Hydroclimatic dynamics of upstream Ubangi river at Mobaye, Central African Republic : comparative study of the role of Savannah and Equatorial forest
Résumé
The rainfall reduction in the 1970s, less marked in Central Africa than in West Africa, still had a major impact on the hydrological regimes of the region's large rivers. The study of the hydropluviometric behavior of the Ubangi River at Mobaye has the advantage of being a study of a basin excluding anthropogenic impact. Forest cover and population density have not changed since at least 1970. Statistical analysis of the breaks in the long rainfall time series to Mobaye (1938-2015) confirms a long period of drought from 1969 to 2006, corresponding to a reduction of 8% in rainfall. Also, the study of the corresponding hydrological series indicates a second downward break in 1981, marking an exceptional hydrological drought. Flows increased in 2013, a few years after the rainfall increase. The statistical study of the annual rainfall/flow series of the upstream basins over the period 1951-1995 (the Kotto River in Kembe and Bria, the Mbomu River in Bangassou and Zemio, and the Uele River + Bili hydrographic system) highlights different hydrological behaviors related to the vegetation cover. On the one hand, the savannah basins show a continuous hydrological deficit marked by a runoff coefficient (CE) that fell to only 5% from the 1990s. On the other hand, the basins under forest show a runoff increase since 1990, marked by a CE above 10%. Under savannah, the part of the flow infiltrating to recharge the aquifer would have decreased faster than under forest, which results in a runoff CE very significantly negatively correlated with the savannah area present in the studied watershed.