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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 Q_t = 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)