Humanity’s water footprint

On 11 February 1942, three days after they’d landed on Singapore’s weakly defended northern shores, Japanese forces under General Tomoyuki Yamashita seized control of the island’s reservoirs of drinking water. The position of the defending British-led forces became precarious. Four days later, British General Arthur Percival and 80 000 British, Indian, and Australian soldiers surrendered in the largest capitulation in British military history.

Singapore’s modern leaders have not forgotten the strategic importance of a secure water supply. When I visited the island nation two years ago, I toured a Siemens lab whose focus was developing membrane technologies for recycling wastewater. NEWater, as the reprocessed sewage is appealingly named, now accounts for 30% of Singapore’s consumption. Thanks to desalinated seawater (10% of the supply) and other measures, Singapore has reduced its dependence on water imported from neighboring Malaysia from 50% of the total to 40%.

Arjen Hoekstra of the University of Twente in the Netherlands is also aware of water’s strategic importance, but on a global scale. He and his collaborators have developed the concept of a water footprint—that is, the amount of water a country consumes to sustain its population and economy.

What makes Hoekstra’s work so interesting—and, I believe, important—is that he doesn’t just tally countries’ domestic sources and sinks of water. Raising beef cattle consumes a lot of water. The global average, according to Hoekstra , is 15 500 liters of water per kilogram of beef. So if the inhabitants of a country eat a lot of imported beef, then its water footprint rises thanks to the corresponding virtual inflow of water.

In the latest of a series of papers, Hoekstra and his colleague Mesfin Mekonnen take a detailed look at the water footprints of countries whose population exceeds 5 million. They identify three kinds of freshwater: “blue” (drawn from lakes, rivers and other natural sources), “green” (rainfall), and gray (polluted water). Farmers in California’s San Fernando Valley, say, use more blue water than green water when they irrigate their crops. Semiconductor companies in Silicon Valley produce gray water when they make computer chips.

Hoekstra and Mekonnen estimate that the world’s annual average water footprint for the 1996–2005 was 9087 giga cubic meters per year, of which 74% was green, 11% blue, and 15% gray. By far the biggest contributor to the world’s water footprint was agriculture, at 92%. Industrial production and domestic water supply made up 4.4% and 3.6%, respectively.

As you might expect, the countries with the largest population have the largest water footprints. China, India, and the US have footprints of 1207, 1182 and 1053 Gm³/y, respectively. What’s less obvious is that the total trade in virtual water, at 2320 Gm³/y, amounts to a quarter of the total world footprint. Some countries that have big agricultural sectors are net exporters of virtual water. Net importers tend to be either rich and industrialized or poor and dry.

The huge size of virtual flows has several strategic implications. China is currently a net exporter of virtual water, but its modest per capita footprint of 1089 m³/y is 87% of the value for the UK, but it’s only 38% of the US figure. If China’s economy comes to resemble America’s more than Britain’s, the impact on global flows of virtual water could be significant.

The US, which is simultaneously the world’s biggest exporter and importer, runs a large net export surplus. Given the droughts that have gripped Texas and other states, continuing that surplus might be unwise. Now, thanks to Hoekstra and Mekonnen’s work, we can determine how unwise.