complex shows an intermediate picture that does not allow the identification of specific hydrological behavior that could be linked to the significant presence of tropical rainforest.
The analysis of statistical breaks (Table 6.4) makes it possible to identify the two upstream basins (Kotto at Bria and Mbomu at Zemio), which show no hydrological break in 1970, but a break in 1985 at Bria and in 1981 at Zemio. On the other hand, the Kotto basin at Kembe and the Mbomu basin at Bangassou were both impacted by a hydrological break in 1970, with a continuous dry sequence from 1970 to 1993, this sequence being more accentuated on the Kotto at Kembe since 1985. The hydrological deficit is much more marked on the Kotto river basins, a savannah area: –50% at Bria and –74% at Kembe. In the Mbomu basins, the hydrological deficits are –49% at Zemio and –33% at Bangassou.
Figure 6.5 Hydrologic index from 1948 to 1995 for the hydrological basins of the Ubangi at Mobaye, the Kotto at Kembe and Bria, and from 1951 to 1995 for the hydrological basins of the Mbomu at Bangassou and at Zemio, estimated hydrologic index of the hydrological system Uele + Bili at the confluence with the Mbomu.
6.5. DISCUSSION
6.5.1. Comparative Interannual Evolution of the Ubangi at Mobaye with the Ubangi River at Bangui
The interannual evolution of the water flow from the Ubangi at Mobaye is similar to that of the Ubangi at Bangui (Figure 6.6), with a very marked hydrological break in 1971 and a semblance of resumption of flows from 2013 onwards (Nguimalet & Orange, 2019). The flow deficit on the Ubangi at Bangui is slightly more marked than at Mobaye, with an average value of the hydrological index around –1 for Bangui and –0.8 for Mobaye. Thus the hydrological deficit of the Ubangi at Bangui is –27% between the wet period 1960–1970 (4,877 m3/s) and the first dry period of 1971–1982 (3,577 m3/s), while the deficit is slightly lower at Mobaye, –24% between the wet period of 1957–1968 (3919 m3/s) and the dry period of 1969–1980 (2,986 m3/s). The beginning of the resumption of flows recorded in 2013 comes, as on the Ubangi River at Bangui, a few years after the rainfall recovery of 2008 (Figure 6.3), but a year ahead of schedule (Nguimalet & Orange, 2019). This one‐year delay indicates a difference in the upstream–downstream hydrological behavior of the Ubangi: the hydrological support of flows is better ensured in the upstream basin.
Figure 6.6 Comparative interannual evolution of the runoff (LE) of the Ubangi basin at Mobaye and Bangui (from 1938 to 2015). The periods 1950, 1958–1969, 1971, and 1975–2014 have been reconstructed from the Ubangi discharges at Bangui.
6.5.2. Diversity of Major Climatic Ruptures in the Ubangi Basin at Mobaye
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.
6.5.3. Opposing Roles of Savannah and Forest on the Hydropluviometric Dynamics of the Sub‐Basins of the Ubangi River at Mobaye
A concerted examination of the statistical breaks in the rainfall and hydrological time series of the sub‐basins making up the Ubangi basin at Mobaye reveals a difference in hydropluviometric behavior between the Kotto basin, north–south oriented, with a large savannah domain, and the Mbomu, NE–SW oriented with 10% of equatorial forest and 30% of wooded savannah: i) on the Kotto, the hydropluviometric deficit has only increased since 1970 with a marked increase in the hydrological deficit from 1985 onwards (Table 6.3) and with no sign of resumption of flows despite a resumption of rainfall around 1992 (Figure 6.4); and ii)
