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.

The foliations, lineations, fold axes trajectories and shear zones kinematics vary from the Archean Ntem and Paleoproterozoic Nyong complexes to Neoproterozoic Oubanguide complex. The S 1 , S 2 and S 3 typified foliations are folded in F 2 , F 3 and F 4 cartographic folds equivalent to thrust slices while the L 1 , L 2 and L 3 lineations are oriented SSE-NNW, westeast and sub-north-south in the above complexes, respectively. Both, the foliations and lineations confirm the transport top-to-the east of the Nyong nappe onto the Congo craton during the Eburnean orogeny under amphibolitic conditions and as the transport top-tothe SSE of the Yaounde nappe onto the Ntem and Nyong complexes during the Pan-African event under granulitic to green schist conditions. The paleostress of fault and shear zones display crustal thinning and horizontal extension oriented east-west, NW-SE and subnorth-south in the Ntem, Nyong and Oubanguide complexes. All these tectonic features, completed by north-south and east-west cross sections, demonstrate litho-chronostratigraphical, structural and angular discordances between the three complexes.

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A study of environmental chloride and groundwater balance has been carried out in order to estimate their relative value for measuring average groundwater recharge under a humid climatic environment with a relatively shallow water table. The hybrid water fluctuation method allowed the split of the hydrologic year into two seasons of recharge (wet season) and no recharge (dry season) to appraise specific yield during the dry season and, second, to estimate recharge from the water table rise during the wet season. This well elaborated and suitable method has then been used as a standard to assess the effectiveness of the chloride method under forest humid climatic environment. Effective specific yield of 0.08 was obtained for the study area. It reflects an effective basinwide process and is insensitive to local heterogeneities in the aquifer system. The hybrid water fluctuation method gives an average recharge value of 87.14 mm/year at the basin scale, which represents 5.7% of the annual rainfall. Recharge value estimated based on the chloride method varies between 16.24 and 236.95 mm/year with an average value of 108.45 mm/year. It represents 7% of the mean annual precipitation. The discrepancy observed between recharge value estimated by the hybrid water fluctuation and the chloride mass balance methods appears to be very important, which could imply the ineffectiveness of the chloride mass balance method for this present humid environment.