Climate zones in West Africa
The climate in West Africa rapidly changes from north to south. Rainfall is very limited at the edge of the Sahara and consequently there is hardly any vegetation. About 1000 km to the south annual rainfall amounts to 2000 mm and here contiguous (rain)forests were found in the past. The ecosystems along this gradient are often classified into bio-climate zones (e.g. Keay 1959, Le Houérou 1989), with annual rainfall as long-term determinant of the potential vegetation. Classifications differ in the level of detail and delineation, but in general reflect the same pattern. The delineation between climate zones is not stationary due to shifting rainfall patterns between years and landscape modifications through land use intensification. In the Observatory we use a simplified layout based on exiting classifications:
Annual rainfall between 200-500 mm (some authors use 100-600 mm), mostly grazed steppes with scattered Acacia trees (Acacia tortilis, A. senegal) and (Balanites aegyptiaca) trees. Agriculture is limited to depressions, lake shores and floodplains along the Niger river. The Inner Niger Delta is largely situated in the Sahel zone. Today cropland is expanding in the southern part of this zone.
Dry savanna - Sudan zone
Annual rainfall between 500 – 900 mm. This zone is often classified as Sudan or Sahelo-Sudan zone (Arbonnier 2004, Le Houérou 1989). This zone consists mostly of grazed, dry and scarcely wooded savanna with Acacia trees (Acacia seyal and other). Agriculture – cropland – in the form of agroforestry parklands (with Shea Vitellaria paradoxa and Faidherbia albida) is the main form of landuse in this zone. The northern part of this zone is important to the system of transhumance, especially in the dry season. A large part of the Upper Basin is situated in this zone, in particular of the Bani catchment.
Moist savanna - Guinea zone
Annual rainfall 900 – 1800 mm. This zone is often classified as Sudano-Guinea and/or Guinea zone (Arbonnier 2004, Le Houérou 1989). This zone can be characterised as wooded savanna where Acacia trees are scares abound. Grazing is very limited in this zone, caused by the presence of the tsetse fly (> 1000 mm rainfall), which causes sleeping sickness among cattle. Agriculture in agroforestry parklands is the main form of landuse. The Guinean part of the Upper Niger Basin is situated in this zone.
Forest transition zone
Annual rainfall > 1800/2000 mm. In this zone rainfall is high and humid conditions prevail. Contiguous forest were present in the past, with gallery forest in the river valleys. Today, the zone consists mainly of wooded savanna with scattered forest patches. The most southern part of the Upper Niger Basin is part of this zone, while the upper reaches of the Milo tributary even reach into the Forest zone where still fragmented rainforests are present.
Elevation and soils
Large parts of West Africa are flat and not mountainous. The major exceptions are the Guinean Highlands where the Niger River originates, and the mountains of western and central Cameroon where the Bénoué River rises. Elevation in the Upper Niger Basin is shown in the viewers as background map.
The source of the Niger River is situated in the Guinean Highlands near the border with Sierra Leone, at an altitude of around 750 m a.s.l (above sea level). To the northwest the Guinean Highlands extend into the Fouta Djalon mountains, which rise up to 1,500 m. To the southeast are the Kourandou and Simandou mountain ranges and Mt Nimba, at 1,752 m the highest peak of Guinea. The altitude decreases as the Niger flows eastwards towards Mali. The section of the basin northeast of Bamako is mostly flat and is situated at an altitude of around 200 – 300 m a.s.l.
Three major soil types occur in the basin: ferralitic soils, tropical ferruginous soils and hydromorphic soils. Ferralitic soils are heavily weathered and leached soils that are generally rich in iron and bauxite (aluminium). These soils are mostly located in the humid forest zone, like the Upper Guinea forests, and the southern Guinea savanna zone with relatively high rainfall. Ferralitic soils are present in the Guinean Highlands and in the lower Niger Basin in southern Nigeria. More to the north, the savanna zone is characterised by ferruginous soils. Fertility and organic matter content of these soils are generally low compared to the soils of the humid forest zone. These soils are generally suitable for growing annual crops like maize, millet etc. Hydromorphic soils are found in wetlands, marshes and river beds, and most of the soils in the Inner Niger Delta belong to this type.
The Great Mosque of Djenné in Mali, is the largest mud brick structure in the world (source: Soil Atlas Africa). The structure is built from thousands of sun-baked mud bricks (called ferey) and a mud-based mortar coated with a mud plaster. Also other mosques in the Inner Niger Delta are built in that way. The thick walls (between 40 - 60cm thick) bear the weight of the structure and provide insulation The outside plaster is renewed every year after the erosion caused by heavy showers during the rainy season (source Soil Atlas Africa).
More information: http://africasoils.net/ and http://esdac.jrc.ec.europa.eu/content/soil-map-soil-atlas-africa
Land use patterns
The term ‘land use’ in this Section covers terrestrial land use and the use of water systems by rural communities. This applies in particular for fishing and the growing of wild (floating) rice and bourgou, such as in the Inner Niger Delta.
Terrestrial land use depends on soil fertility but is, in tropical West Africa, very much determined by the duration and amount of rainfall. This is reflected in large differences in land occupation per climate zone (sources CILSS 2016, Zwarts et al. 2009). The distribution of cattle, sheep and goats is generally limited to the zone where annual rainfall ranges between 50 and 1000 mm, in particular the Sahel and adjacent Dry savanna (Le Houérou 1989). With less than 50 mm of rain annually, vegetation is too scarce to support livestock other than camels. In regions with more than 1000 mm of rain, the tsetse fly abounds and sleeping sickness prevents herding the large humped Zebu cattle, although tsetse eradication programs have changed this pattern. The humpless, longhorn Ngama cattle are resistant to sleeping sickness but less adapted to the Sahel. Consequently, Le Houérou (1989) estimates that 80% of the livestock in the transient zone is found in the Sahel and 20% in the Sudan zone, encompassing in our terminology also the dry savanna zone. Cropland is the dominant land use in the dry and moist savanna zones, often in the form of agroforestry parklands.
The occupation patterns in West Africa - in particular the different forms of land use and the lifestyles that go with it - have a long history of development. The traditional forms of land use are often linked to ethnic groups, such as transhumance and fishing in the Inner Niger Delta (e.g. Moorehead 1990). Although the link between land use and ethnic groups is becoming less distinct, this pattern is still largely intact today.
Land use mapping
Terrestrial land use has been mapped for different periods on the basis of satellite data. In 2016 the CILLS (2016: Comité permanent Inter-états de Lutte contre la Sécheresse dans le Sahel) published an analysis of land use changes in West-Africa, on the basis of MODIS satellite data. Also the European Space Agency (ESA) provides land use data and maps from West Africa, from different time series and with different resolutions. ESA Land use maps from 2015 have a resolution of 300 m, while the most recent land use maps of Africa are based on Sentinel II satellite images with a resolution of 20 m. Recent land use maps can thus be obtained from CILSS and ESA.
A comparison of both maps – shown here below – shows the differences in detail and used categories. Digital mapping, using satellite imagery, nicely illustrates the larger pattern but cannot be used for detailed land use characteristics.
Land use map of West Africa in 2013. Source: CILSS 2016. CILSS provides land use maps and change data from West Africa over three periods (1975, 2000, 2013), with in depth information on methodologies used, detail level (resolution) and changes (Cotillon 2017). Data can be downloaded from: https://eros.usgs.gov/westafrica/land-cover/land-use-and-land-cover-trends-west-africa and https://eros.usgs.gov/westafrica/data-downloads
Land use map of West Africa in 2015. Source: ESA 2015. ESA provide land use maps of Africa from several years with different resolutions. Data can be downloaded from:https://eros.usgs.gov/westafrica/land-cover/land-use-and-land-cover-trends-west-africa and https://eros.usgs.gov/westafrica/data-downloads
Land use is briefly outlined here, from south to north, based on the land use maps of CILSS (2016) and ESA (2916), supplemented with more detailed information about the situation along the Niger and in the Inner Niger Delta (Zwarts et al. 2005, Klop et al. 2018). The Sections in the theme Socio-economy provide more detailed information and data on the production of crops and natural resources.
The southern edge of the Upper Niger Basin is part of the Forest transition zone, where the upper reaches of the Niger proper, Niandan, Milo and Sankarani tributaries are found. In the past contiguous forests dominated in this zone. The Haut Guinea National Park is partly situated in this zone. Agriculture and agroforestry parkland is increasing in this zone. Still, well developed gallery forest fringe the river banks. Gallery forests also can be found along the Niger and its tributaries more to the north, while the river itself mainly consists of sandbanks that are partly exposed during the dry season. The gallery forests provide a unique habitat of relatively dense and moist forest in an otherwise open savanna landscape.
The part of the basin in Guinea belongs to the moist savanna as well as the southern part of the Bani catchment in Mali. In this zone predominantly, more or less wooded, savannas and agroforestry parklands are found with large surface areas used for agriculture. The agroforestry parklands mainly consist of Shea parkland (local name Karité, Vitellaria paradoxa) with scattered White acacia (Faidherbia albida) and Parkia (Parkia biglobosa). Characteristic for this zone in Guinea are the bowé, small plateaus in the wooded savanna landscape. Much of this landscape has changed through open mining for the extraction of bauxite, gold and other minerals. More to the north cropland dominates, either in agroforestry parklands or, more to the north, on open savannas. Both in Guinea and Mali, rainfed agriculture is the most important form of terrestrial land use.
Along the Niger and tributaries mainly flood recession cultures and irrigated agriculture are found. Crops consist of rice and legumes like onions and tomatoes. Larger irrigation complexes have been developed near the reservoirs and in the Office du Niger. The most important irrigation complexes, in terms of surface area and production, presently are situated in Mali downstream of the Sélingué reservoir and around Ségou. By far the largest irrigation complex of West Africa is the Irrigation Zone of Office du Niger in a dead branch of the Niger (Delta Mort).
The extraction of gold from riverine soils is increasing. This started as small-scaled artisanal gold digging but increasingly is growing into open mining, in Guinea as well as in the Bani catchment in Mali.
The dry savanna starts in the Mali part of the basin In the south cropland is still important, in a setting of an open savanna landscape. Croplands dominate in agroforestry parklands or, more to the north, on more open savannas. Here also grassland savannas are found. The northern part is situated in the Sahel. This zone mainly consists of grazed steppes. Although livestock raising predominates, cropland is present in depressions, along lake shores and on river floodplains. In the dry north, livestock raising is the most dominant land use system, efficiently exploiting the scarce and highly variable resources. However, also livestock needs sufficient food and water. The Niger and the Inner Niger Delta play a key role in the exploitation of the northern grazing grounds, since the livestock farmers depend on water from the river and the grazing grounds the delta in the dry season (see Section Transhumance).
Land use in the Inner Niger Delta is determined by the seasonal flood pulse. Over time a traditional system of land and water use has been evolved with a distinct separation between user and owner rights. In the past these systems – fisheries, transhumance and agricultures (rice) - were regulated by the DINA law (Moorehead 1990). This traditional system still plays an important role in the organisation and management of land and water use in the delta. In recent years agriculture has ecome increasingly important in the Inner Niger Delta, with a growing development of small-scaled irrigated perimeters. Also in the northern lakes and around Tombouctou agriculture – in particular rice growing but also the production of legumes – has increased.
Changes in land use
The systematic mapping of land use patterns in West Africa by the CILSS (2016) in the periods around 1975 (just before the great droughts), 2000 and 2013, reveals major changes in the area. It must, however, be kept in mind that these changes are analysed on a low resolution levels, and thus must be regarded in that context. In particular the deforestration of savannas is difficult to capture on this scale of mapping. Details on global land use changes can be downloaded from the USGS-site (https://eros.usgs.gov/westafrica/data-downloads). In the table below, data from Mali and Guinea are presented for the most important land use categories.
Land use categories in Mali and Guinea in 1975, 2000 and 2013. Surface area presented in km2. Source CILSS 2016, Collison 2017. https://eros.usgs.gov/westafrica/data-downloads
In general, Sahelian steppes (discontinuous vegetation cover) and dry savannas in the north and (wooded) savannas more to the south declined sharply. Partly this is caused by the Great droughts in the 70s and 80s in the former century and partly this is due to the strong increase of agriculture in the past decades, especially on the dry savannas. In general, in the dry and moist savanna zone a transition took place from (wooded) savannas to rather intensively used agricultural landscapes. Savannas are getting more and more open due to the gradual but ongoing disappearance of trees. Simultaneously with the increase of agriculture, the surface area of wooded savannas and forests (evergreen forests with closed canopy) declined sharply.
The overall surface area of irrigated agriculture has increased considerably. This increase took place on a small scale, and with a scattered distribution, along the Niger and its tributaries. On a large scale large investments resulted in large irrigation complexes near reservoir of the river (Sélingué, Ségou), and in the irrigation zone of Office du Niger. In the Inner Niger Delta too, the area with irrigation increased, especially in the form of small-scale irrigation perimeters (PIVs).
Sources and more information
- ABN 2007. Atlas du Bassin du fleuve Niger. Autorité du Bassin du Niger ABN, Niamey. http://archive.wetlands.org/Portals/0/Atlas.pdf
- Arbonnier M. 2004. Trees, shrubs and lianas of West African dry zones. Markgraf, Weikersheim. http://www.quae.com/en/r367-trees-shrubs-and-lianas-of-west-african-dry-zones.html
- CILSS 2016. Les Paysages de l'Afrique de l'Ouest : Une Fenêtre sur un Monde en Pleine Évolution / Landscapes of West Africa. U.S. Geological Survey EROS, 47914 252nd St, Garretson, SD 57030, UNITED STATES. https://eros.usgs.gov/westafrica/sites/default/files/ebook-English/index.html#p=30
- Cotillon, S.E., 2017. West Africa land use and land cover time series: U.S. Geological Survey Fact Sheet 2017–3004, 4 p., https://doi.org/10.3133/fs20173004.
- Keay, R.W.J., 1959. An Outline of Nigerian Vegetation. 2nd Edn., Government Printer, Lagos.
- Klop, E. & M. Sikkema 2018. Mission report of the field visit to the Upper Niger Basin in Guinea, February 2018. Altenburg & Wymenga ecologisch onderzoek, Feanwâlden.
- Le Houérou, H.N. 1989. The Grazing Land Ecosystems of the African Sahel. Ecological Studies nr. 75. Springer-Verlag Berlin Heidelberg.
- Zwarts, L, Bijlsma RJ, van der Kamp J, Wymenga, E 2009. Living on the Edge. Wetlands and birds in a changing Sahel. KNNV Publishing, Zeist. p. 1-564. http://www.altwym.nl/uploads/file/540_1433753005.pdf