Presentations

During the last five decades, land clearing for increasing the surface area cultivated for growing rain-fed millet in southwestern Niger has resulted in gullying and increased runoff on crusted slopes. In this semiarid, endoreic landscape, runoff concentrates in seasonal ponds and then recharges the aquifer (focused recharge): therefore, higher runoff increased aquifer recharge. Because groundwater pumping remained low (< 1 mm/yr), this has resulted in a long-term rise in the water table (4 m rise from 1963 to 2007). Combined use of various methods (subsurface geophysics, isotope geochemistry, neutron probe soundings, remote sensing, hydrological modeling, piezometric surveys, unsaturated zone sampling) consistently pointed out a threefold increase in surface runoff and a tenfold increase in groundwater recharge through ponds, from ~2 mm/yr in the 1950s (pre-clearing conditions) to ~25 mm/yr in the 1990s-2000s (i.e. ~5 percent of rainfall). This increase in focused recharge also had an influence on groundwater salinity near ponds, as shown by a rising trend in groundwater nitrate concentrations of non-human origin (75 percent of d.15N values range between +4 to and +80). Recently observed increases in irrigation by using fresh pond water would, however, increase return flow to the aquifer and may trigger soil salinization. In this well-documented region of the Sahel, impacts of land use change on water quantity and quality are therefore much greater than the direct impact of climate variability.