Professor Tony Allan, the British geographer who developed the influential concept of “virtual water” or “hidden water,” died on April 15, 2021, at the age of 84. He first introduced the concept in 1993 and received the Stockholm Water Prize in 2008 for this work. [source]
In one of his important papers, Allan argued that water-scarce regions such as the Middle East could reduce pressure on domestic water supplies through the strategic import of water-intensive products such as wheat and rice from countries with abundant water resources. The article connects this idea with the economic principle of using comparative advantages—including what it calls hydrological advantages—while noting that tariffs and restrictions mean global food trade is not completely free and virtual-water trade is consequently more complicated in practice. [source] [source]
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| Image from the original article |
What is virtual water?
The hidden water of a product—whether food, a manufactured good, energy, or a service—is the total volume of water used directly and indirectly through the stages of producing and delivering it. For chicken meat, for example, this includes water associated with the bird's feed, growing the grain it eats, slaughter and processing equipment, washing, and other steps.
In other words, virtual water represents the water consumed throughout a product's production chain. The source gives an average of approximately 1,300 litres of water for one kilogram of wheat. “Virtual” does not mean imaginary: most of the water used during production is simply no longer physically contained in the final product. It is real water that has already been consumed.
The article estimates the lifetime virtual-water requirement of raising an 800–1,000 kg bull at roughly 12–15 million litres. For comparison, it gives about 6 million litres as the virtual water associated with constructing one residential unit.
Several illustrative product estimates are then provided, with the source itself cautioning that the numbers may not be exact:
- One 200 mL glass of milk: about 200 litres
- One 125 mL cup of coffee: about 140 litres
- One 100 g apple: about 70 litres
- One 100 g potato: about 25 litres
- One medium cotton shirt: about 4,100 litres
- One A4 sheet of paper: about 10 litres
- One pair of cow-leather shoes: about 8,000 litres
- One 250 g hamburger: about 2,400 litres
- One 500 g block of cheese: about 2,500 litres
Trading water without moving water itself
A map reproduced by the original article compares global virtual-water trade patterns in 1986 and 2011. Major virtual-water exporters are shown in purple and major net importers in red. The source states that most economies are net importers of virtual water and gives a range of roughly 160–210 national economies classified as major net importers in the underlying material.
Major exporters—including countries in North America, Latin America, and Australia—tend to be located in hydrologically productive regions and also possess infrastructure that supports trade. Arrows in the map show global virtual-water flows and highlight the roles of countries such as the United States, Australia, and Brazil in international food trade.
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| Image from the original article |
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| Image from the original article |
A second diagram compares virtual-water flows in international food trade in 1986 and 2007, measured in billions of cubic metres. Continents are colour-coded according to the origin of the exported virtual water. As one example, the source reports that Asian imports of virtual water from South America rose from roughly 8 billion cubic metres in 1986 to more than 100 billion cubic metres in 2007.
Agriculture and non-renewable groundwater
Agriculture accounts for the largest share of global water consumption. Rising food demand has led to overuse of water resources in many important agricultural regions, including substantial use of non-renewable groundwater for irrigation. Depletion or declining levels in aquifers are particularly concerning because groundwater can provide an important reserve during droughts, which the article notes may become more frequent under climate change.
The source discusses a 2019 study by Carole Dalin, Makoto Taniguchi, and Timothy Green examining global groundwater depletion associated with agricultural activity. The study estimated the use of strategic, non-renewable groundwater for irrigation from 1960 to 2000.
The researchers noted that global food trade increasingly encourages countries to produce agricultural exports without necessarily aligning production with their own climatic and natural-resource potential. Water is distributed very unevenly around the planet: the Middle East is relatively water-scarce, while regions of Latin America, North America, and parts of Africa possess much greater water resources. Water management may be primarily domestic, but international agricultural trade links distant producing regions directly to consumers elsewhere.
Groundwater embedded in international food trade
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| Image from the original article |
Another diagram, drawn from the Dalin and colleagues study, shows groundwater withdrawals embodied in international crop trade around 2010. In effect, it depicts virtual water originating from aquifers that countries used in producing agricultural goods for international trade.
The source highlights an important finding: countries including Mexico, Iran, and Saudi Arabia appeared both among significant exporters of crops irrigated with groundwater and among major importers of foreign products whose production also relied on unsustainable groundwater withdrawals. These countries may therefore face food-supply risks through both domestic production and imported supply chains.
The article further states that Iran was among roughly 11 major exporters of virtual water derived from groundwater, despite the country's low precipitation and its uneven distribution in space and time.
The concept of virtual water therefore makes otherwise hidden connections visible: a food product crossing a border also represents water previously consumed in its place of production, and international food choices can shift water demand—and water risk—from one region to another.