SPATIOTEMPORAL VARIATION OF BASEFLOW AND SURFACE RUNOFF TO ASSESS GROUNDWATER VARIABILITY

The amount and distribution of baseflow in a watershed indicates the contribution of groundwater to streamflow and is critical to sustaining flow in perennial streams. This project employed the Soil and Water Assessment Tool (SWAT) to estimate the spatiotemporal variation of baseflow and streamflow as a proxy to assess the groundwater resource potential and variability in part of the Benin River Basin, Nigeria. The study site consists of two major watersheds, the Ossiomo River and Ethiope River watersheds, which constitute the southern part of the Benin River Basin with an area of 4685 km² and 3910 km², respectively. The studied watersheds have five-gauge stations with limited streamflow observations available from 1989-1994. As a result, the current condition of the water resource in the watersheds is unknown. Quantifying the water resource potential of a watershed is essential for long-term water security planning. The main objectives of this project are (1) to assess the water resource potential of two sub-watersheds, (2) to evaluate trends in baseflow contribution between the upstream and downstream reaches of the two sub-watersheds, and (3) to determine factors controlling water availability within the two sub-watersheds. The SWAT model was evaluated using the available observations to estimate streamflow, baseflow, and groundwater recharge. Model input data includes the Digital Elevation Model (DEM), land use map, soil map, and weather data. It is expected that the calibrated model to be sensitive to rainfall and runoff mainly controlled by baseflow—an increasing trend in baseflow contribution to the rivers. An increasing groundwater withdrawal could intervene in streamflow by reducing baseflow. Most recent in-situ observational data may help determine the actual impact of groundwater on streamflow.