ESTIMATION OF LONG-TERM, DIFFUSE GROUNDWATER DISCHARGE IN THE NORTHERN SAHARA USING STABLE ISOTOPE PROFILES IN SOIL-WATER

Stable isotope profiles in soil water in the Sahara have been interpreted in terms of the hydrologic processes taking place. At one site, where the water table is at about 10 m depth, the isotope data are interpreted using the model of Barnes and Allison to show that the water loss by evaporation from the water table is about 2 mm a** minus **1. At another site nearby there is a net evaporative loss from the water table, but the evaporation is interrupted by occasional infiltration events. It is suggested that accumulation of chloride in soil water can give information about the onset of aridity in the Sahara.