Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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Congo Basin Hydrology, Climate, and Biogeochemistry - Группа авторов


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to the southern basins under equatorial forest. Therefore, we propose here to study the hydropluviometric evolution of the Ubangi basin at the outlet of Mobaye and its constituent hydrological sub‐basins, Mobaye being located 380 km upstream from Bangui with no large tributaries between these two hydrological stations (Orange et al., 1995). The Ubangi basin at Mobaye is without any anthropogenic influence. Half of it is covered by savannah drained by two large rivers (the Kotto and the Mbomu), and half by dense forest drained by the Uele (Orange, 1996).

      The coupled evolution of rainfall (P) and annual flow (Q) over the upstream basin of the Ubangi River at Mobaye is studied in order to discuss the role of the forest compared to the savannah on the hydropluviometric behavior of the Kotto, Mbomu, and Uele basins, and to assess the behavior of aquifers in this hydropluviometric deficit context recorded since 1970 in the region. We approach this analysis by looking for homogeneous hydroclimatic periods of the 1938–2015 sequence on the Ubangi River at Mobaye, and over the 1951–1995 period for its three major tributaries: the Kotto, Mbomu, and Uele rivers.

Schematic illustration of topographic map of the Ubangi basin at Mobaye.

      Source: Based on Orange et al., 1994.

Schematic illustration of vegetation map of the Ubangi basin at Mobaye, the geological bedrock and the hydropluviometric network studied, with the limits of the five studied sub-watersheds.

      Source: Based on Orange et al., 1996.

      In the region, population growth is low: the population in Bangassou on the Mbomu River, the regional capital, increased from 902,205 inhabitants in 2006 to 1,126,730 in 2015, an increase of only 2.5% per year. The entire Ubangi basin at Mobaye thus remains a sparsely populated area, with a population density varying between 10 inhab/km2 and 3.3 inhab/km2. In the Uele basin, the population density was 7 inhab/km2 in 2008 (Haut‐Uélé, 2020). Human activities are essentially primary (slash‐and‐burn agriculture, artisanal mining, production of firewood and charcoal, etc.), concentrated around the Ubangi, Uele, and Mbomu valleys; these activities have changed little since 1970 (Haut‐Uélé, 2020; Nguimalet & Orange, 2019; Orange & Ghiloufi, 1996).

      6.3.1. Hydropluviometric Data

      The hydrological stations studied are the Ubangi River at Mobaye (403,800 km2), the Kotto River at Kembe (78,400 km2, downstream of the basin) and at Bria (61,500 km2, upstream of the basin), the Mbomu River at Bangassou (115,000 km2, downstream of the basin) and at Zemio (29,300 km2, upstream of the basin), and the Uele + Bili hydrographic complex (161,100 km2), whose flows were estimated by deducting the confluences, according to the work of Orange (1995).

      When monthly data are functionally missing, the monthly reconstitution of month i is done by the method of averaging the two years of month i surrounding the missing data. For the periods 1977–1985 and 1995–2015, the missing daily flow data at the Mobaye outlet were reconstructed from the Bangui station, considering that the specific flow is unchanged between Mobaye and Bangui. Finally, for the complete missing years, data for the Kotto and Mbomu rivers were reconstructed from the nearby hydrological station, according to the formula (Jacob et al., 2006; Azouka, 2011):

      (6.1)q equals upper Q left-parenthesis upper A slash a right-parenthesis Superscript 0 comma 8

      where q is the estimated stream discharge; Q is the discharge from the nearest hydrological station; A is the surface area of the basin at the nearest hydrological station; and a


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