Water resource shortage is paradoxical in a context of increasing water need, under the combined effects of demographic growth and the impacts ofclimatic 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 this 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 series ofstream flows to the dam fora 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 inter-annual fluctuations. The results show that overall the model represents in a satisfactory way the form of observed hydrographs.The calculated output of peak is well located in time, even if they are sometimes underestimated or over-estimated. On the other hand,the low levels of water are perfectly reconstituted and there is a good superposition of initial hydrographs of flows and the calculated oneComparatively to the period of 1950 to 1969 it comes out that for the period of 1990 to 2010 precipitations is reduced 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 average pluviometry is reduced at 10%. Indeed,flows depend on precipitations and are 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. finally, the average of the flows has been reduced by 19% for the four last decades.Three rivers sections were now identified for further field hydrological measurements. They are accessible by the road at Binassi, Darnon and Douroube. Request are already introduced to equip those land markstations very soon.
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