A new study conducted by research scientists at the Colorado State University (CSU) has shown the high dependence on evaporated water by some of the world’s largest cities.

The downside of rapid urbanisation is the continuous migration of people from rural areas to urban centres, adding pressure to existing water resources. Instead of groundwater or desalinated water, many cities around the world actually rely on renewable freshwater regularly refilled by precipitation resulting from moisture recycling.

Published in PLOS ONE, the study titled ‘Megacity precipitationsheds reveal tele-connected water security challenges’ finds how 19 of the 29 largest cities in the world depend on evaporation from surrounding terrain for more than a third of their water supplies. ‘Precipitationsheds’, a term coined by the research team at CSU, refers to a watershed of the sky that identifies the origin of precipitation falling in a given region. Dependence on this source of water is higher in the dry years.

These findings have implications for land managers and policymakers responsible for urban water security.

According to CSU research scientist Pat Keys, cities most dependent on moisture recycling include Karachi in Pakistan, and Shanghai, Wuhan and Chongqing in China. At the other end of the spectrum are cities with the least vulnerable moisture recycling such as Cairo, Egypt; Paris, France; Sao Paulo, Brazil; and Chicago, United States.

Keys, a researcher in the School of Global Environmental Sustainability at CSU explains that the less vulnerable cities have significant management processes for water resources and supplies.

Chicago, which has faced water stress in the past, is today well-buffered by water management. But many megacities are unable to buffer themselves from fluctuations in climate and seasonal weather patterns.

Moisture recycling is the term used to define the process when water evaporates from the land and the resulting moisture rises up into the atmosphere, flowing along prevailing wind currents and falling out as precipitation elsewhere.

Observing that actions on land influence the whole course of the water cycle, Keys says changing the amount of water or time of evaporation can impact other places and people.

To conduct the study, CSU researchers assessed municipal water sources in 29 cities (mostly reliant on surface water), representing almost half-a-million people around the world. A moisture tracking model calculated the precipitationshed for these sources of surface water.

The study aimed to highlight the vulnerabilities in cities with limited resources, especially the added vulnerability of their water supply, and the need for these cities to mitigate these changes through solutions such as reservoirs, and treatment and desalination plants.