(1) Yppi et Bambari are assimilated; Ouadda Djalle et Birao are assimilated.
6.3.2. Break Tests for Hydropluviometric Data
The break tests on the rainfall P and hydrometric Q series were performed with the Khronostat 1.01 software (IRD, 1998) to characterize the stability periods of these two long time sequences, establishing if possible homogeneous flow subsequences, or hydroclimatic periods. Pettitt, Lee and Heghinian rank tests, and Hubert’s segmentation were applied (Hubert & Carbonnel, 1987). The combination of these various methods is justified by the fact that the first two methods (Pettitt’s and Lee and Heghinian’s) are tests of homogeneity whose null hypothesis indicates that a time series is homogeneous between two given dates and according to two laws of different distributions. On the one hand, these tests are known for their robustness but do not allow the detection of more than one break in a time series. On the other hand, Hubert’s test gives the year of the different breaks observed in the series with the means and standard deviations of the different segments. This method has been the subject of numerous publications in different regions of Africa (Ardoin et al., 2003; Bodian et al., 2011; Servat et al., 1999) and in Central Africa (Nguimamet & Orange, 2020). These tests have not been applied to the Uele time series, which are series inferred from the surrounding basins.
6.3.3. Calculation of River Depletion Coefficients
The river depletion coefficients of the Ubangi at Mobaye and the volume mobilized by the basin aquifer were calculated from daily data (according to Cosandey et al., 2003; Goula et al., 2006; Nguimalet & Orange, 2019; Savané et al., 2001). However, numerous gaps in the 1960s with exceptional rainfall and after 1976 shortened the series. This approach makes it possible to assess the contribution of surface runoff on aquifer recharge, by homogeneous hydropluviometric period in this deficit context.
The mathematical expression for the river depletion coefficient is written:
(6.3)
where Qt = discharge at time t, Q0 = initial discharge (discharge at the beginning of the depletion), and k = Maillet depletion coefficient. Maillet’s law is modeled on a single reservoir, which corresponds to all the aquifers contributing to flows in the watershed under study (Goula et al., 2006; Nguimalet & Orange, 2019). The integration of this equation over the interval [0, +] gives an estimate of the volume mobilized from the aquifers in the watershed, i.e.:
(6.4)
6.4. RESULTS
6.4.1. Interannual Evolution of Rainfall and Annual Flows of the Ubangi River at Mobaye
The average interannual rainfall on the Ubangi basin at Mobaye is 1,506 mm over the period 1938–2015. On the one hand, the search for breaks with Hubert’s segmentation reveals two breaks (1968 and 2006) and thus three homogeneous periods (1938–1968, 1969–2006, and 2007–2013). On the other hand, a single break appears in 1969 with the Pettitt and Lee‐Heghinian tests (Table 6.3). This time series is thus characterized by a definite rainfall break in 1968/1969 (Figure 6.3), marking the passage from a previous wet period to a dry period, and a probable second break in 2006 to be confirmed in the future, and which would characterize the return to a wetter period. The average rainfall amounts for the periods 1938–1968, 1969–2006, and 2007–2015 are 1,568 mm, 1,436 mm, and 1,583 mm respectively. The dry period of 1969–2006 corresponds to a reduction of 8% in rainfall compared to the previous period. And the current wet period (2007–2015) describes a 10% increase in rainfall compared to the previous period.
Table 6.3 Statistical segmentation of annual rainfall in the Ubangi basin at Mobaye (1938–2015), and in the upstream basins from 1951–1995 (in the Mbomu basin at Bangassou and Zemio, and in the Kotto basin at Kembe and Bria).
| Hydrological basins | Interannual average (mm) | Pettitt | Lee and Heghinian | Segmentation of Hubert (mm) | Standard Deviation | Hydro‐climatic trends | P divided by interannual average (%) |
|---|---|---|---|---|---|---|---|
| Ubangi at Mobaye (403,800 km2) | 1,506 | 1969 | 1969 | 1938–1968: 1,568 1969–2006: 1,436 2007–2013: 1,583 | 109 88 81 | Wet Dry Wet | +4 –5 +5 |
| Mbomu at Bangassou (115,000 km2) | 1,491 | 1970 | 1951–1994: 1,851 |
