Owing to increased urbanisation, 19 of the 29 largest cities in the world depend on evaporation from surrounding lands for more than one-third of their water supply, finds a study. Urbanisation has taken billions of people from the rural countryside to urban centres, adding pressure to existing water resources. Many cities rely on renewable freshwater regularly refilled by precipitation, rather than groundwater or desalinated water. The dependence on this water supply was found to be higher in dry years. The study focussed on how moisture recycling is linked to a city’s water supply. Moisture recycling occurs when water evaporates from the land and rises up into the atmosphere. This moisture then flows along prevailing wind currents through the atmosphere, falling out as precipitation elsewhere, the researchers said.
Cities that are most dependent on this type of recycling include Karachi, in Pakistan, and three cities in China: Shanghai, Wuhan and Chongqing. At the opposite end of the scale, the research team found the cities with the least vulnerable moisture recycling include Cairo, in Egypt; Paris, in France; Sao Paulo, in Brazil; and Chicago, in the US.
“A lot of these cities have complex and significant management processes for water resources and supplies,” said Pat Keys, researcher at the Colorado State University in the US. “Cities and countries have limited resources… 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,” Keys added
For the study, published in the journal PLOS ONE, the researchers evaluated the sources of municipal water for 29 cities representing more than 450 million people around the world, and found that most of these cities relied on surface water. The team then used a moisture-tracking model to calculate the precipitation-shed — a watershed of the sky that identifies the origin of precipitation falling in a given region — for these sources of surface water.
The results showed that cities and countries have limited water resources and, importantly, very few of the cities will shrink in size and more “megacities” will be added to the list. The findings have implications for land managers and policymakers who oversee urban water security, the researchers said.