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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.

dealing with the spatial and temporal variations in Sahelian soil water content as well as with the infiltration of water through deep soil layers of the vadose zone. The purpose of this chapter is to provide an overview of hydrological behaviour throughout West Africa based on point, local, meso and regional scales observations. 2. Background The paradoxical increase in runoff despite drought conditions in sub-Saharan Africa was first noted in a paper by Albergel [3], analysing decadal series of runoff measurements in experimental sites of Burkina Faso. He noticed that this increase was observed in Sahelian areas, but not in the more humid Sudanian regions. The decrease in rainfall during the 1969-1983 period seems to be largely offset by the evolution of surface features in the functioning of small catchments. These changes favoured the conditions of runoff in the Sahelian basins; there are due to both the human actions and the climatic conditions. The reduction of vegetation cover and the widespread crops areas cause soil surface settling and the appearance of impervious superficial layers, as well as the extension of eroded areas. Some sahelian basins have nowadays [in 1987] the common characteristics of basins located northward, with great areas of bare soils; perennial graminaceae are replaced with annual ones, and combretaceae with prickly bush species" [3]. Albergel [3] attributed the contrasting behaviour of Sudanian (mean annual rainfall > 750 mm) and Sahelian (mean annual rainfall < 750 mm) areas to increasing bare soils and decreasing vegetation cover in Sahelian basins. This hypothesis was confirmed in 1999 by Mahé and Olivry [4] and then in 2002 by Olivry [2], who remarked that the discharge of right bank tributaries of Middle Niger River had been increasing since the beginning of the Drought (1968). Similarly, Amani and Nguetora [5] noted that runoff coefficients were increasing significantly in right bank tributaries and showed that the onset of the annual flood was occurring earlier than in previous decades. Mahé et al. [6] analysed the runoff evolution of eight right bank tributaries of the Middle Niger River and noted that the decrease in rainfall did not lead to a decrease in runoff under the Sahelian climate as commonly observed in other basins in the world. Rather, these tributaries exhibited increasing runoff coefficients and in discharges, while "Sudanian" climate tributaries suffered a decrease in discharge and in runoff coefficient [6]. 3. Material and methods This study is mainly based on two sources of data: • field measurements and observations made during the AMMA (African Monsoon Multidisciplinary Analysis) experiment at the Niger experimental site (Niger River middle stretch and Niamey square degree), and: