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Congo River Basin: Geology and soil type influence drought impact

Institut de recherche pour le développement

Two main points emerge: the Congo discharge levels were markedly unstable during the second half of the century, decreasing steeply over the last decade; and, although the discharge fluctuations are induced by rainfall variations, that influence is modified considerably by the soil type and geology of the terrain through which the rivers pass.

The Congo drainage basin is situated in Central Africa. Its hydrological system straddles several countries (Congo and the Democratic Republic of Congo for the most part, but also Angola, Cameroon, the Central African Republic, Zambia and Tanzania, stretching through Lake Tanganyika). The River Congo, the second longest African river after the Nile, second in the world, after the Amazon, in terms of discharge, itself accounts for half the total volume of waters which pour into the Atlantic from that continent. An understanding of how the hydrological system works is indispensable now at the start of a new century when water is such a crucial issue, especially in Africa.

The IRD hydrologists first studied the Congo catchment rainfall data. This basin, just like in the whole of Africa, and especially in the North of the continent, has been hit by a period of drought which started to bite in the second half of the XXth century. The drop in rainfall appeared first in a sub-basin of the Congo, that of its main tributary the Oubangui, from 1960. Precipitation decreased by 3% between the two periods 1951-1959 and 1960-1989. In other sub-catchments (of the tributary Rivers Shangha and Kouyou, further South), rainfall figures began to decrease 10 to 13 years later. For the Congo Basin as a whole, comparison of data for the periods 1951-1969 and 1970-1989 revealed a rainfall loss of 4.5%.

As for discharge, data indicated a series of four distinct phases in the Congo and the Oubangui since the beginning of the XXth century. There was a situation of stable discharges up to 1960. Thereafter they changed with each decade. During the 1960s they increased, overtaking their average over a century. The Congo discharge then fell, returning in 1970 to what had been the normal level, whereas the Oubangui entered a drought phase. This trend accentuated from 1980 and, until 1996, the Congo discharge weakened by 10% (37 400 m3/s in 1992 compared with an average of 40 600 m3 /s over that period as a whole), which was the most dramatic decrease of the century. This fall is much stronger in the Oubangui (- 29%), yet negligible (- 0.2%) in the Kouyou sub-basin. Overall, whereas discharge decrease in the Congo Basin is between two and four times the drop in rainfall, it is nine times that figure in the Oubangui.

Great differences in discharge reduction are therefore evident between the different rivers in the Congo system, in spite of a generalized drought which has been prevailing over the entire region. What can explain this? The researchers have brought evidence here of the strong influence of soil geology on the effect a change of rainfall has on river discharge. The various sub-catchments here indeed differ widely in their geology. In the North, the Oubangui sub-basin is a ferruginous cuirassed peneplain which favours water runoff. Further South, the Shangha sub-basin has a sandy soil where the land becomes partly flooded during periods of heavy rainfall. The third, nearer the river mouth, is where the Kouyou sub-basin borders the Bakétés plateaux consisting of sandstones. These are porous and permeable, aquifers capable of storing an excess of water. The drought has not had the same impact in the three regimes. The nature of the geological substrate of the Oubangui sub-basin amplifies considerably any variation in rainfall; between 1982 and 1993, a 3% drop in rainfall induced a 29% loss of discharge. Conversely, the sandy soils of the Kouyou sub-catchment have a stabilizing effect, in that they store or release water. During the humid period of the 1960s, excess water from heavy precipitation entered them and was held there. When drought followed, the water was released. The decrease in rainfall in this area is about 5.3% but the effect on the discharge is 26 times smaller: it has diminished by only 0.2%!

This study does not call into question the dramatic effect drought has been inflicting on Africa since the 1970s. However, it does bring a more accurate picture of its consequences for water resources. Its size and the diversity of its tributaries provide the Congo Basin with an overall stability against variations in rainfall. If the balance of the hydrological regime of the main river was as delicate as that in the Oubangui sub-basin, it is easy to imagine the consequences the recent drought would be having on this region of Africa.

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