Many large cities depend on evaporation for their water supply

Researchers at Colorado State University have found that most of the world’s largest cities depend on evaporation from surrounding regions for more than 33 percent of their water supply. In dry years, these cities depend even more on surface water.

The researchers determined that the following cities are the most dependent on moisture recycling: Karachi, Pakistan, Shanghai, Wuhan, and Chongqing. The experts also identified the cities which are the least dependent on moisture recycling including Cairo, Paris, Sao Paulo, and Chicago.

“A lot of these cities have complex and significant management processes for water resources and supplies,” said study co-author Pat Keys.

“Cities like Chicago have experienced water stress in the past, but they are well-buffered by water management. On the other hand, many megacities are not able to buffer themselves from fluctuations in climate and seasonal weather patterns, such as Lagos in Nigeria, or Rio de Janeiro in Brazil.”

When water evaporates from the land and rises up into the atmosphere, the moisture is transported by the wind and falls as precipitation in a nearby region.

“What you do on the land influences that whole branch of the water cycle,” said Keys. “If you plant a forest or cropland where there used to be a shrubland or desert, it probably won’t last without substantial irrigation. If you change the amount of water or change when it is evaporated and flows up into the atmosphere, that can have impacts for other places and people.”

The research team analyzed the sources of water for 29 major cities representing more than 450 million people across the globe. They used a moisture tracking model to estimate how much the cities depended on surface water, and found that 19 out of the 29 cities were significantly dependent on precipitation from local evaporation.

The study authors hope that the findings of this research will make people more aware, especially considering that most of the cities evaluated in the study will continue to grow in size.

“Cities and countries have limited resources,” said Keys. “If I were in one of those highly vulnerable cities, I’d want to look at this additional dimension of vulnerability for the water supply.”

“With climate change, and demographic and land use fluctuations, it is important to understand where vulnerabilities exist and have a full picture.”

The study is published in the journal PLOS ONE.

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By Chrissy Sexton, Earth.com Staff Writer

Image Credit: Patrick W. Keys/Colorado State University