Countries shown in green exported more virtual water than they imported from 1995 to 2005. Yellow and red countries were net importers. (Virtual water is water consumed in producing crops or industrial goods.) Credit: Hoekstra and Mekonnen/PNAS 2012
Water runs downhill from mountaintops to streams to rivers to oceans. But downhill isn’t the only way that water moves. A new study measures how water travels from country to country for human consumption. This flow isn’t the type we usually think about. These scientists looked at the water used to grow and make the products that get shipped from nation to nation as imports or exports. They call this a flow of “virtual water.”
We typically think about water as the liquid that flows from a faucet. However, nearly all — 92 percent — of the water used by people goes into growing crops, according to water researcher Arjen Hoekstra at the University of Twente in the Netherlands. He and his team recently studied the hidden travels of virtual water used in products made from things like crops and meats. These products are shipped around the world.
For example, consider a sugary soft drink. Hoekstra and his team estimated that to produce one half-liter of the beverage requires between 170 and 310 liters of water. Only a tiny amount of that — about 1 percent — is the water actually used in the soda. The vast majority of the water — about 95 percent — is used to grow and process the ingredients. Another 4 percent goes into the packaging and labeling, say the scientists. (The exact amount of water required depends on the sweetener being used.) In Hoekstra’s calculation, when one country produces a half-liter of soda and sells it abroad, it exports as much virtual water as would fill a large refrigerator.
According to Hoekstra’s new report, dry countries like Israel and Kuwait, both in the Middle East, get the majority of their virtual water from other countries, through imported products. More surprisingly, some wetter countries, like the Netherlands and the United Kingdom, also get the majority of their virtual water from other places. That means that most of the water used to grow or produce the products and food consumed in those countries came from other countries.
In the United States, most of the virtual water used comes from American sources; only about 20 percent of the virtual water comes from outside national boundaries. In China, even less of the water associated with its products — about 10 percent — came from foreign countries.
Tracing the water trade, Hoekstra and his team turned up other surprising relationships. For instance, often an arid region sends virtual water to a wetter region. Dry areas in northern China, for example, send virtual water to the southern part of that country, which is wetter.
“There are exporting regions that do have a lot of water stress,” he told Science News.
POWER WORDS (adapted from the New Oxford American Dictionary)
arid Having little or no rain; too dry or barren to support vegetation.
agriculture The science or practice of farming, including cultivation of the soil for the growing of crops and the rearing of animals to provide food, wool and other products
