Most current and nearly all future population growth in the developing world will occur in cities. As cities become more populated, they will need more water. Today, many cities are "water stressed"—defined as currently using at least 40 percent of available water.
However, the news is not all grim. A new study, the first to accurately estimate global urban water sources and water stress in large cities, has found that past analyses substantially overestimated water stress
The study was led by Rob McDonald, senior scientist with the Nature Conservancy, and included a team of researchers from nine institutions, including Mark R. Montgomery of the Population Council. The scientists surveyed and mapped the water sources of more than 500 large cities (over 750,000 population) worldwide. The researchers used multiple computer models of global water distribution to estimate water use and water stress based on population and types of industry for each city.
The researchers found that water-stressed large cities exist in both developed and developing countries. The top ten most populous water-stressed cities are: Tokyo, Delhi, Mexico City, Shanghai, Beijing, Kolkata, Karachi, Los Angeles, Rio de Janeiro, and Moscow.
"Interestingly, many cities are not as water stressed as earlier estimates suggested. Previously, experts thought that around 40 percent of cities were water-stressed, but somehow large cities have found ways of getting access to much of the water they need: we found that only around 25 percent lack sufficient water," says Montgomery. "Nevertheless, we should not become complacent. It’s likely that in poor countries many of the smaller cities are severely water-stressed. We couldn’t find enough data on small-city water sources to evaluate their needs. It might be that smaller cities facing difficulties with water supply never grow to become larger cities. We need to know a lot more about conditions in these smaller places—that’s where most urban residents live."
When estimating water stress previously, researchers looked at maps of cities and nearby watersheds, assuming cities obtained water from nearby—but in this study, researchers delved much deeper. In addition to computer models, researchers found water source information by searching online to locate city water utilities and finding utility reports listing water sources and the amount of water withdrawn. This research showed that many cities have built infrastructure—pipes and distribution systems—that allows them to import water from distant watersheds. Past analyses substantially over-estimated water stress, at least for the bigger cities, because they ignored this infrastructure.
"Cities, like deep rooted plants, can reach quite a long distance to acquire the water they need," says McDonald.
The study also explored the extent to which financial and water resources depend upon each other. Cities with adequate financial resources can pipe in water from great distances or invest in technologies such as desalinization. However, many of the world’s fastest growing cities are in poor developing countries, which cannot afford such strategies. To develop the prosperity needed to build this infrastructure, cities need vibrant economies. But without adequate water, it is difficult for economies to thrive.
"It’s a vicious cycle that water-stressed cities in less-developed countries will find difficult to escape without significant international aid and investment," said Montgomery.
McDonald, Robert I., Katherine Weber, Julie Padowski, Martina Florke, Christof Schneider, Pamela Green, Thomas Gleeson, Stephanie Eckman, Bernhard Lehner, Deborah Balk, Timothy Boucher, Gunther Grill, and Mark R. Montgomery. 2014. “Water on an urban planet: Urbanization and the reach of urban water infrastructure,” Global Environmental Change 27: 96–105.
US National Science Foundation