Today more than half the world's inhabitants live in cities, meaning that nearly 3.6 billion people live on less than 1% of the total global land area. Meeting the needs of these highly condensed populations is a major challenge, particularly when it comes to water. But some cities are more vulnerable than others.

In order to investigate urban water supply vulnerability, Julie Padowski now at Washington State University, US, and Steven Gorelick from Stanford University, US, gathered hydrological data on cities that don't have access to groundwater, and rely solely on surface water (rivers or reservoirs). All of the cities had populations of more than 750,000.

Of the 70 cities in the study, located in 39 countries, the researchers found that 36% (25 cities) could be considered to have vulnerable water supplies in 2010. Cities were defined as “vulnerable” if they were unable to provide 4600 litres of water per person per day and had a demand-to-supply ratio of greater than 40%.

“These cities are scattered throughout the world, with several in the US, China and India,” explained Padowski, whose findings are published in Environmental Research Letters (ERL). “In general they tend to lack supply diversity, and are subject to seasonally-limited river flow and reservoir storage limitations.”

Using urban growth projections the researchers also investigated how water supply vulnerability would change in the future. They showed that 44% of cities in their study (a total of 31) would fall into the vulnerable category by 2040. The six cities becoming vulnerable over this time period are Dublin, Ireland; Charlotte, US; Ouagadougou, Burkina Faso; and Guangzhou, Wuhan and Nanjing in China. This is without taking climate change into account, which could exacerbate the problem further.

Most of the cities classified as vulnerable were reliant on a single type of water supply, usually reservoirs. And some of the cities had significant population growth projected by 2040, as well as increasing agricultural needs. So how can these cities reduce their vulnerability? Building new reservoirs, or modifying existing ones is one option, but better still would be to incorporate other sources of water, such as groundwater or desalination. In addition, raising public awareness of the problem, encouraging water conservation and adjusting water pricing are all potential tools to help reduce water scarcity.

But if cities fail to manage their water needs they may have to further tap water sources being used by others. “An extreme example of this type of reallocation would be when a city uses or diverts the entire flow of a river, drying up the subsequent downstream portions,” said Padowski. “The consequence can be severe damage to the environment dependent on that flow.” In the very worst case scenario, cities may have to start relying on expensive water delivered by tanker trucks, as happened in the Indian city of Chennai in response to a severe drought in 2004 to 2005. For the cities on the vulnerable list it is time to start planning ahead.

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