**Abstract**

Groundwater resources are indispensable not only in water scarce or water stressed countries, but globally as a dependable reservoir and an alternative resource of freshwater. This study assessed the spatio-temporal variability of groundwater resources within two of the biggest transboundary aquifers that South Africa shares with its neighbouring countries. Groundwater dynamics in the Karoo-Sedimentary Transboundary Aquifer (KSTA) as well as the Stampriet Transboundary Aquifer System (STAS) were studied over a period of 72 years from 1948-2020. The study explored the use of historical groundwater storage data acquired through the use of Remote Sensing (RS) techniques, coupled with the use of Geographical Information Systems (GIS) to map spatio-temporal variability in groundwater storage. Groundwater resources of the Karoo-Sedimentary Transboundary Aquifer were found to be declining over time, with an overall decline of just over 5.4 km3 , whereas groundwater resources in the Stampriet remained relatively constant, with an overall increase of 0.2 km3 over the past 72 years. The results show that RS techniques coupled with GIS applications are invaluable where there is a dearth of scientific data and information, furthermore, their use in the monitoring, management and protection of groundwater resources can be applicable on the local, regional and international scales.

**Keywords:** transboundary aquifer, southern Africa, groundwater, aquifer monitoring, Stampriet transboundary aquifer, Karoo sedimentary transboundary aquifer
