Four billion people live under conditions of severe water shortages at least one month of the year, according to new research.

Nearly half of these people live in India and China, according to a study by Science Advances. Other populations facing severe water scarcity live in Bangladesh, the United States (mostly in western states), Pakistan and Nigeria.

Of the total figure half a billion people experience water shortages all year round, with the highest number affected in India and Pakistan. In Saudi Arabia and Yemen year-round water scarcity affects all people in the country, putting these parts of the world in “an extremely vulnerable position”.

The findings reveal that the water scarcity is a far more severe problem than previously thought. Previous studies published in the past few years gave estimates between just 1.7 and 3.1 billion.

The discrepancies have been explained by the fact that water shortages were previously measured on an annual rather than a monthly basis, therefore hiding the water scarcity in particular places and specific parts of the year.

“More than a billion people experience severe water scarcity one to three months per year, a fact that definitely affects the people involved but gets lost in annual water scarcity evaluations,” researchers said in the Science Advances study.

“During the last few decades, it has become evident that because of a steadily increasing demand, freshwater scarcity is becoming a threat to sustainable development of human society."

Meeting humanity’s increasing demand for freshwater while also protecting ecosystems will be “one of the most difficult and important challenges of this century,” warned the study.