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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 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).