Do human consumption habits affect groundwater depletion as a result of international food trade? A global analysis indicates that they do, and shows which products and countries have the biggest impact. See Letter p.700
Groundwater levels are being depleted at alarming rates in the world's arid and semi-arid regions, mostly because of farm irrigation that has been implemented over the past 50 years with little planning or control by governmental agencies1. Such irrigation measures have brought numerous socio-economic benefits and are generally less prone to corruption than are surface-water irrigation projects, but they have also led to ecological damage such as land subsidence, seawater intrusion into coastal areas and the loss of springs and wetlands2. These problems seem to be partly related to the international export and consumption of commodities3,4. On page 700, Dalin and colleagues5 report the first quantification of global, non-renewable groundwater depletion associated with irrigation that is embedded in the world's food trade.
The concept of virtual water trade — the water used to produce traded goods — was introduced to explain how food imports can save water in the Middle East6. The idea was that food imports allow the Middle East to use less of its own water for crop production than it would do if it grew all of its own food. More-recent studies have quantified the volumes of virtual water embedded in food trade as a way of linking food consumption and water scarcity in the producing regions3,4,7,8.
Dalin and colleagues now deepen the analysis of the virtual water trade by revealing the non-renewable depletion of groundwater that was embodied in food trade for the period from 2000 to 2010. Their research correlates the amount of irrigation water taken from non-renewable groundwater resources with the related amount of trade. The analysis is likely to influence both integrated management schemes for global water resources and the rapidly evolving research field that studies the socio-economic 'footprints' of global trade (see ref. 9, for example).
The authors show that approximately 11% of global non-renewable groundwater use was exported through agricultural trade during the period studied. Of this, about two-thirds was accounted for by the combined exports of Pakistan (29%), the United States (27%) and India (12%) (Fig. 1). The researchers also find that the vast majority of the world's population lives in countries that source nearly all of their imported staple food from partners that consume non-renewable groundwater. Some countries — such as the United States, Mexico, Iran and China — are particularly exposed to water- and food-security risks related to groundwater depletion, because they both produce and import food irrigated using water from rapidly depleting aquifers.
This work is based mainly on big data and global-scale studies. The numbers reported by Dalin and colleagues are therefore generally average values, and have a wide range of uncertainty. Groundwater evaluations per se are quite uncertain, because the properties of the atmosphere and soil are all highly variable, particularly in arid and semi-arid areas10. This uncertainty explains why results found in different reports can vary, sometimes greatly. One should therefore always be cautious when drawing conclusions from global groundwater-focused studies, especially bearing in mind that the individual uncertainties for quantities estimated in such studies combine to generate even bigger overall uncertainties.
Dalin and co-workers' analysis assumes that technological and social factors affecting groundwater use will not change. The implications of their paper — that groundwater depletion associated with global food trade is unsustainable — might therefore represent a worst-case scenario. Reality is more complex. So even though agriculture often accounts for 70% of groundwater use today2, it is difficult to predict how this will change in the next hundred years.
Global water assessments are useful in highlighting the international need to respond to groundwater depletion, but water is also a local resource that moves within specific river basins or aquifer systems. Smaller-scale studies are therefore needed to make policy-relevant decisions, because the details of local situations are key to understanding the context, drivers and influencing factors that affect groundwater stocks. Hydrogeology is one factor, but the economic, social, cultural and environmental aspects of groundwater use must also be considered10.
For example, in southeastern Spain, intensive groundwater use and mining — continuous groundwater depletion that often exceeds replenishment of supplies — to irrigate cash crops, and increasingly to supply water for tourism, have cumulatively reduced groundwater stocks by about 15 cubic kilometres (ref. 10). This has occasionally caused serious administrative, legal and environmental problems, but has also fostered huge economic and social development10. Such market-driven use of an exhaustible vital resource raises ethical concerns that point to the need for a sea change in the governance of environmental resources.
Dalin and colleagues' study identifies regions and agricultural products most at risk from groundwater depletion. It should serve as a wake-up call for nations and river-basin authorities to consider the influence of agricultural trade on non-renewable groundwater depletion, and on the related sustainability of national consumption. It should also focus the minds of the end-consumers of these products, who typically turn a blind eye to supply chains and the impacts of imported goods.
Such global assessments are a first step towards improving the sustainability of worldwide food production, because they provide fresh data and perspectives on the big picture and on the drivers of water use and abuse. A consideration of trade-related environmental concerns might also suggest new global water-governance solutions, which could be applied by introducing measures to ensure that existing food-trade frameworks of the European Single Market and the World Trade Organization are effective, sustainable and equitable. Footnote 1
Famiglietti, J. S. Nature Clim. Change 4, 945–948 (2014).
Llamas, M. R. UNESCO Ser. Water and Ethics Essay 7 (UNESCO, 2004); see http://www.rac.es/ficheros/doc/00010.pdf
Marston, L., Konar, M., Cai, X. & Troy, T. J. Proc. Natl Acad. Sci. USA 112, 8561–8566 (2015).
Mekonnen, M. M. & Hoekstra, A. Y. Hydrol. Earth Syst. Sci. 14, 1259–1276 (2010).
Dalin, C., Wada, Y., Kastener, T. & Puma, M. J. Nature 543, 700–704 (2017).
Allan, J. Virtual Water: Tackling the Threat to Our Planet's Most Precious Resource (Tauris, 2011).
Dalin, C., Konar, M., Hanasaki, N., Rinaldo, A. & Rodriguez-Iturbe, I. Proc. Natl Acad. Sci. USA 109, 5989–5994 (2012).
Hoekstra, A. Y. & Mekonnen, M. M. Proc. Natl Acad. Sci. USA 109, 3232–3237 (2012).
Zhang, Q. et al. Nature 543, 705–709 (2017).
Custodio, E. et al. Sci. Tot. Environ. 559, 302–316 (2016).
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