Contenu du sommaire : Sahel, la grande secheresse

The area of the Sahel is an environment facing various hazards. The severity of the present crisis results from the conjunction of both natural and anthropological factors : an unprecendented shortage of rainfall, overgrazing, a demographic explosion and the consequences of a modernisation of society which has forced nomads to become fixed settlers and led to uncontrolled urbanisation. To resolve this crisis, action needs to be taken in three areas : improve the management of water resources, control demographic growth and restore the vegetation cover.

The Grande-Côte of Senegal and the Cap-Vert archipelago respond to common dynamic conditions, receiving both winter rainfall and, above all, summer precipitation brought by squall lines and cyclonic depressions. The recent climate change is characterized by a significant modification of numerous meteorological variables. Rainfall has been the most affected, experiencing a substantial reduction. The deterioration in precipitation levels has been particularly dramatic in the Sahel region, and has disadvantaged above all the Cap-Vert archipelago, despite its maritime situation.

The Sudanese-Sahelian region has been affected by drought, a condition characterized by its long duration, its intensity and its southwards extension. In Senegal average isohyets for the periods 1931-1960 and 1961- 1990, 1970-1993, using data from 32 rainfall stations reveal the southern migration of the different climatic regimes. The reduction in rainfall has resulted in damage to the natural environment. Drought leads to destruction of vegetation. Soils suffer from erosion and gullying. Acidification and salinisation are accentuated, especially in the area of Casamance and the Saloum estuary. Wind deflation is intensified.

The streamflow of the Nile river, like those of the others waterways which pass through the Sudano-Sahelian zone, has been considerably strucked down in 1972, 1973, 1983 and 1984. The hydrological drought has brought on a water shortage characterized by a degradation of the streamflow of the river Nile, a lowing of the fluvial underground water table, an abnormal depletion of soil moisture and a damage of the fluvial environment (erosion and undermining of banks, withering of the fluvial vegetation, decrease of the river fishing) in the river Nile valley.

Analysis of the evolution of rainfall in west Africa is updated. This is based on the calculation of new mean rainfall figures for the period 1961- 90. The first stage consisted of analysing results from more than a thousand rain gauges to provide long time-series data to determine the year when trends changed. The justification for the choice of 1968 as the beginning of the drought is discussed. Comparison between the wet period, 1950-67, and the more recent period since 1968 is based on an analysis of medians which shows that probability of a change in rainfall is one year in two. Study of variations shows a dramatic reduction in rainfall on the Atlantic coast. This reduction can exceed an annual total of 400 mm. The monthly distribution of variations shows a contrast between the zone north of the 10th parallel and the maritime region of the Gulf of Guinea.

It has become fashionable to provide explanations for droughts in the Sahel. However, such a major climatic change extends far beyond the African context and could only relate to broader meridional patterns of general atmospheric circulation. The cooling of the Arctic has increased the strength of the moving polar highs and increased the dynamism of the northern meteorological hemisphere, causing the southwards migration of the whole of the tropical rain-bearing structures over a distance of 200-300 kilometres.

The drought offers a convenient "explanation" and justification for regional change. However, it is still necessary to be sure that this is automatically accepted by societies. This is not the case in Mauritania where patterns of migration and permanent settlement respond to processes which are more complex and subtler than the sweeping changes imagined by a decline in rainfall. Even the options for regional development reveal a diversity of situations leading to regional specialisations. In the same context, an ancient form of spatial organisation in the Sahelian - Saharan area has reappeared : local complementarity within a society based more on exchange than on production.

Analysis of rainfall in Mauritania since 1941 aims first to study the variability of precipitation from year to year and, second, to determine spatial variations in the lack of rain during the years of drought, notably in the 1970s. By highlighting regional disparities, the study emphasises the effects of the drought on the natural environment as well as on population movements at a national scale.

The drought has produced an aptitude for change, as well as various initiatives to combat the problem. Thus, the government has organised the agricultural colonisation of valleys by establishing within designated areas thousands of farmers, supervised by a specialised state organisation. Rightly or wrongly this operation has been considered as a means of combating the drought, although irrigation has not been used. Moreover the new settlements of the middle basin of the Nakambé (White Volta) might be considered as a initiative.

Recent periods of drought have followed many other crises, where famine has resulted from drought, epidemics, epizootics and locust invasions. Faced with these catastrophes, shepards have developed numerous strategies, linking mobility and a diversification of resources. Recurrent droughts have led the government and developers to attempt to create groups of breeders similar to the cooperatives which have already been set up in rural areas. Currently shepards are increasingly blamed for practicing a form of pastoralism which is judged outdated.

Reconstituting the vegetation cover is fundamental to the redevelopment of the damaged areas of the Sahel. Attempts at reafforestation have proved difficult and generally inefficient. Although the herbaceous stratum is relatively well-known, it is essential to study the forms of constitution and renewal of seeds in the ground, for these ensure the permanence of annual species. Much work has been carried out on identifying seeds and on the links between the seed potential and the resulting herbaceous cover in relation to variations in rainfall, the nature of soils and methods of cultivation. Amongst the techniques employed to protect the environment, the delimitation of protected areas has proved interesting as it clearly reveals the potentialities of the seeds in the soil. It is important to pursue actively research in this area.

The Climatic Watch Programme aims to create climatological data files to allow climatic surveillance and to assist in agrometeorology. This is to be achieved by the combined utilisation of satellite meteorologie imagery and ground data. Parameters currently available, are the plotting of the Intertropical convergence zone (ITCZ), ocean surface and land temperatures and the frequency of convective clouds (clouds with a cold top). Other important parameters related to radioactive flows and the climatology of clouds are being studied. Major research has already been conducted into estimating rainfall by satellite, enabling an evaluation of the hydric condition of the soil surface.

Exogenous rivers are supposed to provide an important resource for the dry countries they traverse. An analysis involving the two major rivers of Africa to the North of the equator, the Nile and the Senegal, suggests the need for a dramatic revision of such optimistic views due to the present drought : the forecast of the resources use is computed according to the flows of years characterised by important rainfall ; the demand of upstream countries is growing fast ; the riverside users are frequently not able to manage new and changing technologies. Due to the lack of practises designed to conserve water, such as tarification by consumed volume, the perspectives based on the exogenous rivers could be misleading.