Indicateurs des évènements hydroclimatiques extrêmes dans le bassin versant de l'Ouémé à l'exutoire de Bonou en Afrique de l'Ouest

Abstract : This research aims to characterize the indicators relating to extreme hydroclimatic events in Ouémé Watershed at Bonou’s outlet. To achieve this objective, climatological data (daily rain, daily temperatures, potential daily evapotranspiration), hydrometric data (daily flows), reanalysis data (atmospheric pressure, relative humidity, wind speed, West African Monsoon Indices), the ocean surface temperature on the Cotonou coast and land use mapping data of 1986, 2000, 2015 are used. The analysis of the hydroclimatic variability revealed that during the 1951-2015 period, the study area is marked by two breaks stationarity (1968 and 1987) which divided the series into three sub-periods. The rainfall variability reveals a regression of 18 % (in the south), 16 % (in the center) and 15.8 % (in the north) between the first sub-period (1951-1968) and the second sub-period (1969-1987), while the last sub-period (1988-2015) is marked by a rainfall recovery of 15 % (in the south), 16.3 % (in the center) and 15.7 % (in north) relative to the previous sub-period (1969-1987). This is not without consequences on the hydrological dynamics of the study area. In this sense, it is noted that there is a regression of 49.27% of the average annual flows between the 1951-1968 and 1969-1987 sub-periods and a 65 % increase between the 1969-1987 and the 1969-1987 sub-periods. 1988-2015. In the last sub-period (1988-2015), the results also reveal an increase in the indicators of extreme hydroclimatic events likely to cause floods Ouémé Watershed at Bonou’s outlet. The results shows that daily maximum rainfall has an occurrence of 2 years, 5 years, 10 years for strong rainfall events; 20 years, for very heavy rain events, 50 years and over for extremely heavy rain events in the study area. The maximum flow rates in 24 hours at the sametime as the return periods. The maximum flow rates of 50th quantiles occur every two years (02 years), the maximum flows corresponding to the hydrologically strong events, very strong occur every 10 years and the extremely high flows have an occurrence centennial. The conditions of atmospheric, oceanic, continental surface, wind speed, relative humidity, and geographic factors are all elements that interact in the spatio-temporal distribution of rainfall in the study area. The Rainfall of the basin is also influenced by the monsoon flow (north of the study area), to which the southeastern grain lines are associated and are responsible for the stormy rains and showers that generate flooding in the area. In addition, the current climatic context and land-use dynamics are determinants that amplify the occurrence of floods in the study area. The reasearch indicates that the rain-flow conceptual model, of the Rural Engineering with four parameters and the daily time step (GR4J), overestimates the flows observed during the low water period and the sub-peaks. The efficiency and performance criteria NSE, RMSE and KGE, highlighted and calculated on high water flow rates, gave better results in calibration than in validation. Better KGE values range from 83 to 85 % calibration and from 56 to 68 % validation, which gives the GR4J model the efficiency and performance to reproduce extreme flows of floods in the study area. The GR4J can therefore be used as a decision to support tool for updating hydrological standards in the study area. In order to future study about extreme climatic events, this research assessed output projection data from regional climate models from the African CORDEX Program that could be used for hydrological projections in future work in Ouémé Watershed at Bonou’s outlet.
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Domiho Japhet Kodja. Indicateurs des évènements hydroclimatiques extrêmes dans le bassin versant de l'Ouémé à l'exutoire de Bonou en Afrique de l'Ouest. Géographie. Université Montpellier, 2018. Français. ⟨NNT : 2018MONTG014⟩. ⟨tel-01869842⟩

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