Colorado State University has published a new study that shows exactly how urban centers are able to substantiate some of the water for their populations. As it turns out, the water comes from the areas directly surrounding such cities — the very places that most of these population centers draw their people, the rural countryside.
According to the study, urbanization — which has taken billions of people from the rural countryside to urban centers — means many of our cities rely on renewable freshwater regularly refilled by precipitation, rather than groundwater or desalinated water. In other words, evaporation is one of the most substantial sources of water movement throughout the globe as the primary out for urban water supply.
"Nineteen of the 29 largest cities in the world depend on evaporation from surrounding lands for more than one-third of their water supplies," the study's authors noted. "Researchers also found that the dependence on this water supply is higher in dry years, obviously. The findings have implications for land managers and policymakers who oversee urban water security."
The university's research scientist, Pat Keys, is part of a team that had previously coined the term "precipitationsheds," a watershed of the sky that identifies the origin of precipitation falling in a given region. The new study, "Megacity precipitationsheds reveal tele-connected water security challenges," was recently published in PLOS One.
The following graphic, provided by the university, shows the draw of water for a typical urban center:
A key finding suggests that moisture recycling is linked to a city's water supply, Keys said in the report.
Those cities that are most dependent on this type of recycling include Karachi, Pakistan; as well as Shanghai, Wuhan and Chongqing — all in China. Cities with the least vulnerable moisture recycling include Cairo, Egypt; Paris, France; Sao Paulo, Brazil; and Chicago.
Per the study, the effect is caused by evaporation from the surrounding lands that rises up into the atmosphere, with this moisture then "flowing along prevailing wind currents through the atmosphere, falling out as precipitation elsewhere." In other words, a pretty typical, well-understood scientific evaporation — precipitation cycle.
"A lot of these cities have complex and significant management processes for water resources and supplies," said Keys, a member of the School of Global Environmental Sustainability at Colorado State. "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."
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 precipitationshed for these sources of surface water.
"What you do on the land influences that whole branch of the water cycle," Keys said. "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."
In addition, few of the cities highlighted in the study will shrink in size, and more "megacities" will be added to the list. Researchers did not explore climate change as part of the study, which would make an additional difference.
"With climate change, and demographic and land use fluctuations, it is important to understand where vulnerabilities exist and have a full picture," Keys said.
Will the study do anything to result in any specific change? Not really. There's nothing that can be done, and the researchers admit as much. But the study does illustrate an interesting effect of modern urban living on the ecosystem.
