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 coefficient (CE) and the rate of no‐wooded savannah surface in the sub‐basins of the Ubangi at Mobaye.
The time series of flows of the Ubangi at Mobaye confirms the observations made on the downstream station of the Ubangi at Bangui by many authors (Laraque et al., 2001; Nguimalet & Orange, 2019; Orange et al., 1997): even if the 1970 hydrological rupture recognized throughout West and Central Africa can be observed on the Ubangi, the 1981 rupture has a more drastic impact on the capacity of this river to mitigate annual rainfall deficits. Although the current major drought period began in the Ubangi basin more than a decade before the 1981 rupture, the early 1980s in Central Africa witnessed an accentuation of the effect of rainfall deterioration in the forest basins, as confirmed by work on the forest basins of Ivory Coast (Fadika et al., 2008; Goula et al., 2006). These authors revealed a late impact around 1980 of the 1970 rainfall failure on small rivers or elementary basins in West and Central Africa, compared to large rivers, which recorded the major climatic break of 1970 on their flow (e.g. Laraque et al., 1998, 2001).
6.6. CONCLUSION
The rainfall series from the Ubangi Basin at Mobaye seems to confirm the end of the long drought of the 1970s with a probable second break in 2006 that characterizes the return to a wet rainy period. The average rainfall to be retained is therefore 1,568 mm, 1,436 mm, and 1,583 mm for the periods 1938–1968, 1969–2006, and 2007–2015 respectively. The period of rainfall deficit 1969–2006 is exceptionally long (38 years), corresponding to a reduction of 8% in rainfall compared to the previous period. On the other hand, the current wet period of 2007–2015 describes a rainfall increase of +10% compared to the previous period. Rainfall in the Central African Republic has thus returned to its average level of 1938–1968, around 1,570 mm/year. But what about river flows?
It is remarkable that the two dates of rainfall disruption (1968 and 2006) found on both the Ubangi at Bangui and the Ubangi at Mobaye are not found in the time series of annual flows, where only 1970 appears, marking the highly exceptional nature of this disruption on the hydrological functioning in the region. On the other hand, the time series of flows from the Ubangi at Mobaye attests to the importance of a second hydrological downward break in 1981 and a third, this time an upward break in 2013. There are therefore three hydroclimatic periods for the Ubangi at Mobaye: 1954–1968, 1969–1980, and 1981–2013, with, respectively, mean interannual flows of 3,919 m3/s, 2,986 m3/s (–24%) and 2,372 m3/s (–21%).
We will retain two things: (1) On the one hand, the semblance of a resumption of flows, recorded in 2013, occurs a few years after the rainfall resumption of 2006, as on the Ubangi at Bangui, but one year earlier (Nguimalet & Orange, 2019). This time lag is found in the break dates of the beginning of the long period of drought, dated from 1981 in Mobaye and 1983 in Bangui. These differences in the upstream–downstream hydrological behavior of the Ubangi seem to indicate that the hydrological support of the flows is better ensured in the upstream basin, but that this support has deteriorated sharply over the last 25 years. (2) On the other hand, a second break in 1981 shows a hydrological collapse of the river flows indicating the exceptional character of this “drought of the 1970s”!
This second hydrological deficit does not correspond to a new rainfall deficit, so it can only be explained by parameters exogenous to rainfall. As vegetation cover and population pressure have changed little since 1970 in the entire Ubangi basin at Mobaye, this collapse of river flows marks a different functioning of the contribution of aquifers, as announced by Orange et al. (1997). The long period of drought has altered, in a different way, the infiltration mechanisms between the savannah zone in the north and the rainforest zone in the south. Under savannah, the proportion of flow infiltrating to recharge the aquifer would have decreased faster than under forest, which results in a flow coefficient that is very significantly negatively correlated with the area of non‐forested savannah present in the catchment area.
This study showed the opposite behavior between basins under non‐forested savannah (the Kotto) and basins under forest and wooded savannah (the Mbomu in part, and especially the Uele). Savannah and forest play opposing roles in the hydropluviometric behaviour of the catchment areas that make up the Ubangi at Mobaye. Thus the 1970 rainfall failure characteristic of the West and Central African continent, which was well confirmed on the Ubangi at Mobaye, only appears explicitly on the non‐forested savannah basins drained by the Kotto. On the other hand, in the Mbomu and Uele basins, the absence of rainfall breaks is attributed to the impact of forest cover.
ACKNOWLEDGMENTS
The authors would
