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. DATA AND METHODS
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).
Daily flow data were used for the Bangui and Mobaye stations on the Ubangi River from 1938 to 2015. These data come from the IRD database published in the Monography of the Ubangi (Callède et al., 2009) and supplemented by data from Wesselink et al. (1995) over the period 1986–1993, and from the Direction de la Météorologie Nationale from 1994 to 2015. On the one hand, there are no missing data on Ubangi at Bangui. On the other hand, between 1975 and 2014, a total of 21 years is completely missing from Mobaye (Table 6.1). For the four other Central African hydrological stations (Kembe and Bria on the Kotto; Bangassou and Zemio on the Mbomu), the hydrological data used are monthly and cover only the period 1948–1995, taken entirely from the IRD database, published in part (1986–1993) by Wesselink et al (1995). On average, over 45 years, about 15 full years are missing, i.e., a third of the period studied. For the other years, there may be occasional monthly shortfalls (Table 6.1), with the result that hydrological data gaps represent between 43 and 52% of the monthly bank.
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)
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 is the surface area of the basin at the studied hydrological station.
Table 6.1 Hydrological data gaps.
| Hydrological stations | Period | Months missing | Years missing | Missing periods |
|---|---|---|---|---|
| Ubangi / Bangui | 1938–2015 (77 years) | None | None | None |
| Ubangi / Mobaye | 1938–2015 (77 years, 924 months) | 475 months (51%) | 21 years + 20 non complete years (53%) | 1938, 1950, 1958–1969 1971, 1975–2014 |
| Kotto / Kembe | 1948–1995 (47 years, 564 months) | 256 months (45%) | 14 years + 16 non complete years (64%) | 1949–1952, 1956, 1965–1969, 1976–1994 |
| Kotto / Bria | 1948–1995 (47 years, 564 months) | 242 months (43%) | 17 years + 5 non complete years (47%) | 1948–1953, 1964, 1967, 1968, 1977–1989 |
| Mbomu / Bangassou | 1951–1995 (44 years, 528 months) | 274 months (52%) | 14 years + 20 non complete years (77%) | 1951–1954, 1956–1962 1964–1973, 1976–1986, 1992, 1994 |
| Mbomu / Zemio | 1951–1995 (44 years, 528 months) | 100 months (19%) | 4 years + 13 non complete years (39%) | 1951–1953, 1957–1961, 1969, 1976, 1979–1982 1984, 1985, 1993 |
| Uele + Bili | Estimated | No interest | No interest |
Rainfall data are from the Agence pour la Sécurité de la Navigation Aérienne en Afrique et Madagascar (ASECNA) in Bangui for CAR and from the IRD database for Congo. Annual rainfall data were collected from eight stations in CAR and three stations in Congo, spread over the entire Ubangi basin at Mobaye (Figure 6.1), based on the work of Orange et al (1994). From west to east (Figure 6.2), there are: Ippy, Mobaye, Bria, Ouadda Djalle, Yalinga, Bangassou, Rafaï, and Obo in CAR; Bondo, Niangara, and Watsa in Congo. The two rainfall stations at Yalinga and Obo are complete from 1951 to 1995; most of the other stations have common data from 1972 to 1992 (Table 6.2). Reconstruction of the missing data was done by analysis of annual correlations between stations, according to the work of Orange et al. (1994).
The rainfall (P) and hydrology (Q) data have been arranged according to the hydrological year beginning April 1. The annual rainfall for each sub‐basin was calculated using the Thiessen method (Orange et al., 1994). The annual rainfall and hydrological indices (respectively, rainfall index and flow index) are standardized indices. They are calculated at the outlet of each watershed studied according to the following formula:
(6.2)
where Xi is the value of the variable X for year i; Xm and Si are, respectively, the mean and the standard deviation of the variable X observed for the duration of the series studied. The variables considered are the annual precipitated water depth P (expressed in mm) and the annual flow Q (expressed in m3/s). This index makes it possible to define the severity of the drought. Negative annual values indicate drought in relation to the chosen reference period and positive values indicate a wet situation (Ardoin et al., 2003).
Table 6.2 Rainfall data gaps.
| Rainfall stations | Period | Missing years |
Missing periods
|
|---|
