Recorded changes over hydrological extremes of Oubangui at Bangui (Central African Republic): trends analysis
The principal tributary draining northern Congo, the River Oubangui at Bangui, has a transboundary basin extending in the DRC (Democratic Republic of Congo) over 499 000 km2. Four wet tropical climate variants influence its regime according to a South-North rainfall gradient (1 600-1 000 mm), but with the Sudano-Guinean variant prevalence over the major part of the basin. This paper characterizes the extreme discharge (rising and low-water levels) fluctuations of the Oubangui River and recorded trends over time, as affected by the hydrological variability noted since 1971. Annual extremes (maximum and minimum) and daily mean discharge data over the 1911-2014 period were used. Statistical tests (R irregularity indices, research for temporal tendencies, and changes in the dates of flood and drought periods) were applied to these extreme discharges to identify which of the basin’s hydroclimatic periods determines the instability and/or stability of these extreme hydrological regimes. Thus, the Qmax and Qmin recorded an overall stability period, which was followed by a relatively more severe drop of the Qmin (R = 0.64) compared to the Qmax (R = 0.56). This trend is confirmed by the A coefficient evolution (R = 0.56), whereas the R irregularity normal (R = 0.51) and decadal (R = 0.83) indices, and the depletion coefficient (R = 0.59), show some relative degrees of either increasing discharge or hydrological drought. The Qmax/Qmin correlation displays an increasing average trend (R = 0.51), which suggests that the weak reduction of Qmax counteracts the accentuated drop of Qmin. Indeed, this difference in hydrological response is noticeable for the temporal changes in the Oubangui regime: 16% decline for Qmax since 1975 against 47% for Qmin since 1982. This observation confirms the severity of the current drought, with the drying-up or disappearance of sub-tributaries. Also, dates for increasing discharge have passed from October (1911-2014) to November (1971-2014) and those of low-water levels from March-April (1911-2014) to April (1971-2014). Severe floods have become less frequent, in favour of medium floods and increasingly marked low-water levels. This is established by break points (Hubert segmentation) showing a drop of Qmax (R = 0.84) and Qmin (R = 0.81). In this context, an exacerbation of these positive or negative trends (frequency and severity of floods and droughts) will generate noticeable societal impacts in Bangui and along this river.