Urbanization has depleted the rural countryside of its people earlier and now evaporating its water, said a study. Many cities rely on renewable freshwater regularly refilled by precipitation, instead of depending on groundwater or desalinated water, it said.
The study led by Colorado State University researchers found that 19 of the 29 largest cities world over depend on water resources in the surrounding landscape to meet more than one-third of their requirements and it is higher during dry years.
CSU research scientist Pat Keys found how moisture recycling is linked to a city’s water supply and found cities which depend on recycling such as Karachi in Pakistan, and Shanghai, Wuhan and Chongqing in China. On the flip side, the cities with the least vulnerable moisture recycling include Cairo, Egypt; Paris, France; Sao Paulo, Brazil; and Chicago, United States.
"A lot of these cities have complex and significant management processes for water resources and supplies," said Keys, a researcher in the School of Global Environmental Sustainability at CSU. "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," he said.
Moisture recycling occurs when water evaporates from the land and rises up into the atmosphere and then it flows along prevailing wind currents through the atmosphere, falling out as precipitation elsewhere.
"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 study findings are not meant to be a cause for concern, but instead to highlight vulnerabilities that people might not have known about. "Cities and countries have limited resources," he said. "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."
In addition, very few of the cities highlighted in the study will shrink in size, and more "megacities" will be added to the list. "With climate change, and demographic and land use fluctuations, it is important to understand where vulnerabilities exist and have a full picture," he said.
The study, titled "Megacity precipitationsheds reveal tele-connected water security challenges," was published in PLOS.[category, SCIENCE]