Holocene depletion and active recharge of the Kalahari groundwaters — A review and an indicative model

A paramount question in the evaluation of groundwater resources in the Kalahari semi-desert is whether there is any active infiltration of present-day rainfall to the deep aquifers. Previous investigations with isotope analyses gave evidence of recent recharge but no unanimity about the distribution of this process has been reached until now. The present study investigates whether the regional hydraulic gradient means a modern replenishment, or whether the slope of the piezometric surface should be explained as a residual feature resulting from head decay since the last pluvial period. It is concluded that the hydraulic gradient is most probably of a residual nature, and the replenishment has been less than 0.5 mm yr.⁻¹ during at least the last 4000 yr. or so.