result of the high incidence of trypanosomiasis and onchocerciasis. The relatively high agricultural potential of this zone which can grow roots, tubers and cereals has not been widely exploited. Population density ranges from under 4 per km2 in the dry northern areas to over 200 per km2 in parts of southeastern Nigeria.
The annual growth rate of population in West Africa varies from 1.6 in Guinea-Bissau to 5.5 in Ivory Coast and, in general, 45 percent of the population is less than 15 years of age; the proportion of the population engaged in agricultural production ranged from 50 to 87 percent in 1981 and the proportion of the labour force engaged in agriculture ranged from 46 percent in Benin to 93 percent in Guinea-Bissau in 1979. The percentage of urban population in 1980 ranged from 9 percent in Upper Volta to 38 percent in Ivory Coast, while the annual rate of growth of urbanization was estimated at from 3.8 percent in Upper Volta to 8.6 percent in Mauritania.
BASIC STATISTICAL DATA ON WEST AFRICAN COUNTRIES
|Land Area Km × 103||Crop Area|
|Indices of Production (1970=100)||Population 1981 × 106|
|Ag. Prod.||Food Prod.||Per capita Food Prod.|
Source: FAO PRODUCTION Year Book, 1981a
Fig. 2 WEST AFRICA DISTRIBUTION OF POPULATION. (After Jarrett 1980)
Figure 3 shows climatic zones in West Africa and the lengths of the growing season in each zone; except for the humid zone the rainfall is unimodal. Figure 4 relates growing season to vegetation type and farming system.
The Arid Zone
This includes the Sahel or Sahelian zone and has up to 750mm of rain in a single short rainy season and 90 Days of Growing Period (DGP) with an extended dry season of up to 10 months. The dry season sometimes extends into years causing severe droughts. This zone includes northern parts of Senegal, parts of Mali, Upper Volta, Niger and Cameroon. The vegetation is mostly grassland and large numbers of Zebu cattle, sheep and goats are raised by pastoralists. The main food crops are millet, sorghum and groundnuts and cotton is the principal cash crop. Kapok, mango and Acacia spp. are the main trees.
The Semi-Arid Zone
This roughly includes the Sahelo-Sudan zone which covers the southern parts of Senegambia, Mali, Upper Volta, Niger, Chad and upper parts of Guinea-Bissau, Guinea, Togo, Benin, Nigeria, Cameroon and the Central African Republic. The average annual rainfall, 750mm to 1250mm, falls in one season followed by a long dry season. The vegetation is mainly grassland with some shrubs and acacia trees. The zone supports large numbers of livestock, mostly zebu and zebu-Shorthorn crosses, sheep, goats and a few pigs. The main crops are millet, sorghum, groundnuts, cotton, beans and rice; plantation crops are mango, cashew and kapok.
The Subhumid Zone
This zone includes Guinea-Bissau, upper parts of Guinea, the southernmost parts of Mali and Upper Volta and the northern parts of Ghana, Ivory Coast, Cameroon, Sierra Leone, Benin and the central parts of Nigeria. The average annual rainfall is between 1250mm and 1500mm in one season with 180–270 DGP which supports a basically grass and shrub vegetation which is widely tsetse infested. Cattle, as well as sheep and goats, are raised but crop farming is the main agricultural occupation; crops include maize, sorghum, rice, millet, yam, cotton, groundnuts and pulses, and the plantation crops are mango, kapok and cashew. Forests are confined to the river valleys in the southern parts of the zone.
Fig. 3 WEST AFRICA; CLIMATIC ZONES AND GROWING SEASON. (Source: Jahnke, 1982)
|Sahelo-Sudanian: Semi-Arid (90–100 DGP.*)|
|Sudano-Guinean: Sub-Humid ( 180–270 DGP.*)|
( 270–365 DGP.*)
* = Days of Growing Period
** = Bimodal Rainfall
............Approximate limit of Tsetse infested areas
Fig. 4 Humid and arid months, vegetation belts and farming systems in the tropics. (Adapted from Uhlig, 1965 and Andreae, 1980) Okigbo 1981.
The Humid Zone
This zone, which is also infested with tsetse fly, consists of two parts:
The Guinea or derived savannah zone has an annual rainfall of between 1500mm and 1800mm divided into two seasons which alternate with two dry seasons. The natural vegetation is generally grassland and woody transitional forests. The crops are mostly maize, yam, rice, millet, sorghum, groundnuts and cotton; sugar cane is grown in the wetter parts. Livestock, mostly trypanotolerant cattle, sheep and goats are few. This sub-zone includes parts of southeast Guinea, northern Liberia, parts of Ivory Coast, middle Ghana, the middle belt of Nigeria and southern Cameroon.
In the forest zone the annual rainfall is between 1500mm and 2000mm which falls in two wet seasons alternating with two dry seasons; the DGP is from 270 to 365 days. The vegetation is dense tropical forest, which can be cleared to grow oil palms, coconuts, rubber and cocoa. The main food crops are maize, yam, cassava, cocoyam plantain, banana and beans, but coffee, mango, citrus and sugar cane are also grown.
The prospects for integrating crop and livestock production are greatest in areas of high rainfall, long growing periods and low population. In spite of their trypanosomiasis risk it is therefore the subhumid and humid zones which most favour this type of development.