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RESEARCH PRODUCT

Impact de la variabilité climatique et de la dynamique des états de surface sur les écoulements du bassin béninois du fleuve Niger

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This thesis has for principal objective to establish a diagnosis of the hydrological system of the beninese basin of the Niger River, in the context of the rainfall decrease which has severely affected Africa since the 1970s. To achieve this goal, this study was based on three main types of data :weather (rainfall, temperature, relative humidity, potential evapotranspiration, real evapotranspiration), hydrological (stream flow, recharge) and of land surface conditions (land cover types, NDVI).It considers three key aspects. The first one, devoted to the study of hydro-climatic variability over the period 1955- 1992, shows that the rainfall deficits of the 1970s and 1980s were largely amplified in the stream flows of all the rivers (Mekrou, Alibori, Sota) in the beninese basin of the Niger river.The stream flow deficit amount to five to six times that of the rain (Mekrou, Alibori).The rainfall deficit is particularly amplified in the recharge of Sota in the Couberi sub basin : the recharge deficit between the two sub periods 1955-1972 and 1973-1992 represents nearly five times that of the rain.The study also showed the differential behaviour of the sandstones and the basement complex.Over the latter (Mekrou, Alibori), the stream flow is strongly related to the rain, as demonstrated by the high value of the correlation coefficient ;on the other hand, on the sandstones (Sota at Couberi sub basin), it is related to both the rain and the recharge. The second part of the study shows the effectiveness of the GR4J and GR2M models to simulate the average stream flows according to the Nash the criterion applied to the stream flows, which gives satisfactory values (higher than 60 % in general).One can thus say that the GR4J and GR2M models provide reasonable simulations of the stream flow at daily and monthly time scales.However, the Nash criterion on the daily average stream flows using the GR4J model are largely improved at a monthly time scale using the GR2M model. In the Mekrou, Alibori and Sota at Couberi sub basins, the models identified a clear downward trend for many components of the hydrological cycle.There is no suchtrend in the Sota basin at Gbasse though it may partly be related to inadequate statistical tests. The third part is devoted to the impact study of land surface dynamics on the stream flows. It shows that the land cover types underwent great changes between 1979 and 1992, with a significant regression of the natural vegetation formations (forests and woodlands) and a marked increase of anthropised land cover types (mosaic of crops and fallow) for the same period.he search for NDVI/stream flow correlation, and NDVI residuals/stream flow residuals independently of the rain, also underlines a significant relationship in the Mekrou and Sota at Gbasse sub basins.The results demonstrate a covariation between the photosynthetic activity and the stream flow, independently of the rain in these sub basins. However, in the Alibori and Sota at Couberi sub basins, no significant relationship between the photosynthetic activity for different land cover conditions and the stream flow. Nevertheless, our results are restricted by the NDVI data which reflect only the photosynthetic activity of the vegetation cover. Moreover, the corresponding time-series are too short to test efficiently the influence of land surface conditions dynamics on stream flow.