ESTIMATION OF RAINFALL INPUTS AND DIRECT RECHARGE TO THE DEEP UNSATURATED ZONE OF SOUTHERN NIGER USING THE CHLORIDE PROFILE METHOD

An estimate of direct groundwater recharge below a region of natural w oodland (tiger bush) has been made in south-west Niger using the solut e profile technique. Data has been collected from a 77 m deep well dug within the study area covered by HAPEX-Sahel (Hydrological and Atmosp heric Pilot Experiment), an international large-scale energy, water an d carbon balance experiment carried out during the summer of 1992. Dur ing well construction samples were taken from the unsaturated zone at the following intervals: every 25 cm from 0-10 m, every 50 cm from 10- 62.5 m, then every metre to the bottom of the well. Pore water was ext racted from each sample either by centrifugation or elutriation and an alysed for chloride; moisture contents of samples were obtained gravim etrically. These data have been used to produce depth profiles of pore water chloride concentration and moisture content throughout the unsa turated zone. From these profiles it has been possible to derive an es timate of historic direct recharge at the site. The chloride concentra tion of rainfall, which is required to make the estimate, was determin ed from the analysis of 123 rainfall samples collected from five EPSAT (vers une Estimation des Precipitation par Satellite au sahel) rain g auges in 1992. A mean recharge rate of 13 mm year(-1) (range 10-19 mm) is estimated for the upper 70 m of the profile, with a total residenc e time of 790 years (range 520-990 years). This is considered to be a representative estimate of the magnitude of direct recharge taking pla ce below tiger bush areas.