Influence des couvertures lithologiques et végétales sur les régimes et la qualité des eaux des affluents congolais du fleuve Congo Impact of lithological and vegetation covers on flow discharge and water quality of Congolese tributaries of the Congo river

Résumé

Les affluents et sous affluents congolais du fleuve Congo se partagent en deux systèmes hydrologiques, qui, tout en étant proches dans l'espace, sont sensiblement différents dans leur fonctionnement. Cette distinction repose sur la différenciation manifeste entre les deux régions naturelles que sont les "Plateaux Tékés" d'une part et la "Cuvette congolaise" d'autre part, nettement contrastées du point de vue du couvert végétal et lithologique et subsidiairement du climat

La classification hydrologique que nous présentons est basée sur l'étude des coefficients d'écoulement interannuel (rapport de la lame d'eau écoulée sur la lame d'eau précipitée, exprimé en pourcentage) et sur le coefficient d'irrégularité saisonnière moyen des débits qui est le rapport moyen sur les chroniques disponibles, entre débits mensuels maximum et débits mensuels minimum par année hydrologique. Pour les rivières Tékés, le premier paramètre présente des valeurs comprises entre 45% et 60% contre 20% à 30% pour la "Cuvette Congolaise", alors que le deuxième paramètre varie respectivement pour ces deux régions de 1,1 à 1,5 et de 2,5 à 5,5.

Cette classification présente l'avantage d'être à la fois adaptée à ces deux contextes régionaux voisins mais aussi de mettre en évidence le rôle prépondérant des formations géologiques et des couvertures végétales sur les écoulements des cours d'eau concernés, ainsi que sur la qualité des eaux.

En effet, après son passage au travers d'un important aquifère gréseux, les eaux des Plateaux Tékés à couverture de savane font partie des "eaux claires", à très faible minéralisation, marquée essentiellement par la présence de silice dissoute. En revanche, les " eaux noires " couleur de thé qui drainent la Cuvette Congolaise, couverte de forêts, sont très riches en matière organique et très acides

Mots-clés:

• Régime hydrologique,
• hydrogramme,
• coefficient d'écoulement,
• débit spécifique,
• module,
• Congo,
• plateaux Tékés,
• Cuvette congolaise,
• qualité des eaux

Mots-clés:

• Hydrological regime,
• hydrograph,
• runoff coefficient,
• specific discharge,
• module,
• Congo,
• Téké plateaus,
• Congolese Basin,
• water quality

Abstract

The Congolese tributaries and sub-tributaries of the Congo river come from two adjacent regions that have similar annual rainfall volumes (1,700 and 1,900 mm.year-¹) but very different physiographic characteristics. These latter characteristics are the main reasons accounting for the differences in their hydrological behavior. The purpose of this work is to introduce a regional hydrological classification - adapted to the particularities of the regions that are crossed by these tributaries - that points out the major influence of the geological formations and the vegetative cover on the runoff of the rivers, as well as on their water qualities.

The two natural regions concerned are the Téké plateaus and the Congolese basin. The first region, centered on Congo but also spread into Gabon in its western part and into Congo on its eastern side, consists of sand-gritty formations of the Tertiary going from 200 to 400 m in thickness, covered with savanna. The second region builds up a continuation of the wide Congo depression that occupies the whole center of the Congo river basin. It comprises Quaternary alluvial deposits, covered with swamps and dense equatorial rain forests.

The hydrological characterization we present here is based on two hydrological coefficients that do not have physical linkages but illustrate the differences in the regimes of their respective flows. These are the interannual runoff coefficient (KE) and the average seasonal variability of the discharges (CIS) observed over a period of forty years. Concerning the Téké rivers, the first parameter (KE) has values ranging between 45 % and 60 % versus 20 % o 30 % for the Congolese basin. The second parameter (CIS) has values ranging from 1.1 to 1.5 for the Téké rivers, versus 2.5 to 5.5 for the Congolese basin. The specific discharges of the first region vary from 25 to 35 l.s-¹.km-² versus 10 and 15 l.s-¹.km-² for the second region.

Concerning the Téké plateaus, the weak average seasonal variations of the discharges can only be explained by a great capacity for storage, and consequently for flow regulation, of the sand-gritty aquifer. Infiltration of the rainfall waters is important and they are mostly replaced by this important aquifer which attenuates the floods and helps minimize the droughts. This is also the reason why it is in this region that some of the most regular rivers of the planet are encountered.

Concerning the Congolese basin, the lower permeability of the soils, the interception of rain waters, the evapotranspiration of the forest cover, and the direct evaporation on the floodable areas as well as the swamps, lead to an important water deficit. As there is no aquifer, at least none as important and regulating as in the Téké plateaus, the hydrological regime is more similar to the regional rainfall rhythm. The Téké hydrological regimes are for their part practically independent of the regional rainfall regime.

From a qualitative point of view, these waters are also very distinct. Indeed, after crossing the important gritty aquifer, the waters of the Téké plateau - which is covered with savanna - consist of highly diluted clear waters, relatively rich in dissolved silica. These waters, with dissolved inorganic matter ranging from 1 to 3 mg.l-¹ (not taking into account the dissolved silica), are similar in composition to rainfall waters. These are among the most dilute surface waters of the world, the soluble ions of the mentioned formations having been almost totally leached.

On the other hand, waters draining the Congolese basin originate from a long residence time under the forest cover. They are very rich in organic matter (up to 44 % of particulate organic carbon) and very acid (pH can be lower than 4). They are truly qualified as "black rivers". Their mineralization, although low (11 to 30 mg.l-¹, without dissolved silica), can be considered high when compared to the plateaus' waters, and also of a greater diversity. For both regions, the content in dissolved silica is identical to the world wide average, varying between 8 and 10 mg.l-¹.

If the specific export rates in dissolved matter are low, they are nevertheless identical in the two geographical domains despite the important differences in concentrations. For matter of mineral and organic origins, we calculate respectively 10 t.km-².year-¹ in the basin versus 9.7 t.km-2.year-1 on the plateaus, and 19.3 t.km-².year-¹ in the forest versus 13.5 t.km-².year-¹ in the savanna. This apparent similitude is obviously linked to the compensation by the specific discharges of the Plateau rivers, more than twice as much as in the basin.