In this paper, we study the impact of climate change on river regimes in several parts of Africa, and we look at the most probable causes of these changes either climatically or anthropogenically driven. We study time series of updated monthly and annual runoff of rivers of North Africa, West Africa (Sahelian and humid tropical regions) and Central Africa, including the largest river basins: Niger and Volta rivers in West Africa, and Congo and Ogooue rivers in Central Africa. The recent years are studied in the perspective of multi-decadal variability. In West Africa and in a part of Central Africa, the climate has changed since 1970, and rainfall has not returned to previous annual amounts, except in Equatorial Africa. The consequences of the long-lasting drought are, depending on the area concerned, the modification of seasonal regimes (Equatorial area), the groundwater table decrease (Tropical humid area) and the land cover degradation (Sahelian area). The increasing number of dams and of agricultural areas also plays a major role on the modification of river regimes. The population increase will continue to impact on the environment: land cover change, deforestation, agriculture and increasing number of dams will be associated with a reduction of water and sediment discharges to the sea, and major impacts on downstream ecosystems and coastal areas. It seems necessary to share with stakeholders a comprehensive approach of the water cycle from the basin to the sea, to prevent long-lasting damages to ecosystems and infrastructures.

Résumé L'analyse de la variabilité spatio-temporelle de totaux pluviométriques, de débits et de coefficients d'écoulements, aux pas de temps mensuels, saisonniers et annuels dans le sud équatorial du Cameroun, révèle que les pluies et les débits annuels ont diminué lors des années très sèches (1972-1973 et 1983-1984). Le changement climatique le plus significatif réside dans la modification des régimes de pluie des saisons sèches, modifiant le cycle hydrologique annuel. La diminution de la pluviométrie de la saison sèche de Décembre à Février entraîne une baisse des coefficients d'écoulement durant la petite saison des pluies qui suit (Mars-Juin). Inversement, alors que la pluviométrie a peu varié au cours de la saison des pluies de Septembre-Novembre, les écoulements augmentent, probablement du fait que les pluies de la petite saison sèche (Juillet et Août) sont plus élevées depuis quelques décennies. Ceci indique une évolution du climat dans le sudouest du Cameroun, visible également au Gabon et au Congo. Sur le bassin de la Kienké, la disparition progressive de la petite saison sèche de Juillet-Août tend à joindre les deux saisons des pluies en une seule. Abstract Analysis of the spatio-temporal variability of rainfall amounts, discharge and runoff coefficients, for monthly, seasonal and annual time steps in equatorial south Cameroon, reveals that annual values of rainfall and discharge have decreased during the years of greatest drought (1972-1973 and 1983-1984). The most significant climatic change is a modification of rainfall regime during the dry seasons, which led to a change in the annual hydrological cycle. The decrease in rainfall during the dry season in December-February generates a decrease in the runoff coefficients during the subsequent short rainy season (March-June). Conversely, although the corresponding rainfall has not changed much during the rainy season in September-November, the discharge has increased, probably due to the fact that rainfall during the short dry season (July and August) has been much higher for several decades. This is assumed to indicate a change in climate in southwest Cameroon, also evident in Gabon and Congo. Over the Kienke basin, the progressive disappearance of the short dry season in July-August has resulted in a tendency for the two rainy seasons to merge into one.

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The river Niger is the 3 rd longest river in Africa, with a stream length of 4200 km, a drainage basin of 2,170,500 km 2 of which 1,500,000 km 2 is an active basin, and an average discharge of about 6000 m 3 /s. The natural variability of its rainfall and discharge is analyzed for several major sub-basins, in the context of the West African drought which has lasted for nearly 40 years, showing two paradoxes: the increase of Sahelian runoff since the beginning of the drought due to land degradation, and the steep decrease of sudanian runoff over the same period, substantiated by the long-lasting decrease of the groundwater tables. Much information about the water resources available in the basin is collected and analysed by the NBA 1 , which is summarized in this paper including surface water resources, rainfall over the basin, existing and projected dams. The river Niger is deficient in dams to control water, especially in its upper and middle basins. Nigeria has many dams, including large dams, while Burkina Faso has many small dams, but there are only a few dams upstream of the river Niger in Mali/Guinea/Ivory Coast. It is therefore likely that several dams will be built in the Niger basin in the coming years, and several are in the project phase. All of these will have a large impact on the river Niger regime and the environment, especially the Fomi dam which will change significantly the river regime upstream of the inner Delta, inducing an important reduction of the flooded area, and the Tossaye dam on the Saharan border of Mali which could promote a very significant level of evaporation. It is very important before building these dams to take into account the past years variability of climate and river regime.