About four billion people, or two-thirds of the world’s population, face severe water shortages during at least one month every year, far more than was previously thought, according to Arjen Y. Hoekstra, a professor of water management at the University of Twente in the Netherlands.

In a paper published Friday in the journal Science Advances, Dr. Hoekstra and his colleague Mesfin M. Mekonnen designed a computer model to create what they say is a more accurate picture of water scarcity around the world. Severe water scarcity can lead to crop failure and low crop yields, which could cause food price increases as well as famine and widespread starvation.

An area experiences severe water scarcity when its farms, industries and households consume double the amount of water available in that area.

“That means that groundwater levels are falling, lakes are drying up, less water is flowing in rivers, and water supplies for industry and farmers are threatened,” Dr. Hoekstra said in an email.