INTEGRATED HYDROLOGICAL ASSESSMENT FOR THE DESIGN OF A MULTIPURPOSE SMALL RESERVOIR IN THE SIRBA RIVER BASIN, NIGER

Water scarcity and increasing hydroclimatic variability represent major constraints to sustainable development in the Sahel, where rural livelihoods depend largely on rain-fed agriculture and seasonal water availability. In southwestern Niger, the Sirba River watershed experiences recurrent floods during the rainy season and severe water shortages during the prolonged dry season, highlighting the need for efficient water storage infrastructures. This study assessed the hydrological feasibility of constructing a multipurpose small dam in the Sirba River watershed using an integrated hydrological and geomorphological approach. Long-term rainfall and hydrological data (1960-2020), a 30-m Shuttle Radar Topography Mission (SRTM) Digital Elevation Model, Geographic Information Systems (GIS), and regional empirical hydrological methods (ORSTOM, CIEH and Gradex) were employed to characterize the watershed, estimate design floods and evaluate the reservoir water balance. Rainfall frequency analysis using the Generalized Extreme Value (GEV) distribution indicated design rainfall depths ranging from 210 mm for the 5-year return period to 289 mm for the 100-year return period. Morphometric analysis showed that the watershed covers 39091 km², with an elongated geometry (Gravelius coefficient = 2.04) and a low drainage density (0.06 km km⁻²), characteristics that contribute to moderate flood concentration. The estimated 10-year design flood reached 100.04 m³ s⁻¹, while the mean annual runoff volume was estimated at 86.12 × 10⁶ m³, decreasing to 40.36.10⁶ m³ under 10-year dry conditions. Although evaporation and sedimentation constitute the principal water losses, the reservoir water balance remained positive under average hydrological conditions, indicating sufficient water availability to satisfy projected domestic, livestock and agricultural demands. Overall, the results demonstrate that the proposed small dam is hydrologically feasible and could significantly enhance water security, support irrigated agriculture, reduce flood risks and strengthen climate resilience in the Sirba River watershed. Future studies should incorporate physically based hydrological modelling, sediment yield assessment and climate change scenarios to optimize reservoir design and long-term operation.