Predictive Modelling and Low-Flow Augmentation Strategies for the Sota Basin at Coubéri in Benin (West Africa)

This study evaluates a low-flow regulation strategy for the Sota River basin, aiming to maintain a target discharge that satisfies concurrent irrigation demands and ecosystem sustainability. The methodology involved a three-step process: 1) characterizing historical low flows using the annual minimum monthly discharge (QMNA), 2) simulating future discharge regimes using the HEC-HMS hydrological model forced with climate projections bias-corrected via the Distribution Mapping method, and 3) assessing the feasibility of implementing low-flow support measures. Results reveal strong interannual variability in low flows, with two breakpoints in stationarity (1971, 2003) yet no significant monotonic trends within the resulting sub-periods. Frequency analysis identified the Weibull distribution as the best fit, yielding a mean low-flow value of 4.18 m3 s−1. Hydrological simulations from the calibrated HEC-HMS model, which demonstrated reliable performance (Nash-Sutcliffe efficiency, NSE = 0.75 in calibration, 0.70 in validation), project a decline in discharges from 2030 to 2080, with annual reductions of up to 34%. A critical finding emerged from the comparison of climate models: significant divergence exists in the estimated required low-flow support. Projections from the RCA4 model indicate that low-flow augmentation would need to supply between 12% and 39% of the mean annual flow, depending on the ecological flow scenario. In contrast, projections from the CCLM and REMO models suggest a substantially higher requirement, between 30% and 90% of the mean annual flow. This divergence underscores that, while the required volume is model-dependent, the implementation of low-flow support remains a hydrologically realistic management objective across the range of climate scenarios evaluated. These findings point to realistic management options for low-flow regulation and contribute to more resilient water resource governance in the Sota basin.