Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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 like to thank the Direction de la Météorologie Nationale and ASECNA for access to their data banks, as well as IRD and the PEGI/GBF‐RCA programmes (Programme d’Etude de l’Environnement de la Géosphère Intertropicale, Grands Bassins Fluviaux en Centrafrique; Bangui, IRD, from 1986 to 1993) and UNDP/WMO CAF/91/021 (Assistance Agrohydrométérologie et Surveillance de l’Environnement; Bangui, from 1991 to 1995). The authors thank the reviewers.

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