2.3 Groundwater Resources in Southern Africa

Due to the limited availability of surface water resources, groundwater is critical to integrated water resources management, particularly in rural areas not close to large rivers or urban water supply networks. Groundwater occurrence in Southern Africa is characterized by the large variety of geological structures. This is illustrated in Figure 2 from the Hydrogeology Map for Southern Africa (SADC, 2009). Discussion of the main hydrogeological characteristics is largely taken from the Explanatory Brochure accompanying this map (SADC, 2009).

Figure 2 – Hydrogeology Map for Southern Africa (SADC, 2009).

Approximately 55 percent of the region is covered by low permeability formations (brown colors, D1 and D2). These are largely basement rocks with aquifer systems developed in the weathered overburden and fractured bedrock. The aquifers developed in these areas are unconfined, locally developed and not spatially extensive. In general, only modest groundwater supplies can be abstracted sustainably from these aquifers and large-scale groundwater well field developments are not feasible.

Fissured aquifer systems (green colors, B1 and B2) are associated particularly with the Karoo formations (interlayered shales and sandstones) found extensively throughout Southern Africa. The formations are normally low-yielding, but where the rocks have been subjected to deformation and intrusion of dolerites, a secondary permeability resulting in good aquifers may be found.

The unconsolidated intergranular aquifer systems (blue colors, A1 and A2) occur as large inland basins, such as the Kalahari and Congo basins, in coastal aquifers and in alluvial aquifers in river channels, banks and flood plains. The Kalahari basin consists of complex deposits of unconsolidated to semi-consolidated sediments, including sands, calcrete, aeolianite, gravel, clay and silcrete. Saline groundwater tends to occur in these basins in the more arid areas in Botswana and Namibia.

Karst aquifers (yellow colors, C1 and C2) constitute some of the most productive aquifers in Namibia, Botswana, Zimbabwe and South Africa. They occur in highly soluble rock, most notably limestone and dolomite. Groundwater flow is concentrated along secondarily enlarged fractures and fissures and other connected openings, where water from slightly acidic rain can dissolve minerals.

The recharge potential of groundwater is related to climatic conditions such as average temperatures and evapotranspiration and geological factors such as porosity and infiltration rates. Recharge potential is relatively low across much of Southern Africa although it improves to the north due mostly to increased precipitation. Figure 3 shows reported recharge values, including those from more humid Southern African regions, in the widely quoted review by Beekman and Xu (2003). In arid regions, groundwater recharge may be 3 percent or less of rainfall; in regions where rainfall exceeds 600 mm/year, groundwater recharge can reach 20 percent. Determination of groundwater recharge in arid and semi-arid areas is neither straightforward nor easy, Xu and Beekman (2019). This is a consequence of the temporal variability of precipitation in arid and semi-arid climates, and spatial variability in soil characteristics, topography, vegetation and land use.

Figure 3 – Results of various groundwater recharge studies in Southern Africa, indicating the region and type of study (Beekman and Xu, 2003).