Water: Why Water Resources Assessment? (4) For the world beneath us is also complex

In times when the temperature variation and hydrological cycles become intensified, rendering the surface water supplies more unpredictable, it makes sense to put a greater focus on groundwater as part of the WRA. According to Taylor et al 2013, in vast region of semi-arid and arid Africa, people could only reliably source their water from underground due to the ephemeral nature of river flow. Without groundwater, it is very difficult for people to obtain water that is drinkable or potable. In addition, as they could be accessed by simple hand-dug well, they are much more economically affordable than buying bottled water.

But given the complexity of both physical and human landscape in Africa, is the exploitation of groundwater resources a cure-all panacea for the lack of water? The answer, as always, would be both yes and no.
According to MacDonald et al (2012), An estimated 0.66 million km3 of freshwater capacity have been found in Africa, with the reserve being most concentrated in countries such as Lybia, Algeria and Chad in Northern Africa (See Figure) . To put this into perspective, this is around 30 and 20 times the volume found in precipitation and lakes above ground respectively (Shiklomanov and Rodda 2003)!

Figure1. Map of groundwater storage in Africa.

What is more astonishing is despite an approximate 9-fold increase of abstraction volume in Makutapora Wellfield, the aquifer is not depleted. Taylor et al (2013) suggests that this could be related to the episodic recharge events that replenish the aquifer. Their results show that a non-linear relationship exist between the intensity of rainfall and groundwater recharge (see Figure2). Therefore on one hand, although climate change makes surface water supplies more unpredictable, on the other hand, it leads to more frequent and intense rainfall, which in the long run, could help replenish the groundwater and provide an alternative solution to the lack of freshwater.

Figure2. Scatter plot of rainy season rainfall against the volume of groundwater recharge.

So that was Mr.Brightside talking, now let’s turn our attention to some other aspects of exploiting and utilising groundwater resources. According to both Giordano (2006) and MacDonald et al (2012), there are many physical and social constraints to this.

To be able to exploit groundwater, there needs to be sufficient yield of water extraction and this varies depending on its purpose. For example, to support domestic usage in a rural community, a simple handpump with a supply greater than 0.1 ls-1 is enough. The requirement rises to at least 5 ls-1 in both urban settings where its population is more concentrated and growing at a much faster rate and in areas of small scale irrigation scheme. Furthermore, for large scale commercial irrigation schemes, it could be upto 50 ls-1 (e.g. in the US).

Some areas, although store significant volume of water, they could not possibly be extracted at rates that meet the requirement for two reasons. Firstly, the yield also faces the issues of very high uncertainty as illustrated by Figure 3(a) and (b) that shows the possible range of productivity with different geological environment in Africa. This means the government and investors will be more reluctant in developing a new drill. Secondly, as the depth of the groundwater increase, so does the cost of development for drilling. Figure 4 shows both the depth and the likely interquartile range of aquifer productivity across Africa. Its uneven nature means although most areas are able to afford installing handpumps due to the shallow nature of groundwater levels, the largest reserve situated in northern Africa faces issues of extraction due to its depth of more than 250m and the fact that it situates far from site of usage (e.g. cities or intensive agricultural schemes). Also, there is limited area outside of large sedimentary basins with groundwater extraction rate of higher than 5 ls-1. The authors therefore concluded that to ensure a more successful groundwater exploitation, the development plan should be predicated on a very detailed knowledge of local groundwater conditions.