Groundwater’s Biggest Problem: Uneven Distribution Part 2

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).

Groundwater's Climate Resilience

Global climate change is expected to affect both temperature and precipitation across Africa. Temperatures are expected to rise by 3-4°C, increasing evapotranspiration rates, whilst mean annual rainfall is predicted to vary by up to ±10-20% (Carter and Parker 2009). Both inter-annual and seasonal variability in rainfall patterns are likely to increase in a region already characterized by variable rainfall (Carter and Parker 2009). Extreme weather events such as flooding and drought are set to intensify (Kusangaya et al. 2013). All of this contributes to making Africa’s water supply even more difficult to manage. Increasing the reliability of supply is key. Africa’s agriculture is particularly vulnerable to climate change due to its widespread reliance on rain fed agriculture, its extreme intra and inter-season climate variability and recurrent dry and wet seasons, coupled with its extensive poverty and consequent low ability to adapt (IPCC 2014b).

More than 95% of food grown in Sub-Saharan Africa is rain-fed. Climate change, therefore, threatens food security because surface water supplies are more likely to be affected by it than groundwater. They are significantly affected by changes in rainfall (Pavelic et al. 2013). With most areas of Africa expected to face decreases in precipitation, this decrease will be passed onto surface water supplies, particularly run-off (Ziervogel et al. 2010). Streamflow projections show they are likely to decrease under climate change – river runoff faces declines of 10-30% in the dry tropics and most conclusions have them set to decrease more generally overall in Africa (Kusangaya et al. 2013). The magnitude of this decrease is uncertain. Additionally, evaporative increases, due to temperature increases, could reduce outflows from reservoirs (Kusangaya et al. 2013). When faced with drought or prolonged dryness, surface water supplies are prone to fail and can prove insufficient (Calow and MacDonald 2009). Changes is surface water supply have proved rather destructive in the past, leaving people without access to water, and consequently food (deWit and Stankiewicz 2006). The vulnerability of surface water to climatic changes means they are not a suitable resource to rely upon in the uncertain face of global climate change.

A well dug in Kunjuru - the prolonged drought has meant that the river has dried up

Source: UN Multimedia

Deep groundwater stores generally prove to be more resilient to changes in rainfall - its “buffering capacity” makes it a more consistent source of water for irrigation in a region facing increasingly variable precipitation (Calow and MacDonald 2009: 1, Conway et al. 2008). Sub-surface aquifers' water storage capacity is often greater than their yearly recharge, meaning they are able to continue to supply water even when rainfall supply is low for an extended period (Calow and MacDonald 2009). Groundwater recharge can be stored for several decades (MacDonald et al. 2012). During periods of prolonged drought, deep groundwater supplies prove more reliable than their shallow counterparts and surface supplies (Calow and MacDonald 2009). Climate change will also likely result in an intensification of precipitation, which will benefit groundwater recharge. The relationship between rainfall and recharge is non-linear, meaning that extreme rainfall is required to generate significant groundwater storage, which is required for agricultural uses (Taylor et al. 2013, Calow and MacDonald 2009).

Groundwater stores are not immune to the effects of climate change. However, it does appear to be a better insulated water supply, therefore proving more reliable water supply for agricultural needs (Kusangaya et al. 2013). 

We need a sustainable solution

Agriculture is by far the most dominant motivation for freshwater withdrawals in Africa, accounting for 83.1% of withdrawals (Wada et al. 2011). So finding a sustainable solution seems vital in order to improve and maintain food security in a region that is set to potentially be drastically affected by climate change and continues to suffer from vast socio-economic issues.

Source: http://www.un.org/africarenewal/magazine/april-2011/investing-africas-farms-—-and-its-future

Groundwater is proposed by many as an abundant, resilient and less variable alternative to rain-fed irrigation, especially in particularly seasonal and variable climates, predicted to be exacerbated by climate change (Pavelic et al. 2013). 

Source: http://www.hydrology.nl/iahpublications/201-groundwater-cartoons.html

Over the next few months (and perhaps longer!), this blog seeks to explore the possibility of, and make a case for, groundwater as a future tool and resource for agricultural production and irrigation in Africa.