One of today's key research challenges is now to provide and anticipate climatic changes so that we can respond and adapt to these future developments. Global reheating of the earth and weather forecasts are a growing risk of climatic changes that are not without consequences on the ecosystems whose the watershed of Oueme. The objective of this study is to analyze the availability of water resources with the climatic variability in the lower Oueme valley in south of Benin. The data and methods used are rainfall, ETP, planimetric data, land use extracted from Landsat 8 OLI / TIRS 2016, ArcGIS 10.1 software for mapping, Khronostat 1.01 for detection of ruptures and field observations, etc. This information after treatment helped to determine the physiognomy of the climate in the lower Oueme valley. The results obtained show an unequal distribution of rainfall marked by the continuous changes of the natural conditions of the basin. The study identifies three phases in the evolution of rainfall. The first 1987-1990 is marked by rainfall surpluses. The second is characterized by rainfall deficits between the period 1990-2006 and the last one characterized by a very strong instability in the evolution of the rainfall, concerns the period 2006-2016. The application of Pettitt test to this chronologic series revealed a break in stationarity at the 95% threshold, thus highlighting two sub-periods, 1987-2006 and 2007 -2016. This rainfall decrease anchor decrease of superficial flow between 0,2% and 1% in the lower Oueme valley.