About four billion people worldwide live for at least one month with an extreme lack of water, and some 500 million people experience it year round.

According to The Washington Post, a pair of researchers from the University of Twente in the Netherlands named Mesfin Mekonnen and Arjen Hoekstra used computer models to detect levels of "blue water" around the world. They then compared the availability of the blue water to its demand.

The researchers detailed this "countercyclical" relationship in a study published in the journal Science Advances.

"Many of the water systems that keep ecosystems thriving and feed a growing human population have become stressed. Rivers, lakes and aquifers are drying up or becoming too polluted to use," the World Wildlife Fund stated on its website. "More than half the world's wetlands have disappeared. Agriculture consumes more water than any other source and wastes much of that through inefficiencies. Climate change is altering patterns of weather and water around the world, causing shortages and droughts in some areas and floods in others.

"At the current consumption rate, this situation will only get worse. By 2025, two-thirds of the world's population may face water shortages. And ecosystems around the world will suffer even more."

Blue water is underground and surface freshwater, and it is used for a variety of functions including basic household needs, The Post reported.

"High water scarcity levels appear to prevail in areas with either high population density...or the presence of much irrigated agriculture... or both," the researchers wrote in the study. "Blue water consumption and blue water availability are countercyclical, with water consumption being highest when water availability is lowest."