It’s no secret that our planet is undergoing a serious water crisis, with population growth putting increasing stress upon the supply of fresh water. But the situation is even worse than we thought.

About 4 billion people across the world have serious difficulty getting enough water for at least one month of every year, according to a just-published study in the journal Science Advances.

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Co-authors Arjen Y. Hoekstra, a professor of water management at the University of Twente in the Netherlands, and post-doctoral researcher Mesfin Mekonnen used computer modeling to study a growing shortage that they say is being driven by population growth, improved living standards, changing consumption patterns and the expansion of irrigation in agriculture.

Unlike previous studies that looked at longer-term patterns of availability, which the researchers say underestimated the impact of water scarcity, they looked at how the supply of “blue” water -- that is, fresh surface water and groundwater -- fluctuates in the course of a year. 

The result was that in addition to areas with year-round water scarcity -- such as Somalia and northern Mexico, for example -- the researchers identified places where water becomes scarce during certain seasons. In Africa, for example, the researchers identified a band roughly between 5 and 15 degrees northern latitude with low water scarcity from May or June to January but moderate to severe water scarcity from February to April. They found other spots -- such as India and northern China -- which experience water shortages during the spring and summer.

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Severe scarcity is defined as the demand being double the supply available in an area.

The researchers also found that about 66 percent of the world’s population suffers from severe water scarcity for at least one month during the year. Of those 4 billion people, 1 billion live in India and another 900 million live in China. About 130 million live in the United States, mostly in western and southern states.

As Hoekstra told the New York Times, water scarcity isn’t just a problem for those who don’t have enough water. “Since the remaining people in the world receive part of their food from the affected areas, it involves us all,” he said.