Estimation of natural groundwater recharge in the Karoo aquifers of South Africa

Abstract

A 3-year project, to study the natural groundwater recharge of aquifers in the semi-arid Karoo formations of South Africa, was undertaken. Two typical Karoo aquifers, at Dewetsdorp and De Aar, were selected for study purposes. Data were collected from both the saturated and unsaturated zone. Neutron probe measurements showed that there was no increase in the water content beneath a depth of ∼ 1m below the surface. Even with the exceptionally high rainfall in February 1988, neutron measurements indicated that very little soil matrix flow occurred, which implied that most of the recharge occurred along preferred pathways. A triangular finite element network was used to determine the relative saturated volume fluctuations (SVF) of the aquifers from the observed water-level changes over a period of time, which allowed determination of the storativity S and the recharge. The main advantage of the method lies in determining recharge and storativity which are both unknowns in the water balance equation and which both contribute to the water-level response. The SVF method showed that the recharge in the Karoo formations of South Africa varies between 2 and ∼5% of the annual rainfall. In areas which are overlain by a thick soil cover, the recharge is less than 3%, while recharge in hilly areas with a thin soil cover may be of the order of 5%.