**3. Results and discussion**

The insufficient availability of groundwater data (such as storage, quality and recharge and discharge patterns) results in a knowledge gap regarding the variability of groundwater resources globally. This is exacerbated by overexploitation as well as well as contamination that is usually unaccounted for and that can also go unnoticed for decades.

Thus, the limited information on the quantity (availability) and quality (contamination) of groundwater resources, coupled with anthropogenic activities as well as climate change, threatens the protection and management as well as the sustainable use of groundwater resources [40]. Accordingly, the use of earth observation or remote sensing techniques in monitoring groundwater resources has become invaluable in groundwater monitoring studies.

#### **3.1 The Karoo-Sedimentary Transboundary Aquifer (KSTA)**

Groundwater storage within the area of the Karoo Sedimentary aquifer is depicted in **Figure 7**. The volume of groundwater is presented over a period of 72 years, from the 1 January, 1948 to 31 December, 2020.

The volume of groundwater in the area fluctuated (between 95 and 120 km3 ) throughout the study period, showing periods of decreasing and increasing volumes, with 1975 and 1978 depicting higher volumes of 128 and 134 km3 , respectively. The aquifer's largest increase in volume was recorded between 1970 and 1980, and again between 2000 and 2010, with the largest decrease in volume recorded between 2010 and 2020. The monthly fluctuations in groundwater volume are depicted in **Figure 8**.

Groundwater volumes were higher between January and July where average volumes were above 105 km3 . There was a steady decline in average volumes between August and November each year, and volumes start to increase again in December each year. These seasonal fluctuations in groundwater volumes correspond to the rainy season in the region. The region receives summer rainfall, starting in December through to March/April each year.
