Évolution récente des ressources en eau de l'Afrique atlantique Water resource variations of the atlantic river basins of africa : the long term effects of rain shortage

Résumé

Dans le contexte pluviométrique déficitaire de l'Afrique intertropicale, l'hydraulicité des fleuves soudano-sahéliens a beaucoup diminué depuis 25 ans. Les fleuves des régions humides, relativement épargnés jusqu'en 1980, montrent aussi, avec un retard de plusieurs années une hydraulicité qui baisse dans des proportions importantes, alors qu'un retour à la normale du régime des pluies paraît amorcé. Le déficit des apports est pour l'Afrique humide de 16% pour la décennie 80 (365 km3 an-1), contre 7% pour la décennie 70, et pour l'Afrique sèche de 27% (65 km3 an-1) pour la décennie 80 contre 13% pour la décennie 70. Les maximums annuels de crue sont systématiquement plus faibles dans la période récente. En régime équatorial, la crue de printemps est la plus affectée par le déficit hydrologique et la différence entre crue de printemps et crue d'automne a triplé entre les années 1950-1960 et les années 1980. L'ensemble de la région connaît des étiages exceptionnels répétés. En zone soudano-sahélienne, le tarissement s'est considérablement accéléré et montre une vidange des écoulements de base deux fois plus rapide dans la période récente. Le phénomène est également sensible en Afrique humide et traduit un amenuisement des réserves souterraines dû aux effets cumulés des déficits pluviométriques et décalé dans le temps, que l'on retrouve dans la baisse du niveau piezométrique des nappes phréatiques. Un effet mémoire est donc observé sur les écoulements de base, avec pour conséquence un un appauvrissement durable des ressources hydriques. Celui-ci restera indépendant, pour un temps, d'une éventuelle amélioration des conditions climatiques, comme le montrent les observations des années les plus récentes.

Mots-clés:

• Afrique tropicale,
• précipitation,
• ressources en eau,
• déficits

Abstract

The extent of the current drought phenomenon in Sudano-Sahelian regions no longer needs to be proven. The study of rainfall deficits in intertropical Africa shows a global climatic degradation which has been going on for about twenty five years. These deficits concern also the humid areas of Western and Central Africa and finally all the african rivers draining the flows to the Atlantic Ocean. Various studies have showed that the current phenomenon greatly differed, in intensity and duration, from other 20th century deficit situations (Sircoulon et Olivry, 1986, Lamb,1985, Nicholson et al, 1988). Despite very great fluctuations which could remind pseudocyclic variations, the current data pointed out a global downward trend since 1860 (Olivry, 1987). Moreover, some authors have seen a statistical break in the rainfall conditions in 1970 (Carbonnel et al, 1985). The necessity to obtain common continual data series led to select the period from 1950 to 1990 with a large number of operational rainfall and jauging stations (Mahe, 1993) (figures 2 et 3).

Without expecting a continuation in this trend, or, on the contrary, a return to a humid period, the current drought will obviously have durable consequences on some hydrological parameters. After a relative improvement of rainfall conditions, observed in 1985 and 1986 and even more recently, as shown by the evolution of the Lamb index (1985). African drought and rainfall deficits observed since 25 years have important repercussions on the flow regime of rivers and especially on the annual runoff and the strenght of annual floods. This phenomenon has intensified during the last ten years while, paradoxically, rainfall deficit greatly has diminished. For instance, the upper Niger river ,with the Bani river, (surface basins 250,000 km2) shows an annual runoff deficit of 20% during the 70's and of 46% during the 80's and an annual rainfall deficit respectively of 15 and 20%. In the regional context of Western and Central Africa, the global runoff deficit varies from -7% to -16% for the decades 1970 and 1980 in the humid areas, and from -13% to -27% in the dry areas (deviations calculated in relation to the 1951-1990 mean values) (Olivry,1993 ). The global annual runoff deficit during the 80ís decade reached 430 km3 year-1 for the atlantic african tributaries (figures 4 et 5, table 1).

During the two last decades, flood hydrographs of large river basins have been greatly reduced in volume and duration; maximum values are much smaller. the flood hydrographs showed lower annual maximum discharges and shorter high water seasons than during the preceding period. In the equatorial areas, with two flood periods by year, the datas showed that the spring floods were more affected than the autumn floods by the climatic changes. (figure 6). A frequential study of flood maxima shows a clear break in data series, and emphasizes the danger of flooding estimations which are based upon recent years only. The long term effects of rain shortage are more marked still for the low flow regimes and different studies have showed that, in addition to the immediate effects of the drought, a large reduction of the groundwater storage can explain the persistent fall of the hydrological resource. In the case of recession-depletion period and low flows, the memory effect of the current deficit period is particularly clear. An unusual repetition of exceptionally dry low flows can be observed, especially an abrupt change in the depletion regime, unprecedented in hydrological time series. It also may be observed that a return to more favourable climatic conditions does not involve an immediate return to the earlier hydrosystem . The figure 7 shows two representative curves of the variability of the low flows (absolute minimum daily discharge) between 1950 and 1990 for a Sudano-Sahelian basin (the Bani river) and for a tropico-equatorial basin (the Sangha river, tributary of the Congo river). It is clear that the dry Africa is more affected by drought and rainfall deficits than the humid Africa, but during the last decade the low flows of the Sangha river have been divided by 2 in relation to the low waters observed before 1970 (Laraque et al,1996). The large deficit of the low flows was also observed on the flows of the Ubangi, Congo/ZaÔre, Ogowe, Sanaga, Kwilu, Bandama and Konkure rivers in the humid areas during the last decade (Olivry, 1993) .

In the dry Africa the deficit of low flows is larger still and it has been observed during the last two decades. After the annual flood and the recession, which occur especially as fast as the rainy season ends, there is a classical pattern of flow drop in the rivers in this region. This corresponds to the basin aquifers draining, also called depletion. This depletion is linked to an exponential law : Q_i=Q_o e^-a(ti-to) with the flow Qo on a to day, the flow Qi on a ti day, and (, a coefficient expressed in days-1, according to the physical and geometrical characteristics of the aquifers. The studies show a significant degradation of the depletion.. This phenomenon, resulting from a deterioration of aquifers, was first noted on the Senegal river in 1983 (Olivry, 1987). Since then, it has been observed again and specified, in relation to all the flows in the Sudano-Sahelian region. On a regional scale, it can be considered as a likely model of the progressive decrease of flows, In the Senegal river (218,000 km_ at Bakel), the depletion coefficient has been relatively stable for over seventy years during humid as well as dry periods. The average is 0.0186 day-1 and the variation coefficient is only 0.075 . With the current drought, the alteration in the pattern only becomes significant after 1975. The depletion coefficient rapidly rises, to reach values such as 0.04 day-1 around 1985. The Niger river (120,000 km_ at Koulikoro and data since 1907) shows before 1975 an average coefficient of 0.023 day-1. The current period shows an average coefficient over the first 8 years as being around 0.028 day-1 and then an abrupt increase to values as high as 0.040 day-1 in the beginning of the 90's.The Bani river(102,000 km_ at Douna), tributary of the Niger river, shows a significant evolution (figures 8 et 9). It is easy to give many examples in the Sudano-Sahelian region and the phenomenon is still likely in humid tropical areas, even it is less marked. Thus in the case of the Ubangui river at Bangui (500,000 km_); the average coefficient from 1935 to 1975 was 0.021 day-1. It has been equal to 0.025 day-1 during the last fifteen years. The increase of the depletion coefficient of the Sangha river is lower, but in the equatorial regions, it is often difficult to observe the depletion periods, the flows being affected by rains during a short dry season ( figure 10) . The depletions similarity in the case of different rivers and even of streams which was observed in the flow time series before the current period must be underlined. The ( values of 0.02 day-1 are characteristic of a low water supply deriving from small hillslope aquifers. These aquifers are typical of the intertropical african geomorphology, where there is no large groundwaters. A good relationship appears between the coefficients variations of recession and of drainage ( Olivry, 1976). According to Darcy's law, the deviations from the norm would therefore mainly correspond to the variations in the aquifers width. The hydrogeological datas showed a drop of 10 m and more of the water table above hillslope aquifers in Senegal, Mali and Burkina Faso during the 80ís. Consequently, there is nothing surprising in the fact that the decrease of these small aquifers results in a comparable ´†hyperdepletion†ª in the basins of various areas affected by the same climatic hazards. A return to better sustained discharges during the dry season first requires the aquifers reconstitution. This cannot happen immediately, as shown by the low effect of the last better rainfall on depletion coefficients. There is no exact convergence between the variations of Lamb's rainfall index and the depletion coefficient ones. Through the aquifers, reactions to the low water restitution are slower ; they require a response time of several years and only become effective with the accumulation of similar climatic variations. The groundwater supply deficit in rivers, considered here, which is noticeable in the dry season, is obviously underlying during the flood period and this also explains the low annual runoff and the poor flood maxima we still observe despite some rainfall recovery. The"memory effect" of the droughtness indicates the important part taken by the base flow in the flood hydrographs of large rivers. A specific stream drought, which could be refered to as a "phreatic drought", is to be added to the climatic drought with a pluriannual gap. The hydrogeological laws are such that, under good rainfall conditions, a restoring of the hydrological regime should last as long as it took to bring it to its current degradation.

Keywords:

• Tropical Africa,
• precipitation,
• rainfall,
• water resources,
• deficits