Humanity uses a lot of fresh water, but we didn't realize just how much until now. And surprisingly, a lot of the water that we seemingly consume is instead lost through evaporation.
In a study recently published in the journal Science, researchers calculated that humans use up to 18 percent more water than previous estimates. That works out to about 10,700 cubic kilometers annually, more than the combined volume of Lakes Michigan, Huron, Ontario and Erie combined.
The study's authors, geographer Fernando Jaramillo and hydrologist Gia Destouni of Stockholm University, based their finding on analysis of 100 large hydrological basins –such as drainage areas around rivers - from 1901 to 2008. They found that more and more water has been evaporating, in large part due to human activities such as irrigation and building dams on rivers to create reservoirs.
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To scientists, water consumption includes any water taken from its source by humans –whether they drink it, use it to irrigate crops, or allow it to evaporate.
Creating reservoirs can increase evaporation, because it spreads standing water over a large surface area. Irrigation also can increase evaporation, because it leads to more plants in an area. Those plants draw in water and then release it into the air through their leaves, as a Washington Post article on the research notes.
Previous research also has shown water use increasing dramatically during the 20th Century, and found that more water evaporates from reservoirs than is purposefully consumed by humans for industrial or domestic purposes.
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Water that evaporates eventually returns to Earth at some point as precipitation. But it doesn't necessarily come down in the same location or amount, which can create shortages in supply.
Destouni told the Post that tracking the world's water use is crucial to making more efficient use of the resource. According to the United Nations, about 1.2 billion people live in parts of the world where water is scarce.