“We find that 4 billion people live in areas that experience severe water scarcity at least part of the year, which is more than previously thought, based on those earlier studies done on an annual basis,” says Hoekstra, who published the work in Science Advances Friday. “You have to look really month by month, in order to get the scarcity.”

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Those prior studies had given totals of about 1.7 to 3.1 billion, rather than the current 4 billion. The new total includes 120 million people living in the United States, principally in California as well as other western states.

Water demand is increasing across the globe as populations and agriculture expand, but the study says that there is enough total global water for all needs. However, the problem is that there isn’t always enough in every place where it is needed, when it is needed.

“High water scarcity levels appear to prevail in areas with either high population density…or the presence of much irrigated agriculture…or both,” write the authors. Indeed, they also note that in key parts of the world — including the Ganges and Limpopo river basins — “blue water consumption and blue water availability are countercyclical, with water consumption being highest when water availability is lowest.”

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By far the leading source of human water demand is agriculture, says Hoekstra, followed by businesses. Individual human homes tend to require the least water overall, about 1 to 4 percent of the total, he says.

In the study, conditions of severe water scarcity were determined to exist in an area when there was twice as much water demand as there was availability.

It’s important to stress, Hoekstra says, that water scarcity does not mean humans will suddenly go without drinking water. First, facing conditions of scarcity, it is possible to withdraw more water than is ultimately sustainable from rivers, lakes, or groundwater systems. That’s precisely what happened, the study notes, in the Aral Sea in Central Asia (pictured above), which the paper calls “the most prominent example of a disappearing lake as a result of reduced river inflow.”

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Moreover, if severe water shortages occur, the main impact is usually upon agricultural systems and farmers, since they need the most water to begin with.

“You can never say that people have no drinking water because of water scarcity,” says Hoekstra. “It’s really about limited water to agriculture, so it impacts farmers, and food security.” Thus, what the research really underscores is which regions may face food crises in the future if they get particularly low amounts of rainfall or enter into an extended period of drought.

In such situations, what matters next is relative wealth or poverty. Rich societies can buy their food and their water from elsewhere. Poor ones are in a totally different situation.

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“If it’s a poor country, then the country will be really in trouble, and people will have no food,” Hoekstra says.