More groundwater abstraction = more irrigation = more food. This all sounds great, but is there a limit? We have a tendency to view groundwater as a vast unlimited resource... but to what extent is this really true? Will there come a point where increasing groundwater abstraction in Africa becomes unsustainable? This post aims to find out.
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Fig. 1 KNOW YOUR GROUNDWATER EXTRACTION LIMITS!!
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How do we establish sustainable abstraction rates?
Originally, it was assumed that sustainable abstraction rates (or ‘sustainable yield’) could be equated to natural groundwater recharge. This seems to make sense: if abstraction rates exceed recharge rates then the volume of water stored in aquifers will decrease, a process referred to as groundwater depletion (Aeschbach-Hertig & Gleeson, 2012). However, Bredehoeft (1982; 1997; 2002) provocatively stated that sustainable groundwater developments have almost nothing to do with natural recharge. He called this “The Water Budget Myth”.
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Fig. 3 The Water Budget … is a MYTH!
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The myth occurs because groundwater abstraction actually alters natural recharge and discharge rates. Groundwater systems are dynamic and react to perturbations caused by pumping. Therefore, many of the original sustainable abstraction rates were incorrect. Calculating more accurate sustainable extraction rates requires complex modelling of local groundwater systems. This is further complicated by the fact that groundwater is by definition underground… so quite hard to measure. In reality, no one really knows what the sustainable abstraction rates should be for many localities in Africa and a lot of research is currently being carried out to try to establish them. In the meantime, we can do two things: (1) acknowledge that ultimately there is an abstraction rate limit, even though we don't yet know what that is and (2) examine current groundwater depletion data to see whether this limit has already been crossed...
Is groundwater depletion already a problem in Africa?
At a global scale, groundwater depletion doesn’t seem to be a cause for concern. Global groundwater extractions are small (~1,500 km3
yr-1) (Doll et al., 2012)
compared to global groundwater recharge (~12, 600 km3 yr-1)
(Doll & Fiedler, 2008;
Doll, 2009).
Therefore the issue of groundwater depletion is often dismissed. However, global values hide
spatial variations in abstraction and renewal rates. At a local/regional scale groundwater
depletion often is a significant and
growing issue, as shown by the '3D mountains of groundwater depletion' in the map below (Aeschbach-Hertig & Gleeson, 2012). This map suggests that groundwater depletion is currently a big problem in Asia but not yet in Africa. This is hardly surprising, given that only 2% of Africa's groundwater resources are currently being accessed. However, Asia's situation is worrying because in my previous posts I’ve portrayed the extent of irrigation and use of groundwater in Asia as something that Africa should aspire towards. Yet, clearly their abstraction rates are unsustainable. Would increasing abstraction rates in Africa lead to similar problems? In all honesty, I have no idea. However, I have so far promoted small-scale groundwater pumping to support irrigation on rural smallholder plots in Africa rather than large-scale commercial operations. This is unlikely to be of a large enough scale to significantly deplete groundwater resources.
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Fig. 2 Map to show a recent estimate of the global distribution of
current groundwater depletion. The map is based on data from Wada et al (2012)
but has been remapped by Aeschbach-Hertig & Gleeson (2012) to show 3D
‘mountains of groundwater depletion’.
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What are the other options?
Groundwater is not the
only source of water available to farmers in Africa. Let's not forget about rain! My next post will investigate how we can improve rainwater harvesting.
Hi Becca - nice blog. I have been researching how climate change has the ability to impact groundwater reserves; I found that in fact climate change is expected to increase groundwater recharge and thus, generally groundwater reserves offer a possible adaptive strategy to climate change in Africa.
ReplyDeleteBut, yes groundwater reserves are actually at risk of depletion from over-extraction, which complicates this issue further. However, instead of just looking at alternative freshwater sources, will you be examining the issue of over exploitation and how this can be mitigated? Groundwater reserves are after all a greater water source (in both scale and reliability) compared to surface water supplies.
Hi Ruth - thanks for reading! Really interesting point... I didn't know that climate change may actually increase groundwater recharge!
ReplyDeleteI think you're right: groundwater is (in general) still the largest, most well distributed and most reliable source of water for African farmers. However, as I mentioned in the post, I think small-scale farmers are unlikely to significantly deplete these resources because they extract such small quantities of water. Nevertheless preventing/mitigating overexploitation is still something to bear in mind if significantly more farmers start extracting water. My last post promoted small-scale groundwater pumps that are fundamentally limited in the rate and extent of water that they can extract. I think that encouraging the use of these kinds of pumps as opposed to large-scale extraction technologies is one way to mitigate over extraction. The other main way is to increase water use efficiency. This would allow farmers to maintain yields, whilst reducing extraction rates. Use efficiency is definitely something I will be exploring later on in my blog. Finally, whilst groundwater remains, as you put it, a "greater water resource" than surface water supplies... these supplies are still important and efficient water management should make use of all resources available.