Groundwater recharge in the Kalahari, with reference to paleo-hydrologic conditions

The Kalahari is situated in the semi-arid center of southern Africa and can be characterized as a savannah with a sandy subsurface, deep groundwater tables and annual rainfall ranging from 250 mm in the southwest to 550 mm in the northeast. A high infiltration rate and high retention storage during the wet season and subsequent high transpiration by the dense vegetation during the dry season, make that very little water passes the root zone and contributes to aquifer recharge. A lively debate has continued for almost a century on the question whether the Kalahari aquifers are being replenished at all under present climatic conditions. The present paper reports on results of an extensive recharge research project at the eastern fringe of the Kalahari, which is the most favorable part for groundwater replenishment. Additional observations were made in the central Kalahari. Environmental tracer studies and groundwater flow modeling indicate that present-day recharge is in the order of 5 mm yr(-1) at the eastern fringe of the Kalahari where annual rainfall exceeds 400 mm. Figures in the order of 1 mm were obtained from the central Kalahari with lower precipitation. A dry valley system refers to more humid paleo-climatic conditions with a higher groundwater recharge. A tentative reconstruction of the groundwater depletion history suggests a time lapse of several thousands of years since the end of the last wet period. (C) 2000 Elsevier Science B.V. All rights reserved.