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Over the period studied 1951–1995, only the two basins of the Kotto (at Bria and Kembe) show a downward trend in their flows despite the resumption of rainfall in the early 1990s (Figs. 6.4 and 6.5). On the other hand, in the 1990s flows in the two Mbomu basins are increasing and seem to explain the increase observed on the Ubangi at Mobaye. This is explained by the impact of forest cover on the hydrological regime of the Mbomu, compared to the savannah environment drained by the Kotto. Furthermore, the results show much more marked hydrological deficits on the Kotto than on the Mbomu.

Coupled analysis of the rainfall and hydrological series showed an early climatic rupture in the Ubangi basin at Mobaye and thus the current drought of one year, in 1968 and 1969, respectively, in the face of the major break of 1970 unanimously accepted in Central and West Africa. However, this break is not explicit in the two upstream basins studied, the Kotto at Bria and the Mbomu at Zemio. Moreover, the 1968 break in the rainfall series occurred at the end of a homogeneous wet period (1938–1968), recording only a surplus of +4% compared to the interannual average. This wet rainy period produced two homogeneous periods of annual flows, one moderately wet with a hydrological surplus of +10% (1938–1956) and the other of a very wet decade with a surplus flow of +32% (1957–1968). This shows an example of disparity in the response of the flow to the rainfall variable, due to the good functioning of the aquifer reserves at that time, as announced by Orange et al. (1997) from the study of water balances by sub‐basin. The long period of drought modi fied the infiltration mechanisms differently between the savannah zone in the North and the tropical forest zone in the South. Under savannah, the proportion of runoff infiltrating to recharge the aquifer would have decreased faster than under forest.

Figure 6.7 Interannual evolution of river depletion coefficient from the Ubangi at Mobaye and water volume of the aquifer (1938–1976).

The availability of daily hydrological data enabled the calculation of the drying coefficient on the Ubangi at Mobaye from 1938 to 1974 (Figure 6.7). Over this period, the mean interannual drying coefficient is 0.019/day, with a maximum of 0.026/day (1972–1973) and a minimum of 0.015/day (1968–1969). These drying coefficients are slightly higher than those calculated for Ubangi in Bangui from 1935 to 2015 (Nguimalet & Orange, 2019). In Bangui, drying up coefficients started to increase from 0.018/day in the late 1960s to reach a maximum of 0.025/day in 2000, and then decreased somewhat. These figures confirm that Ubangi’s drying up dynamics in Mobaye and Bangui are similar. Maximum low‐water levels can therefore be assumed from the 2000s onwards. From the recorded data, the average volume mobilized by the Ubangi basin aquifer at Mobaye would be 36.3 km³, the maximum recorded being 62 km³ in 1968–1969, and the minimum 20 km³ in 1972–1973 (Figure 6.7). By comparison with the Ubangi at Bangui, the minimum contribution is estimated at 17 km³ since 2000.