Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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) on the Mbomu, a semblance of resumption of flows in the early 1990s.

To clarify this behavior, we calculated the relationship between the water flow and the precipitation per decade, from 1951 to 1995, and for each of the six basins studied (Figure 6.8). Only the two basins of the Kotto show a positive correlation between these two variables; for the other four basins the correlation is negative. A positive slope is characteristic of a Hortonian‐type hydrological functioning: surface runoff increases with rainfall. Conversely, a negative slope indicates basins with a capacity to retain water in their soils (to infiltrate), a characteristic often linked to the presence of forest cover.

The calculation of flow coefficients per decade and for each tributary confirms this behavior. The ten‐year flow coefficients of the Kotto decreased continuously from 1961 to 1995 to reach only 5%, whereas in the other basins studied the FT increased over the last decade (Figure 6.9). The average over the period 1951–1995 is 10% for the Kotto at both Kembe and Bria; 14% and 13% on the Mbomu at Bangassou and Zemio, respectively; 14% on the Ubangi at Mobaye; and 20% on the Uele + Bili hydrological system. The value of the mean flow coefficient is very significantly negatively correlated to the area of non‐forested savannah present in the watershed (Figure 6.10).

The probable impact of forest cover (forest + wooded savannah) may explain the evolution of the runoff coefficient. Over the period studied from 1951–1995, only the two Kotto basins (at Bria and Kembe) show a continuous downward trend in their runoff without any recovery due to the resumption of rainfall in the early 1990s, whereas the two Mbomu basins have increased runoffs with a significant increase in runoff coefficients, from less than 10% to nearly 15%. The flow coefficients of the Kotto were only 5% in 1995. This observation can be explained by the impact of the forest cover on the hydrological regime of the Mbomu, compared to the non‐forested savannah cover drained by the Kotto.

Figure 6.8 Relationship of water runoff as a function of the precipitated water level (in mm) on the Ubangi basin at Mobaye and its sub‐basins (per decade, from 1951 to 1995).

Figure 6.9 Decennial evolution of runoff coefficients (CE) from the Ubangi at Mobaye and from its tributaries.

Figure 6.10 Relationship between the runoff