Contents - Previous - Next
This is the old United Nations University website. Visit the new site at http://unu.edu
1. The Sahel: Human and environmental overview
Vegetation, land use, and resource development
In the oral tradition of Niger, drought and hunger are familiar topics. Before foreign conquest there were the periods known as Ize Mere, "the sale of children"; Goosi Borgo, "grinding up the water gourd"; and Yollo Moron, "sit and stroke your plaits"-for there was nothing more that could be done. Colonization did not change the pattern, for there was famine first in 1913-1914 and then in 1931-1932, when the locusts came in hordes, leaving the people with hunger. In 1942 west of Niger there was Wande-Maasu, "leave the wife, push her aside"; and in 1951-1952 there was Gaari-Jire, "cassava meal", when the only way to survive was to eat high-starch cassava meal brought in from Dahomey (now Benin) and Nigeria (Salifour 1975). The drought that lasted from 1968 to 1973 might have been unusual in its length (UNCOD 1977a) but cannot be considered unique (Nicholson 1983).
It was the 1968-1973 drought that focused the world's attention on the region immediately south of the Sahara. An unusually poor series of wet seasons led to the failure of crops and the death of livestock on a massive scale, and the pictures and stories of malnourished children and dead cattle stimulated a tremendous outpouring of emergency relief. This short-term reaction has been followed by an unparalleled international effort to aid the long-term development of the countries that are now commonly called the 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.
South of the Sahel zone we can define a more wooded Sudanian zone, where the average annual precipitation of about 600-950 mm permits a more varied and less risky rainfed agriculture, often integrated with the raising of livestock. Still further south is the rather heavily wooded Guinea zone, with its much higher productivity but also a much greater incidence of debilitating human and livestock diseases. The Guina zone could be considered the beginning of the rain forest, perhaps patchy at first because of natural and anthropogenic fires, but rapidly becoming an unbroken canopy in the natural state.
It should be emphasized that this definition of the Sahelian, Sudanian, and Guinea zones is arbitrary.
FIG. 1. Map of West Africa, showing isohyets of mean annual precipitation in millimetres (Adapted from Boeckm et al. 1974)
Boundaries in the natural state are indistinct, and they are further blurred both by annual variation in precipitation and by human activities. The general pattern of east/west-trending geographic zones is also disrupted by the major river systems such as the Niger, the Senegal, the Gambia, the Volta, the Chari, and the Logane, as well as well as Lake Chad Nevertheless, the concept of the zones is useful to indicate generalized landscape types which can then be associated with human activities.
In recent years, the term Sahel has come to refer also to the group of countries which encompass this "shore" of the Sahara. While technically this should include the Sudan, in practice the countries primarily referred to are Senegal, Mauritania, Mali, Niger, and Chad. Nigeria and Cameroon are excluded, because only a small percentage of their total area and population falls into the Sahel zone. On the other hand, Burkina Faso is relatively arid and is linked to the other Sahelian countries by both its colonial legacy and its basic socio-economic conditions. Similarly, the Gambia, although slightly more humid and with a British rather than a French colonial heritage, is included because its future is inevitably linked with that of Senegal and it faces very similar problems of development. Cape Verde, an island country more than 600 km west of Senegal, has also been politically linked to the Sahelian countries and shares similar environmental conditions, but because of its geographical isolation and the author's lack of direct experience with it, it will not be considered in any detail in this report.
Thus, these seven countries-Burkina Faso, Chad, Gambia, Mali, Mauritania, Niger, and Senegal-form a relatively cohesive geographical and political grouping. All of them underwent severe dislocations as a result of the 1968-1973 drought and have now banded together in an effort to more effectively improve existing socio-economic conditions and minimize the adverse effects of future droughts. As a group, they are beyond doubt among the "poorest of the poor".
In 1981 life expectancy in these seven countries averaged 44 years, and per capita gross national product ranged from $110* in Chad to $460 in Mauritania. in all seven countries adult literacy rates were below 20 per cent in 1980, and daily caloric intake in 1977 ranged from 5 to 26 per cent below FAO standards. Population per physician was 13,000 or more, and safe drinking water was available to no more than 37 per cent of the population (World Bank 1981, 1983a). Even more unsettling is the fact that per capita food production was lower in all countries in 1979-1981 than in 1969-1971 by between 4 and 24 per cent. From 1960 to 1981 gross national product did not keep up with population growth in Chad, Niger, and Sanegal (table 1).
Even without taking the 1968-1972 drought into consideration, this is clearly one of the poorest areas in the world and one that has made little progress in the effort to improve the standard of living. Continued economic growth in the industrialized world leaves these countries further and further behind in relative and real terms. Some form of external assistance is therefore necessary if they are to meet the basic needs of the existing population, to say nothing of improving the prospects of the next, much larger, generation.
TABLE 1. A statistical overview of the Sahelian countries
|Area (thousand km2)||1,284||11||1,240||1,031||1,267||196||274|
|Average annual population growth rate, 1970-1981 (%)||2.0||2-5b||2.6||2.3||3.3||2.7||2.0|
|GNP per capita (US$)||110||370||190||460||330||430||240|
|Average annual GNP growth rate per capita, 1960-1981 (%)||- 2.2||2.5||1.3||1.5||- 1.6||- 0.3||1.1|
|Life expectancy at birth (years)||43||42||45||43||45||44||44|
|Adult literacy rate (%)c||15||15||10||17||10||10||5|
|Food production index per
(1969-1971 = 100)
|Percentage of labour force in agriculture, 1980||85||-||73||69||91||77||82|
Sources: Manshard 1979;World Bank 1983a
All figures from 1981 unless otherwise indicated.
a Now Burkina Faso
c Dates of estimates range from 1978 to 1982.
In physical terms the Sahel is a mixture of elements from the Saharan and Sudanian zones, with seasonal changes determining which of these is uppermost. Life in the region is essentially dependent on the annual movements of the Intertropical Convergence Zone. In April moisture-bearing winds come in from the south-west, pushing against the hot, dry air mass that covers most of North Africa. The convergence of these air masses causes thunderstorms, and in fits and starts, the Intertropical Discontinuity moves north. In the southern part of the Sahel the first rains usually fall in late May or early June, while in the north the rain may begin only in July. Peak rainfall is almost invariably in August, and the retreating intertropical front means a cessation of rain in early September in the north and early October in the south.
Following the rainy season there is a brief transitional period of warm temperatures that is often accompanied by dry winds from the north-east. The cold season lasts approximately from late October to early March, and during this time surface water disappears and the vegetation dries up. Another transitional period is between March and the onset of the rainy season. The lack of cloud cover causes this transitional period to be the hottest time of the year, and it is usually also the time of greatest stress. Both water and forage are scarce, which means that milk production is at its lowest ebb and the animals are at their weakest. Depending on the previous year's harvest, the granaries may also be nearly empty, although the grain must hold out through the rainy season until the new harvest is ready.
One of the primary characteristics of arid and semi-arid zones is extreme variability in rainfall, and the Sahel is no exception. Generally variability increases as one goes from south to north, at least until the hyper-arid (less than 100 mm) zone is reached. It is still a matter of controversy whether the low rainfall years are due to stronger highs over the Indian Ocean (Schove 1977) or lower surface temperatures in the Atlantic (WMO 1975), but neither of these are likely to have much value in terms of forecasting drought. Different authors have also analysed the available climatic data in an effort to find cyclical patterns (Faure and Gac 1981; Schove 1977) and these studies have helped us to understand the extent and frequency of variability. However, the cyclical patterns that have emerged are weak and based on a relatively short record; so again the predictive value is severely limited. Figure 2 shows the variation in annual rainfall for the different zones from 1901 to 1980.
Climatic predictions are further hampered by the fact that changes in the movement of the Intertropical Convergence Zone are not well understood (Rasool 1982). Some authors have postulated a positive feedback loop, whereby drought causes a decrease in the vegetative cover, which then may increase the albedo, increase the dust content, or decrease the amount of freezing nuclei. These in turn would cause a further decline in precipitation (Nicholson 1983). While current models indicate that such processes could have an effect, there is no real evidence that this is the case. The fact that droughts don't persist indefinitely suggest that there are other, more powerful processes at work.
FIG. 2 Surface levels of Lake Chad (annual maxima and minima), 1870-1970 (From Sikes 1972)
Climatic change is another popular incantation that is brought forward to "explain" the recent droughts in the Sahel, but this carries little validity except over the very long term. From 20,000 to 12,500 years ago, for example, the climate of the Sahel was probably very much drier, with active dunes 500 km south of where they are now (Grove 1978). These fossil dunes, now stable and covered with vegetation, are an important feature in today's landscape. From approximately 12,500 to 9,500 years ago, the climate fluctuated, but gradually became wetter than at present, and this relatively humid period confined until some 4,000-5,000 years ago. Since then the climate in the Sahel has been roughly similar to today's climate. Wetter periods do exist, but these do not exceed the historical variation nor provide any indication of a long-term trend. Figure 3 shows the annual and long-term variation in the level of Lake Chad, and this does not show any major drying trend prior to 1970.
While annual precipitation means, such as those indicated in figure 1, can provide a valuable first approximation to the climate in the Sahel, they can also be very misleading. First, the amount of precipitation must be considered in the context of the extremely high annual evaporation figures (3,000-4,000 mm). Second, one must take into account the extreme variability of precipitation in arid lands, both spatial and temporal. As an exemple, it is worthwhile to look at tow station in depth, Agadez and Madona in Niger. At Agadez, in the western part of Niger (170N, 80E), annual precipitation has a 53-year mean of 158 mm, with a maximum of 288 mm in 1958 (182 per cent of the mean) and a minimum of 40 mm in 1970 (25 per cent of mean). Table 2 present monthly average temperatures and precipitation and evaporation-rate figure for Agadez.
FIG.3 Sufrace levels of Lake Chard (annual maxima), 1870-1970 (From Sikes 1972)
TABLE 2. Climatic data for Adages, Niger
|Mean temperature (°C)a||Evaporation rate (mm)b||Rainfallc|
Source: UNCOD 1977a
c Mean 1921-1954
d Number of days with rainfall over 1 mm.
With a standard deviation of 57.2 mm, in any giver year the
chance are one out the three that rainfall will exceed 215 mm or
fall below 101 mm (UNCOD 1977a). At Madona, in south-central
Niger, records from 1944 to 1974 indicate a mean of about 450 mm
per year but with a maximum of 825 mm in 1950 (183 per cent of
the mean) (Faulkingram 1977).
From a plant's point of view (or, for that matter, a dryland farmert's) the timing of the precipitation is at least as important as the total amount. A dry spell of tea days or more can seriously retard the development of a crop, and if prolonged, will kill it. Such dry spells after the onset of the rains were documented in 1970, 1972, and 1973 in a village near Madona (table 3), and each time the dry spell necessitated a second or even a third sowing, further depleting whatever stored grain was available until the next harvest.
TABLE 3. Rainfall (mm) over ten-day periods in Tudu, Niger, 1969-1974
|Month and day||1969||1970||1971||1972||1973||1974|
|1 - 10||13.0||0||0||8.2||0||2.0|
|1 - 10||29.4||52.0||47.7||104.9||25.6||159.7|
|1 - 10||35.9||9.5||26.4||0||27.6||26.8|
Source: Faulkingham 1977
Pasture species are usually much more resistant to drought, but a dry spell after the onset of spring growth will reduce the number of annuals and tower the reserves of perennials (UNCOD 1977a). Undoubtedly annual seeds have a large degree of variability with regard to time and moisture requirements for germination, but a series of such breaks in the rainy season can lead to a relative increase in perennials. The timing and intensity of rainfall interacts with other factors such as slope and soil type to indirectly affect not only germination and primary productivity but also soil erosion, species composition, patterns of grazing, etc.
Spatial variation in precipitation is also very important, as indicated in table 4. P1, P2, and P3 were stations located less than 10 km apart in a small drainage basin 30 km from Agadez. The data for August again suggest that variability tends to be inversely proportional to rainfall, as one storm on 14 August 1973 dropped 50 mm on P1 and P3 but only 24 mm on P2. In areas with a higher average annual rainfall, these variations would tend to even out.
TABLE 4. Rainfall (mm) at stations near Agadez, Niger, 1973
Source: UNCOD 1977a
In summary, rainfall in the Sahel is highly variable with regard to its timing, amount, and spatial distribution. In general, as the annual average precipitation decreases, the variability increases. In turn, this amount of variability suggests that one or more dry years will occur with a certain statistical frequency. This does not, however, imply any sort of periodicity or regularity of drought, nor any predictive capability other than an estimation of probabilities based on the past record.
This variability also makes it mathematically impossible to make any claims of short-term climatic change. In all likelihood any past climatic change took place over hundreds of years, so the change over a period as short as 50 years would be at most a few millimetres of rainfall. Our understanding of the global weather system is such that this also doesn't allow any predictive capability. It may be that dry years tend to come in series rather than randomly (Nicholson 1983), but the cause of this is uncertain. Finally, this variability in climate, combined with the lack of predictability, has critical implications for resource use and development planning.
Contents - Previous - Next