Human lives and activities in certain Asian and North American watersheds could face extreme risk during the next prolonged drought or heatwave.

Yue Qin at the University of California, Irvine, and her colleagues estimated, for the period from 1980 to 2016, how much water evaporated from reservoirs and how much was used to grow crops, cool power plants and quench the thirst of people and animals.

Their calculations show that nearly one-fifth of the world’s population lives in river basins affected by severe water stress. These regions, such as the San Joaquin basin in California and the Tigris–Euphrates river basin in Asia, account for 31% of the world’s total water consumption and are likely to suffer the most from climate change.

The study suggests that regions where fresh water is already scarce could avert the worst effects of shortages by taking measures such as relocating crops and switching to less water-intensive technologies to produce electricity.