3 National Directorate of Meteorology, Ministry of Transport and Civil Aviation, Bangui, Central African Republic
ABSTRACT
The rainfall reduction in the 1970s, less marked in Central Africa than in West Africa, still had a major impact on the hydrological regimes of the region’s large rivers. The study of the hydropluviometric behavior of the Ubangi River at Mobaye has the advantage of being a study of a basin excluding anthropogenic impact. Forest cover and population density have not changed since at least 1970. Statistical analysis of the breaks in the long rainfall time series to Mobaye (1938–2015) confirms a long period of drought from 1969 to 2006, corresponding to a reduction of 8% in rainfall. Also, the study of the corresponding hydrological series indicates a second downward break in 1981, marking an exceptional hydrological drought. Flows increased in 2013, a few years after the rainfall increase. The statistical study of the annual rainfall/flow series of the upstream basins over the period 1951–1995 (the Kotto River in Kembe and Bria, the Mbomu River in Bangassou and Zemio, and the Uele River + Bili hydrographic system) highlights different hydrological behaviors related to the vegetation cover. On the one hand, the savannah basins show a continuous hydrological deficit marked by a runoff coefficient (CE) that fell to only 5% from the 1990s. On the other hand, the basins under forest show a runoff increase since 1990, marked by a CE above 10%. Under savannah, the part of the flow infiltrating to recharge the aquifer would have decreased faster than under forest, which results in a runoff CE very significantly negatively correlated with the savannah area present in the studied watershed.
6.1. INTRODUCTION
The rainfall break of the 1970s was less marked in Central Africa than in West Africa. Nevertheless, it has largely impacted the hydrological regimes of the region’s major rivers (Laraque et al., 1998, 2001; Nguimalet & Orange, 2013; Olivry et al., 1998; Orange et al., 1997; Paturel et al., 1998, 2007; Servat et al., 1999; Sighomnou et al., 2007; Wesselink et al., 1996). On the Ubangi River at Bangui, a large savannah‐dominated basin, the slight drop in rainfall of 5 to 6% resulted in a 21% runoff deficit over the period 1983–2013 (Orange et al., 1994). On the Congo River, Laraque et al. (2013) noted that after a hydrological deficit observed in the 1980s, the runoff has returned to normal since 1990. According to Nguimalet & Orange (2019), the rainfall amount in the Ubangi basin at Bangui has shown a significant increase since 2009, but still without any real hydrological impact on the course of the Ubangi River at Bangui. Recently, Nguimalet and Orange (2020) showed that hydrological behaviors in small basins of 2,500 to 5,000 km2 in the north of the Central African Republic, in the Sudanian savannah zone, were all impacted by the 1970 drought, at different levels and without any common hydroclimatic period. It remains to be seen whether this is specific to the northern basins with savannah vegetation, in contrast 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.
6.2. STUDY AREA: UPPER BASIN OF THE UBANGI RIVER AT MOBAYE
The Mobaye outlet on the Ubangi River is located 151 km from the confluence of the Mbomu River (CAR) (Central African Republic) and the Uele River (DRC) (Democratic Republic of the Congo), the confluence from which the Ubangi River originates at Kemba (Figure 6.1). The Ubangi is the largest right‐bank tributary of the Congo and a natural border between the CAR to the north and the DRC to the south. The Ubangi at Mobaye has a watershed of 403,800 km2. The SE (Uele Highlands) and N (Kotto Highlands) ends of the basin are dominated by mountains with respective altitudes of 1,700 m and 1,300 m, nested between two plateaus (the Uele Sandstone Plateau in the south and the Central African Surface in the north), then the plain of the Ubangi foothills, which is quite narrow, confines the downstream valleys of the Mbomu, Bili, and Uele rivers up to Mobaye, including the downstream valley of the Kotto. In our study, the Bili River of only 21,400 km2 is integrated into the Uele basin (i.e. Orange, 1996).
Figure 6.1 Topographic map of the Ubangi basin at Mobaye, and of the hydropluviometric network studied, with the limits of the five studied sub‐watersheds (at the outlets of Bondo for the Uele River, Zemio and Bangassou for the Mbomu River, and Bria and Kembe for the Kotto River).
Source: Based on Orange et al., 1994.
Figure 6.2 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 (at the outlets of Bondo for the Uele River, Zemio and Bangassou for the Mbomu River, and Bria and Kembe for the Kotto River).
Source: Based on Orange et al., 1996.
The rainfall gradient decreases from south to north, from 1,700 mm on the Uele to 1,000 mm on average over the upper Kotto basin (Nguimalet & Ndjendolé, 2008). The basin of the Ubangi at Mobaye is a well‐marked latitudinal stratification of the edaphic stages (Figure 6.2). From south to north, one observes the tropical rainforest domain of the equatorial forest with 15% primary forest (Global Forest Watch, 2019), followed by the wooded savannah zone, successively wooded, shrubby, and grassy in the extreme north (Nguimalet, 2017). The latest 2018 surveys by Global Forest Watch confirm that in the Central African Republic and Northern Congo the forest area has decreased very little (–1.5% between 2001 and 2018), with a slightly greater loss of primary forest (–1.9%). This study considers that the forest area has not changed. Thus, according to Orange & Ghiloufi (1996), the Uele basin (including Bili) is 70% covered by tropical forest and 30% by wooded and tree‐covered savannah, the Mbomu basin has only 10% tropical forest and 30% wooded savannah (according to Orange, 1996), and the Kotto basin is representative of wooded and tree‐covered savannah (Figure 6.2). The humid forest thus represents 30% of the Ubangi basin at Mobaye.
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,
