|Sustaining the Future. Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 p.)|
|Part 2: Environmental issues and futures|
|Drought, desertification, and water management in Sub-Saharan Africa|
Recurrent droughts are a salient feature of the semi-arid lands of Sub-Saharan Africa, especially the Sahel. Rainfall fluctuates widely in time and space in a way that has not yet been understood and therefore cannot be easily forecast. The implication of this extreme variability for planning and development is that the concept of "mean" or "average" rainfall has little value.
Some of the main characteristics of the drought-prone climate of the region are presented below.
Rainfall fluctuation in time
The recent drought in the Sahel is not unique in the history of the region. Droughts of this magnitude and extent have occurred in the past. Nicholson (1982), using records of harvest quality, lake levels and river flow, rainfall data, and climatic description, reconstructed past episodes of droughts and good rainfall (see figs. 8.1 and 8.2).
For example, a wet episode lasted from about 1870 to 1895. Harvests were consistently good in the semi-arid regions of Namibia, southern Angola, and South Africa. North of the equator, the Niger Bend region near Timbuktoo, Mali, in the Sahel yielded abundant crops and the region became the "bread basket" of West Africa. Today, annual rainfall in the area is only about 200 mm, below the rain-fed agriculture boundary.
Fig. 8.1 Trends of African indicators of lake and river levels, rainfall, and harvest quality, 18801920 (Source: Nicholson 1982)
Fig. 8.2 Annual rainfall fluctuations in the Sahel, 1900-1982 (Source: Sircoulon 1992)
About 1895, a major change towards more arid conditions occurred and the "desiccation" culminated in severe drought around 19131914.
In more recent decades, rainfall was more abundant in the 1950s, with records as high as 30 to 60 per cent above "normal." But this episode ended abruptly towards the end of the 1960s, giving way to the extreme drought of the early 1970s when rainfall went down 15 to 35 per cent below normal in the Sahel. In some areas, rainfall during the 1968-1973 period was 50 per cent lower than during the 1950s.
Fluctuations are often abrupt and extreme, with droughts recurring at irregular intervals. The change from wetter conditions to persistent drought cannot as yet be forecast. Thus the implication for planning and development is that the carrying capacity of the land should be that of the driest years, not the "average" year.
Clustering and persistence of abnormal years
An unusual feature of rainfall fluctuations in the region is their extreme persistence for one to two decades or more.
In most humid areas, dry and wet years are generally randomly interspersed. In some arid regions, however, abnormal years tend to cluster together. In the Sahel, this characteristic is extreme: wet or dry conditions may persist for one or two decades. As an example, the period 1960-1980 was rather consistently dry. Droughts lasting one or two decades are also evident in the historical records mentioned earlier.
A study by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT 1988) illustrates this feature. Change in rainfall variability in semi-arid lands in India and the Sahel was compared by analysing long-term climatic records for Niamey in Niger and Hyderabad in India. The variability in annual rainfall at the two locations is shown in figure 8.3. In 87 years (1901-1987) at Hyderabad, the time-series showed no significant trend, and the rainfall in a given year is not correlated with that of the preceding or the following years. At Niamey in the Sahel, variability of annual rainfall from 1905 to 1987 revealed a different pattern. Compared with Hyderabad, Niamey has longer sequences of consecutive "dry" years with below-average rainfall, and much fewer "wet" years. Droughts are longer in the Sahelian zone.
The clustering of abnormal ("wet" or "dry") years seems to be unique to the Sahel. Other semi-arid lands in Africa do not exhibit this characteristic. Comparison of climatic data between the Kalahari region in southern Africa and the Sahel showed that, although drier (and wetter) periods tend to occur synchronously in both areas, the decadal persistence observed in the Sahel is not a distinct feature of rainfall fluctuation in the semi-arid lands of southern Africa; dry episodes coincide in the two regions but the years of most intense drought do not (Nicholson 1982). Although the recurrence of drought has been shown to be cyclic in some semi-arid regions, this has not been clearly established for the West African Sahel.
High spatial variability is a well-known characteristic of Sahelian rainfall (figs. 8.4 and 8.5). The reason for this spotty distribution is the convective nature of the rains, i.e. the showery type, as shown in figure 8.6; the falling of rain from a cloud generally signals the onset of dissipation of the cloud, which may grow again and produce intense rainfall further away.
Fig. 8.3 Variability in annual rainfall in Niamey, Niger, and Hyderabad, India (Source: ICRISAT 1988)
Implications for planning and development
The implication of this extreme spottiness for regional planning is frightening. Two villages only 1 km apart can experience entirely different rainfall regimes at one time or another during the season, even during a year when total rainfall is comparable in both locations. For a given rainstorm, one village may be drenched while its neighbour remains dry. If this happens at a crucial time for crop development - e.g. when seedlings are establishing themselves or grain is maturing - this can mean a good harvest for one village and total crop failure for its neighbour.
Fig. 8.4 Spatial variability of rainfall on 12 and 13 June 1986 in Ouagadougou, Burkina Faso (Source: Rochette 1989)
Other features of the Sahelian climate described earlier pose equally serious constraints for planning. Persistent years of aboveaverage rainfall can create a false sense of the true climatic conditions and mislead farmers and pastoralists to extend their activities into the marginal desert fringe beyond the true agronomic dry boundary. They may then become trapped in this fragile environment during a drier period; the environmental damage that then occurs is intensified. This may have contributed to the disaster accompanying the droughts of the 1970s, which succeeded a very "wet" period in the 1950s.
As Nicholson (1982) summed it up, the characteristics of the Sahelian climate that should be kept in mind include the low and highly variable rainfall, the prevalence of dry years, the extreme magnitude of the variability, the rapidity with which new persistent conditions can be established, and the spottiness of rainfall even in "normal" years. The "persistence" feature of the Sahelian climate is a clear message to planners that technologies and strategies that have produced good results in other semi-arid regions of the world may not be expected necessarily to work in the Sahel.
Fig. 8.5 Spatial variability of rainfall shown as isoheyets measured on a 400 m grid over 500 ha at the research station of the ICRISAT Sahelian Centre, Niger, 22 July 1986 (Source: ICRISAT 1988)
The task ahead is immense. A complete understanding of the climatic peculiarities of the semi-arid lands of Sub-Saharan Africa and the tuning of development planning to these characteristics deserve sustained efforts. On-going programmes/projects and recent initiatives in this direction include those by the Regional AgrometeorologicalHydrological Centre, the Sahelian Centre of ICRISAT, the African Centre of Meteorological Applications for Development, the Sahara and Sahel Observatory, and the International Geosphere-Biosphere Programme in the region. This development is encouraging, but a breakthrough is still awaited.
Fig. 8.6 Continual growth and decay of a typical convective cloud as it moves downstream with the wind (Source: Nicholson 1982)