Figure 6.3 Rainfall index and flow index of the Ubangi basin at Mobaye, 1938–2015.
The 1938–2015 time series of annual flows from the Ubangi at Mobaye is marked by a sharp decline in flows in 1970 (Figure 6.2). Statistical tests indicate only one break with the Pettitt and Lee‐Heghinian tests: it is noted in 1981 on the 1938–2015 series, and in 1970 on the 1948–1995 series. And Hubert’s segmentation describes five homogeneous periods for the two series tested (1938–2015 and 1948–1995) but only the years 1968 and 1980 appear for the two hydrological series tested, attesting to the importance of the statistical break at these two dates. Three hydroclimatic periods are thus retained for the Ubangi at Mobaye (Table 6.4): 1954–1968, 1969–1980, 1981–2013, with interannual mean flows of 3,919 m3/s, 2,986 m3/s (–24%) and 2,372 m3/s (–21%), respectively.
6.4.2. Interannual Evolution of Rainfall and Annual Flows in Sub‐Basins of the Ubangi at Mobaye
The rainfall series of the Uele basin and the Mbomu basin at Bangassou have a similar evolution to that of the Ubangi basin at Mobaye, and to a lesser extent the Kotto basin at Kembe. On the other hand, the rainfall series of the Mbomu at Zemio and the Kotto at Bria do not show a clear separation between a wet and a dry phase (Figure 6.4). The last two basins drain the wooded and shrubby savannah, while the first two are of East–West extension and drain a significant part of the rainforest. Overall, in all these basins, the 1960s were wet, the 1980s were dry, and there seems to have been a rainfall recovery since the early 1990s, perhaps more marked on the Uele basin.
Table 6.4 Statistical segmentation of the annual flows of the Ubangi River at Mobaye (1948–1995) and in the upstream basins from 1951–1995 (from the Mbomu at Bangassou and Zémio, from the Kotto at Kembé and Bria).
| Streams | Interannual average (m3/s) | Pettitt | Lee and Heghinian | Segmentation of Hubert (m3/s) | Standard Deviation | Hydroclimatic trends | Q divided by interannual average (%) |
|---|---|---|---|---|---|---|---|
| Ubangi at Mobaye (403,800 km2) | 3,059 | 1970 | 1970 | 1948–1953: 2,925 1954–1967: 3,804 1968–1968: 5,524 1969–1980: 2,986 1981–1994: 2,208 | 232 513 0 414 288 | Dry Wet Hyper Wet Dry Hyper Dry | –4 +24 +81 –2 –28 |
| Mbomu at Bangassou (115,000 km2) | 782 | 1975 | 1951–1960: 731 1961–1969: 1,051 1970–1993: 706 | 129 187 185 | Dry Hyper Wet Sèche | –7 +34 –10 | |
| Mbomu at Zemio (29,300 km2) | 171 | 1981 | 1981 | 1951–1961: 165 1962–1968: 249 1969–1980: 180 1981–1993: 128 | 22 49 37 35 | Dry Hyper Wet Wet Hyper Dry | –4 +46 +5 –25 |
| Kotto at Kembe (77,500 km2) | 360 | 1970 | 1948–1965: 401 1965–1969: 651 1970–1984: 345 1985–1993: 169 | 67 38 43 27 | Wet Hyper Wet Dry Hyper Dry | +11 +81 –4 –53 | |
| Kotto at Bria (61,500 km2) | 244 | 1985 | 1948–1984: 268 1985–1993: 134 | 40 22 | Wet Hyper Dry | +10 –45 |
Figure 6.4 Rainfall index of upstream basins of the Ubangi at Mobaye, from 1951 to 1995.
Statistical tests show rainfall breaks only on the two basins of the Kotto; Hubert’s segmentation showed three homogeneous rainfall periods at different dates, only the recent period 1992–1993 is noted as wet. However, the 1970 rainfall break characteristic of the Central African continent (Laraque et al., 2001) is not confirmed. It can only be found in the analysis of the rainfall time series from the Kotto at Bria, for which the dry period is marked over a period of nearly 25 years (from 1967 to 1991) with a significant rainfall deterioration of –12%. The Kotto at Bria is the northernmost of the six basins studied.
It is remarkable that only the Pettitt test detects a rainfall break in 1970 on the Mbomu basin at both Bangassou and Zemio. This absence of a break in the Mbomu sub‐basins is thought to be due to the impact of the forest cover that regulates rainfall systems, as also observed in the Tomi basin at Sibut in north‐central CAR (Nguimalet & Orange, 2020).
Time series of annual flows of the Ubangi River at Mobaye and the Kotto River at Kembe and Bria show a continuous overall decrease from 1948 to 1994, with no semblance of recovery in the 1990s, in contrast to the Mbomu basins at Bangassou and Zemio (Figure 6.5). Flows were generally above average until 1988 before plunging sharply.
