Geographic variations, coupled with climate change, result in uneven distribution of rainfall and freshwater sources, with deserts and rainforests highlighting these water availability extremes (Figure 9.1 and Figure 9.4). Groundwater is the major drinking water source for the majority of people globally, particularly in arid regions and during drought. The estimated available renewable groundwater resource in Africa is more than 100 times that of total annual renewable surface-water resources (MacDonald et al. 2012, p. 5). However, deeper aquifer water is constrained by exploration and abstraction costs. Abstraction of very ancient ‘fossil groundwater’ is unsustainable, because this is not a renewable resource.