**2. Materials and methods**

Southern Africa is a semi-arid region characterised by low and erratic rainfall, occasional flooding, high evaporation rates, cold winters and prolonged droughts [2]. The region is mostly underlain by the 2.5 million km2 Kalahari Basin [32] and the 500,000 km2 Karoo Basin [33]. The Karoo Basin covers the entirety of the Kingdom of Lesotho, and most of South Africa, whereas the Kalahari Basin is located in parts of Angola, most of Botswana, small patches in the Democratic Republic of Congo, some parts of Eastern Namibia and parts of the Northern-Cape in South Africa, as well parts of Zambia and small patches in Zimbabwe [34]. The KSTA is located within the Karoo Basin, and the STAS is located within the Kalahari Basin.

#### **2.1 The Karoo Sedimentary Transboundary Aquifer**

The Karoo Sedimentary Transboundary Aquifer (KSTA) is 165,936 km<sup>2</sup> , covering all of Lesotho, most of the Eastern Cape, and parts of Kwa-Zulu Natal, the Free State and the Northern Cape provinces in South Africa (**Figure 4**). It is the largest TBA between South Africa and a neighbouring state, with an estimated population of just approximately 2.2 million people [35].

**Figure 4.** *Map of the Karoo-Sedimentary Transboundary Aquifer (KSTA) showing the location of the aquifer.*

#### **2.2 The Stampriet Transboundary Aquifer System**

The Stampriet Transboundary Aquifer System (STAS) is 102,401 km<sup>2</sup> and straddles the shared border between Botswana, Namibia and South Africa. Most of the aquifer is in central Namibia, which accounts for 73% of the surface area and just over 90% of the population (total population estimated at 50,000) [36]. Botswana makes up 19%, while a small part of the Northern Cape in South Africa (which is entirely a National Park) constitutes the rest (**Figure 5**). The aquifer is named after the village of Stampriet, which is located in Hardap, Namibia.

The two transboundary aquifers are located below the Orange-Senqu transboundary river basin shared by Botswana, Lesotho, Namibia and South Africa (**Figure 6**). The majority of the Karoo Sedimentary TBA lies beneath the Orange-Senqu River Basin, while the entire Stampriet TBA system is located beneath the transboundary river basin, with some of the upper aquifers hydraulically linked to the basin, whose management, conservation, governance, protection and development are administered through the Orange-Senqu River Commission (ORASECOM).

#### **2.3 Groundwater storage**

The study used National Aeronautics and Space Administration (NASA)'s Goddard Earth Sciences Data and Information Services Center (GES DISC) to access and download data used in the study. NASA's Goddard Earth Sciences Data and Information Services Center developed the Goddard Interactive Online Visualisation and analysis Infrastructure or "Giovanni." Giovanni is an application that allows the visualisation of selected geophysical parameters [37]. It supports

*Groundwater Dynamics in Transboundary Aquifers of Southern Africa DOI: http://dx.doi.org/10.5772/intechopen.109906*

**Figure 5.** *Map of the Stampriet Transboundary Aquifer System (STAS) showing the location of the aquifer.*

**Figure 6.** *Map of the Orange-Senqu basin showing the location of the KSTA and the STAS within the basin.*

single- and multi-parameter visualisations as well as statistical analysis, providing an interactive interface for comparing data from a multitude of sources [37]. The data used in the study were accessed and downloaded from Giovanni. Groundwater storage data were retrieved from the Global Land Data Assimilation System Version 2 (GLDAS-2), which currently covers data from January 1948 to March 2022. The study utilised data from January 1948 to December 2020, covering a span of 72 years.

The study utilised gridded GLDAS Model CLMS025 groundwater storage data maps of 0.25° resolution (27. 2 km) to map groundwater storage in the two selected aquifers in Southern Africa. Groundwater storage (GWS) is calculated by subtracting snow water equivalent (SWE), root zone soil moisture (RZSM), and canopy interception storage (CIS), from the total water storage (TWS), using Eqs. (1) and (2) below.

$$\text{TWS} = \text{CIS} + \text{GWS} + \text{RZSM} + \text{SWE} \tag{1}$$

Thus,

$$\text{GWS} = \text{TWS} - \text{CIS} - \text{RZSM} - \text{SWE}.\tag{2}$$

Annual data (January–December 1948) were downloaded as raster image files, showing the total groundwater storage for that year [38, 39]. Data for subsequent years (1949–2020) were also downloaded.

A geographical information systems' application was used for the extraction and manipulation of the data. Before the extraction of data, shapefiles for each aquifer were created, and these were saved as vector files. For each of the study area aquifers, a shapefile was used for masking/clipping the GLDAS-gridded raster images. For each aquifer, groundwater storage data were extracted in 10-year intervals for a period of seven decades (1948–2018), a total of 72 years when including the years 2019 and 2020.

The raster images for two periods to be compared (e.g., 1948 and 1958) were differenced using the map algebra function in the GIS application. This involved subtracting the raster 1948 from the 1958 raster. The differenced map represents the change in storage for that period. The maps showing the change in groundwater storage for each aquifer and the surrounds were saved as .PNG files, and the images are presented in Section 3, showing areas of significant recharge and discharge in each study area. The storage data are provided in the raster file as depth in millimetres. The depth was converted to volume in cubic kilometres using the following Eq. (3) below:

$$\text{Volume} \left(\text{km}^3\right) = \frac{\text{depth} \left(mm\right)}{\text{1,000,000}} \times \text{Area of the aquifier} \left(km^2\right) . \tag{3}$$

1 km = 1,000,000 mm; hence, in (2), there is division by 1,000,000 in order to convert the millimetres to kilometres.

Since each raster image grid cell size in this case is 0.25° × 0.25°, which is equivalent to 27. 2 km × 27.2 km, the size of each grid is, therefore, an area of 739.84 km2 . This area was multiplied by the number of grid cells for each study area in order to calculate the size of each aquifer.

For the purposes of this study, groundwater is defined as subsurface freshwater found within confined and unconfined aquifers. Thus, changes in groundwater depth refer to changes in the depth of the water table, whereas changes in groundwater volume refer to changes in the volume of groundwater within the demarcated area of each transboundary aquifer. Moreover, mentioning groundwater volume or storage capacity of the Karoo (KSTA) or the Stampriet (STAS) refers to the volume of groundwater within each aquifer/aquifer system.
