The number of large cities reporting water stress is smaller than previously thought, according to a new study published in the journal Global Environmental Change. Still, one in four of the 200 cities surveyed was classified as experiencing water stress.

The term water-stressed is used to denote cities that use at least 40 percent of their available water supplies on an annual basis. Earlier estimates suggested that about 40 percent of all cities fell in that category, according to The Nature Conservancy.

Tokyo, Delhi, Mexico City, Shanghai, Beijing, Los Angeles, Rio de Janeiro and Moscow are among the cities that face the biggest water stress. The study pointed out that the actual number of cities reporting water stress was not as large as expected because previous estimates did not take into account water from more distant watersheds reaching cities through pipes.

But the issue is set to become more pressing over the coming decades, as projected population growth is expected to push up demand for water. According to the Huffington Post, some 2 billion people are expected to move into cities in the next 30 years.

The study's findings show that cities representing about 5 percent of global GDP are under water stress. Overall, these cities account for $4.2 trillion in economic activity — money that directly or indirectly depends on the successful delivery of 167 billion liters of water per day, the study noted.