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
