Quantifying the Contribution of Groundwater to Streamflow in Semi-Arid River Basins of Northern Nigeria Using Multi-Tracer Hydrograph Separation

The baseflow contribution of groundwater to streamflow in the semi-arid river basins of northern Nigeria is poorly constrained, creating significant uncertainty in water resource planning models that must balance surface water and groundwater allocation decisions for agriculture, industry, and domestic supply under projected climate drying. This study quantifies groundwater contributions to streamflow in three representative semi-arid river basins spanning the Sokoto-Rima, Komadugu-Yobe, and Hadejia-Jama'are river systems using a multi-tracer hydrograph separation approach. Intensive sampling campaigns were conducted during rising limb, peak flow, and recession periods across multiple hydrological years. Tracers including electrical conductivity, major ions, silica, radon-222, oxygen-18, and deuterium were measured simultaneously at gauging stations to enable end-member mixing analysis with multiple independent tracer constraints. Catchment-specific hydrograph separation was performed using a Bayesian mixing model that propagates end-member uncertainty and provides probability distributions of groundwater fraction estimates. Temporal variability in groundwater contribution was related to antecedent soil moisture, land use, and inter-annual rainfall variability using generalised additive modelling. The study identifies systematic differences in baseflow fractions between basement and sedimentary catchments attributable to contrasting aquifer storage coefficients. The derived baseflow index values are incorporated into a regional groundwater recharge assessment framework submitted to the Hadejia-Jama'are River Basin Development Authority. Keywords: groundwater-streamflow interaction, hydrograph separation, environmental tracers, semi-arid Nigeria, baseflow.