Hydrological Functions of Gallery Forests Along the Sokoto River: Implications for Water Security

Gallery forests fringing the Sokoto River in northwestern Nigeria deliver critical hydrological functions including seasonal flow regulation, groundwater recharge, bank stabilisation, and water quality filtering, yet these ecosystems are rapidly declining due to agricultural encroachment and fuelwood extraction, raising water security concerns for the densely populated Sokoto basin. This study quantifies the hydrological functions of gallery forest along a 90-kilometre segment of the Sokoto River and models the water security implications of ongoing deforestation through scenario analysis. Paired catchment monitoring was established in forested and adjacent deforested gallery reaches over a two-year hydrological period. Stream flow, suspended sediment load, groundwater depth, and riverbank erosion rates were measured at monthly intervals. Stable isotope analysis differentiated precipitation, surface runoff, and groundwater contributions to streamflow. A lumped hydrological model was calibrated and used to simulate streamflow under alternative gallery forest cover scenarios. Results show that forested gallery reaches maintained streamflow 34% higher during the dry season compared to deforested sections through sustained groundwater contribution. Suspended sediment loads were 6.7 times higher in deforested reaches, with implications for reservoir siltation downstream. Groundwater tables in adjacent farmland were significantly shallower near intact gallery forest, suggesting recharge augmentation. Model projections indicate that complete gallery forest clearance would reduce dry season streamflow by 47% compared to current conditions. Restoration of gallery forest within 50 metres of the river channel is recommended as an urgent water security intervention. Keywords: gallery forest, hydrological functions, Sokoto River, water security, riparian