How do we know how much groundwater is stored in south-western Cape mountains?

Abstract

Isotopes of water (D, O-18) in rain and streams were used to obtain an estimate of the amount of ground water in the south-western Cape Mountains. It was assumed that the groundwater reservoir is well-mixed and that the water isotope signals in local streams are the same as those of groundwater. Analysis suggests that reservoir volume can be several times that of local rainfall minus local streamflow and up to 2-3 times that of local rainfall. This in turn suggests considerable residence times of water to allow this quality of water to accumulate. This has implications for hydrology as well as for understanding the influence of vegetation on streamflow.