Integrated surface water-groundwater modelling in the context of increasing water reserves of a regional Sahelian aquifer

Despite the Sahelian drought of the 1970s-1990s, the unconfined aquifer in southwest Niger exhibits a multidecadal increase in groundwater reserves. Recent changes in land surface conditions have enhanced runoff and thus indirect groundwater recharge below endorheic ponds. This paper presents a model-based investigation of surface runoff and groundwater recharge at mesoscale (similar to 5000 km(2)). A new lumped-conceptual runoff model applicable to the large number of ungauged endorheic catchments is specially developed, derived from an existing fine-scale, physically-based hydrologic model. Runoff simulated for sites identified as groundwater recharge sources are used to derive recharge forcing for a Modflow-based model of the aquifer. The rising water table trend and its spatial distribution over the period 1992-2003 are generally well simulated, albeit smoothed year-to-year dynamics. Comparison with alternative methods of recharge estimation suggests, however, that there may presently exist more recharging sites and/or contributing surfaces than those considered so far.