Water resource shortage is paradoxial in a contexte of increasing water need, under the combined effects of demographic growth and the impacts of climatic changes inducing water resources degradation. So there was an awakening of conscience about water resources management which presents a capital stake for all water actors around Okpara dam in Benin.The major concern in the study area always remains the lack of knowledge which requires the availability of quantity and quality of data for all water related sciences. In this context, Principle of least actions is used to determine the serie of streamflows to the dam for a best monitoring of water Supply system at Parakou.That makes it possible to have a long time series of flows that is enough to estimate water resource and its seasonal and interannual fluctuations. The results show that overall the model represents in a satisfactory way the form of observed hydrogrammes. The calculated output of peak is well located in time, even if they are sometimes tunderestimated or over-estimated. On the other hand, The low levels of water are perfectly reconstituted and there is a good superposition of initial hydrogrammes of flows and the calculated one.Comparatively to the period of 1950 to 1969 it comes out that for the period of 1990 to 2010 precipitations is reduction in frequency and intensity: For an intensity of rain superior to 2 mm, 10 mm, 20 mm and 30 mm respectively, the decrease correspond to 7%, 8%, 18% and 41% respectively; whereas the annual pluviometric average is reduced at 10%. Indeed, flows dependent on precipitations and know a downward trend more amplified: over the four last decades compared to the wettest period of 1951 to 1970, the tendency is with the fall, as well for the minimal flows (17%) as for the maximum flows which are reduced by 22% for the period of 1971-1990, and of 18% for the period of 1971-1990 per report/ratio.Into final, the average of the flows has been reduced 19% for the four last decades.Three rivers sections were now identified for real field hydrological mesurement. They are accessible by the road at Binassi, Darnon and Douroube. Request are already introduce to equip those land mark very soon.
Keywords: : ungauged basin, IWRM, Nanon, Okpara dam at Kpassa, climatic changes, principle of the least actions
