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Sahel

Originally the term Sahel - an Arabic word literally meaning "shore" or "bank"-had a specific geographic reference: the southern boundary, or "shore", of the Sahara, the variable zone between the extremely arid desert and the more humid savannahs to the south. As a somewhat arbitrary zone within the continuum between desert and rain forest, it has been variously defined by different authors (Grove 1978; Bernus 1971). No matter what rainfall or phyto-geographical limits are used, it should always be kept in mind that rainfall is extremely variable from year to year, and this has a direct effect on the vegetation and the types of human activities that can take place.

For the moment we shall define the Sahel as that area immediately south of the Sahara which has an average annual precipitation of more than 100 mm and less than 600 mm. The northern half of this area can be considered useful primarily for nomadic grazing, with a gradation from camels to goats, cattle, and sheep as one moves from north to south. Rain-fed agriculture is possible only on the wetter sites, and then only in the wetter years. In the southern portion of the Sahel rain-fed agriculture is possible in most years, but yields will vary considerably.

On average this zone is some 450 km wide, and, by the definition above, it should extend-at least in a physiographic sense-in an east-west band for roughly 5,500 km between the Atlantic Ocean and the Red Sea, including significant parts of Senegal, Mauritania, Mali, Niger, Chad, and the Sudan as well as the northernmost parts of Burkina Faso,* Nigeria, Cameroon, and Ethiopia. Bernus (undated, citing Monod 1975), suggests that this transitional zone could even be considered to extend around the Ethiopian plateau to northeast Kenya. While it can be argued that the 1968-1973 drought had at least as severe an effect on certain provinces within Ethiopia, semi-arid Ethiopia is generally considered separately from the Sahel; and even the Sudan is separated on political grounds from the West African Sahel, despite the obvious geographical similarities. Figure 1 is a map of West Africa south of the Sahara showing estimated isohyets of mean annual precipitation.

Lee H. MacDonald. The Sahel: Human and environmental overview in Natural Resources Development in the Sahel: The Role of the United Nations System. United Nations University Press (1986) retrieved 2009-0219

Sahelian and Sudano-Sahelian zones with 200 to 600 mm rainfall  J.-M. Boffa Agroforestry parklands in sub-Saharan Africa. FAO CONSERVATION GUIDE 34. Food and Agriculture Organization of the United Nations: Rome (1999) ISBN 92-5-104376-0

Milich, L., 1997. Mean Annual Rainfall and Rainfall Variability in the Drylands of Sub-Saharan North Africa. http://ag.arizona.edu/~lmilich/climzone.html.

e Sahel is the zone with a mean annual rainfall range of 100 mm to 300 mm, with the Sahelo-Saharan zone at the lower end of this amount. The Sahelo-Sudanian zone corresponds with a rainfall range of 300 mm to 600 mm, while the Sudanian zone itself has a southern boundary defined by the 1200 mm mean annual isohyet (see, e.g., Malo, A.R. and S.N. Nicholson, 1990. A study of rainfall and vegetation dynamics in the African Sahel using normalized difference vegetation index. Journal of Arid Environments 19, 1-24.)

[http://books.google.com/books?id=P2UrAAAAYAAJ Environmental Change in the West African Sahel By National Research Council (U.S.). Advisory Committee on the Sahel Published by National Academy Press, 1984

pp3-4, 6 Sahel (Sahil) is an Arabic term signifying coast or border. In this report the area referred to as the Sahel is a zone approximately 200-400 km wide, centered on latitude 15°N in sub-Saharan Africa. Although examples of environmental change from various regions of this extensive zone are used in this report, the focus of the study is the West African Sahel (Figure 1). Within West Africa, several Sahelian enclaves are found beyond the strict limits of the zone; for example, in the Adrar des Iforas, the AÃ¯r, and in the southwestern Tibesti highlands. Within the Sahel, the interior delta of the Niger River and Lake Chad are atypical of the region as a whole.

Since the initiation of international efforts to combat the effects of drought in the arid and semiarid zones of West Africa in the 1970s, the term "Sahel" has been more broadly applied to non- Sahelian regions of the member states of the Permanent Interstate Committee for Drought Control in the Sahel (CILSS). The states include Cape Verde, Senegal, The Gambia, Mauritania, Mali, Upper Volta, Niger, and Chad. In these states, the Sahel proper covers approximately 2 million square kilometers, constituting 27 percent of Senegal, 39 percent of Mauritania, 40 percent of Mali, 7 percent of Upper Volta, 50 percent of Niger, and 32 percent of Chad.

Recent research on the long-term climatic characteristics of the Sahel indicates that the region was repeatedly subject to change during the Late Quaternary. For example, wind-driven sand encroached far into the present Sudanian and Guinean zones to the south of the Sahel between 20,000 and 12,000 years ago (B.P.), while between 9000 and 5000 B.P. the rivers of the region had greater discharge and the lakes substantially greater volumes. The climate of the Sahel has been relatively constant during the last 2,500 years. It has been characterized by short- to medium-term oscillations toward drier or more humid conditions (Figure 2), with rapid, highly variable climatic change occurring within these more predictable fluctuations.

FIGURE 1 The West African Sahel.

Variation in precipitation from year to year is also a characteristic of the Sahel; successive years of severe drought are often followed by years with torrential rains. The Sahelian drought that began in 1968 falls within the norm of at least six earlier periods of low rainfall verified since 1400 A.D. In addition to secular, short-term droughts, some researchers believe that major droughts occur cyclically in the Sahel at intervals of approximately 30 years.

Although precipitation varies with latitude and local conditions, it is generally restricted to a period of 3-5 months. Storms are frequently violent in nature, and the rain is often erratically distributed-both in terms of when and where it falls-with variability increasing from south to north within the Sahel. The rainy season is followed by an extended, unrelieved dry season.

In some regions of the Sahel, precipitation variability is partially compensated for by groundwater reserves. The most important regional aquifer is the Intercalary Continental found in Niger and in northern Mali. Another important aquifer is the post-Eocene Terminal Continental. This formation is largely associated with the Adrar des Iforas in Mali, southwestern Niger, various areas north of the Niger River, and Senegal. The same aquifer apparently exists in southeastern Mauritania, but it is too deep for practical exploitation. Similarly, erratic rainfall in the western Chadian Sahel is compensated for by the northerly trending subsurface discharge of Lake Chad, a major body of water fed largely by the Chari and Logone rivers issuing from the Central African highlands. The Sahel is also traversed by two major rivers, the Senegal and the Niger, whose sources are in the highlands of Guinea.

FIGURE 2 Rainfall fluctuations (1901-1980) for the Sahelo-Saharan, Sahelian, Sudanian, and Sudano-Guinean zones (percentage above or below

normal). (Source: Nicholson 1982)

Most of the Sahel lies below 400 meters in elevation. Its surface is largely composed of worn, folded Precambrian rocks. These formations are characteristically aligned from northeast to southwest, with the alignment reflected in the limited relief of the region. The soils of the region range from ferruginous tropical soils in the south, to brown tropical dryland soils and weakly developed sandy soils on shifting and stabilized sands in the north.

The modern Sahel has often been described as a transitional zone-a territory in which an enriched Sahara and an impoverished Sudan meet in a hybrid no-man's land lacking its own characteristics. This view has been challenged by Theodore Monod, Edmond Bernus, and others who have studied the region. They view the Sahel as an ecoclimatically autonomous region largely defined by its highly variable precipitation and its characteristic seasonal therophyte ("summer prairie") vegetative communities.

There are no universally accepted delimiting criteria or terms for the various climatic and vegetation zones of West Africa. For the purposes of this report, the Sahel is defined as a zone roughly bracketed by the l00 and 500 mm isohyets. It is described as containing three phytogeographical divisions: the northerly Sahelo-Saharan zone (grass steppe), between the 100 and 200 mm isohyets; the Sahel proper (tree steppe), between the 200 and 400 mm isohyets; and the southerly Sudano-Sahelian borderlands (shrub savanna), extending to the 500 mm isohyet. The Sahel is bounded to the north by the Sahara Desert, and to the south by the Sudanian zone (wooded savanna). Finally, the Guinean zone (woodland) extends from the Sudanian zone to the Gulf of Guinea. Most zones contain coastal subregions (such as the Senegalese Coastal Sahel) and are commonly referred to in combination (for example, the Sudano-Guinean zone)-sometimes to describe a transitional area between zones, perhaps more often to group zones as a matter of convenience.

The Sahelo-Saharan zone has relatively few trees; examples include Acacia ehrenbergiana, A. tortilis, and Balanites aegyptiaca. Sparse grass such as Panicum turgidum is found on dunes. The Sahel proper is more heavily vegetated. Characteristic species include Acacia ehrenbergiana, A. laeta, A. nilotica, A. senegal, A. tortilis, Balanites aegyptiaca, Maerua crassifolia, Salvadora persica, Zizyphus mauritiana. Annuals such as Aristida adscensionis, A. funiculata, Panicum laetum, and Schoenefeldia gracilis are found on silty soils; Aristida mutabilis, Cenchrus biflorus, and Tribulus terrestris are found on sandy soils. The vegetative cover increases in the Sudano-Sahelian zone, reaching 10-12 percent on sandy soils and over 60 percent on silty soils. Andropogon gayanus and Zornia glochidiata are representative grasses of the Sudano-Sahelian zone; representative trees include Acacia al, A. seyal, Adansonia digitata, and Combretum glutinosum.

R. Bellefontaine, A. Gaston and Y. Petrucci, et al. Management of Natural Forests of Dry Tropical Zones. FAO CONSERVATION GUIDE 32. Food and Agriculture Organization of the United Nations: Rome (2000) ISBN 92-5-103970-4

Chapter II: Diversity of Woody-Vegetation Formations and Summary Breakdown 1. The various classifications used

Sahelian-Sudanian biogeographic zone  in the Sahelian domain, particularly with a Sahelian-Saharan climate, the steppes found are Acacia tortilis ssp. raddiana, A. senegal, A. ehrenbergiana, Balanites aegyptiaca, Calotropis procera, Leptadenia pyrotechnica steppes.

http://www.fao.org/docrep/004/X3004E/x3004e00.HTM

SAHEL WEATHER AND CROP SITUATION 1999 Global Information and Early Warning System on food and agriculture Report No 4 - 10 September 1999    Economic and Social Development Department, FAO

Sahelian zone: Where average annual precipitation ranges between 250 and 500 mm. This zone is at the limit of perennial vegetation. In parts where precipitation is less than 350 mm, only pastures and occasional short-cycle drought-resistant cereal crops are grown; all cropping in this zone is subject to high risk.

Sudano-Sahelian zone: Where average annual precipitation ranges from 500 to 900 mm. In those parts of this zone where precipitation is less than 700 mm, mostly crops with a short growing cycle of 90 days are generally cultivated predominantly sorghum and millet.

Sudanian zone: Where average annual precipitation ranges from 900 to 1 100 mm. In this zone, most cereal crops have a growing cycle of 120 days or more. Most cereals, notably maize, root and cash crops are grown in this zone.

http://www.oecd.org/dataoecd/41/49/38409502.pdf The ecological vulnerable zone of the Sahelian countries], April 2006, part of the Web Atlas on Regional Integration in West Africa ECOWAS-SWAC/OECD    200-600

Marie-Lise SABRIE & Yann L'HÔTE Sahel: une sécheresse persistante. Fiche Scientifique no. 178 - Juin 2003 IRD, France

Dès la création en 1946 du Service Hydrologique de l'Orstom, aujourd'hui IRD, les chercheurs et techniciens se sont attachés à collecter, entre autres, les relevés journaliers de l'ensemble des stations météorologiques et pluviométriques des Etats d'Afrique de l'Ouest et centrale. Outre la création de banques de données informatisées disponibles dans treize pays africains, de nombreuses études de synthèse ont été et sont encore effectuées grâce à ces données, parmi lesquelles la publication d'une carte d'isohyètes [2] moyennes annuelles sur le sous-continent. Cette dernière met en particulier en évidence un net déplacement vers le sud des isohyètes caractéristiques de la zone sahélienne (300 à 750 mm de pluie par an)

West Africa: Backgrounder On the Sahel IRIN/United Nations. 2 June 2008

"defined as the region between 12.5°N and the Sahara desert"

Wang, G. Eltahir, E. A. B. Foley, J. A. Pollard, D. Levis, S. Decadal variability of rainfall in the Sahel: results from the coupled GENESIS-IBIS atmosphere-biosphere model. CLIMATE DYNAMICS 2004, VOL 22; NUMB 6/7, pages 625-638

"defined as the region between 12.5°N and the Sahara desert"

Aiguo Dai, Peter J. Lamb, Kevin E. Trenberth, Mike Hulme, Philip D. Jones, Pingping Xie. The recent Sahel drought is real International Journal of Climatology Volume 24, no. 11 (2004) pp. 1323-1331 defined as the land region 10?20 ° N and 20 ° W?20 ° E

15W-20E, 13N-20N, which approximately corresponds to the Sahel region as defined by SE Nicholson [in Natural Climate Variability on

climate 9season and vegetation growth lengths), edaphic conditions (plant communities that are distinguished by soil conditions rather than by the climat), vegetaion, precipitation gradient, soil diversity

Jean Koechlin Ecological Conditions and Degredation Factors in the Sahel. pp. 12-36

Societies and Nature in the Sahel: Rethinking Environmental Degradation By Claude Raynaut, Emmanuel Grégoire, Stockholm Environment Institute Contributor Emmanuel Grégoire Edition: illustrated Published by Routledge, 1997

The Sahel vegetation differs from other arid zone vegetations in that it has very few perennial grasses herbaceous and almost no succulents. It is dominated instead by annual herbaceous, mostly C 4 grasses, associated with scattered woody plants. ISBN 0415141028, 9780415141024

Working the Sahel: Environment and Society in Northern Nigeria By Michael Mortimore, William Mark Adams Edition: illustrated Published by Routledge, 1999 ISBN 041514096X, 9780415140966

The Sahel-Sudan climatic zone south of the Sahara Desert is characterized by a strong north-south gradient of seasonal (summer) rainfall varying from 100 mm annual rainfall in the extreme north to over 1000 mm in the extreme south. The region's rainfall is characterized by extreme seasonal and annual temporal and spatial variability.

A Bidirectional Corridor in the Sahel-Sudan Belt and the Distinctive Features of the Chad Basin Populations: A History Revealed by the Mitochondrial DNA Genome V. ?erný, A. Salas, M. Hájek, M. ?aloudková, R. Brdi?ka Annals of Human Genetics Volume 71, Issue 4, Pages433 - 452 2007