Groundwater's Biggest Problem: Uneven Distribution Part 1

As shown in Figure 3, SSA has approximately three times the per capita groundwater availability of China and nearly six times that of India, the world's two biggest players in groundwater-fed irrigation systems (Giordano 2006). However, only 6% of cultivated land in Africa is irrigated (PAVE Irrigation Systems 2015). One of the reasons it has not been exploited to data is because Africa’s groundwater distribution is highly variable, with the majority of groundwater storage in North Africa. An intermix of factors have caused this spatial variability including geology and climate.

Geology

Supplies are dependent upon the ability of underlying rock to store and transfer water. A large proportion groundwater is located in hard rock areas or at extremely deep distances below the ground, making abstraction difficult or costly. Giordano (2006) outlines four geological zones, of which Africa is made up of (see Figure 5 for their distribution across the region), that alter the ability of the ground to store and transfer water. A summary of these zones can be found in Table 1.  

Table 1: Four geological zones across SSA, information summarized from Giordano (2006)

Hydrogeologic Zone	% of Region	Potential as a Groundwater Reserve
Crystalline Basement Rock	40	Poor supply source for groundwater due to its low transmissivity.
Consolidated Sedimentary Rock	32	Can hold large amounts of water so high potential.
Unconsolidated Sediments	22	Groundwater is often held in hold in unconstrained conditions within sands and gravels, and often found in river beds, so easily accessible. However, “many African river systems are typified by fine and very fine sediments, rather than coarse sand and gravel, reducing extraction possibilities” (312).
Volcanic Rock	6	Can produce high groundwater yields and supply springs.

However, hard rock aquifers should not be totally written off. Even those with low yield potential may be suitable for small-scale irrigation. For a community water supply fitted with a hand pump, only a sustained supply of >0.1 l s-1 is needed, in comparison to >51 l s-1 for boreholes for commercial irrigation schemes (MacDonald et al. 2012). Supplies or recharge rates may mean that these larger yields may be unreachable, but could hold potential for smaller scale, less intensive activities.

Climate

  

Groundwater also depends on recharge potential, in order to make it a sustainable resource. Unfortunately, due to past and current climatic conditions, groundwater distribution is highly correlated with rainfall patterns, meaning it tends to be highest in areas of high rainfall and lowest in areas of low rainfall (Calow and MacDonald 2009). This depletes the human value of groundwater. 

Depth to Groundwater

The depth of the groundwater below the surface is again, unsurprisingly, highly variable. Depth affects both the practical accessibility of the resource, and whether it is economically viable (MacDonald et al. 2012). Levels deeper than 50m are not easily accessible by a hand pump and for those >100m, the cost of extraction increases dramatically because more advanced drilling equipment is required (MacDonald et al. 2012).