Characterization of groundwater –surface water interactions using high resolution integrated 3D hydrological model in semiarid urban watershed of Niamey, Niger

Abstract

This study investigates groundwater-surface water interactions using an equivalent porous medium approach in a data scarce and semi-arid hydrogeological watershed located south-west Niger. A large scale fully-integrated hydrologic model was built and calibrated using HydroGeoSphere with a sequential approach of increasing levels of temporal resolution: 1) steady state average conditions; 2) dynamic equilibrium with repeating monthly normal forcing data; and 3) fully transient conditions. This approach provided a useful and straightforward method for reducing the calibration effort of the large-scale fully-integrated hydrologic model. River-aquifer exchange flux dynamics, water balance components for different land use classes, as well as basin average groundwater recharge were computed from the model. Simulation results show that exchange flux between groundwater and surface water are important processes in the basin, with the Niger River acting primarily as a gaining stream, with local losing zones. Ephemeral streams constitute important focused groundwater recharge areas, while ponds exhibit either groundwater discharge behavior, or a recharge zone profile depending on local topography. The basin average water balance highlights the importance of plant transpiration (58% of total rainfall) over surface evaporation (8%), with groundwater recharge of up to 5% of total rainfall. Overland flow and infiltration account for 11% and 16% of the total annual rainfall respectively, and groundwater discharge to the river is 2% of the total rainfall.