London has been ranked 15^th amongst the world’s most water stressed cities in a new study of more than 500 cities globally.

The study, published in Global Environmental Change, providing the first global look at the water infrastructure that serves the world’s large cities - scientists have developed the first global database of urban water sources and stress by  surveying and mapping the water sources of more than 500 cities globally.

A team of scientists working at the Socio-environmental Synthesis Center, led by Rob McDonald, senior scientist with the Nature Conservancy, used computer models to estimate the water use based on population and types of industry for each city and defined water stressed cities as those using at least 40 percent of the water they have available.

As more people move to urban areas, cities around the world are experiencing increased water stress and looking for additional water supplies to support their continued grow.

The study, which estimates that cities move 504 billion litres of water a distance of 27,000 kilometers every day, says that while large cities occupy only 1 percent of global land surface, they  draw their water resources from almost half of that surface - 41%.

The study finds that the ten largest cities under water stress are in order of priority are Tokyo, Delhi, Mexico City, Shanghai, Beijing, Kolkata, Karachi, Los Angeles, Rio de Janeiro and Moscow. London comes in at 15^th behind Moscow. Some of the fastest growing cities of the 21st century are in areas with a geographical limitation in water availability

However, while earlier estimates put approximately 40 percent of cities into the water-stressed category, the analysis has the number at 25 percent, containing almost 400 million people.

Previous estimates of urban water stress were based only on the watershed in which each city was located, but many cities draw heavily on watersheds well beyond their boundaries. The 20 largest inter-basin transfers in 2010 totalled over 42 billion litres of water per day.

The study also highlights the extent to which financial resources and water resources are intertwined. It points out that while is possible for a city to build itself out of water scarcity — either by piping in water from greater and greater distances or by investing in technologies such as desalinization — but many of the fastest growing cities are also economically stressed and will find it difficult to deliver adequate water to residents without international aid and investment.

Rob McDonald commented:

“Cities, like deep rooted plants, can reach quite a long distance to acquire the water they need. However, the poorest cities find themselves in a real race to build water infrastructure to keep up with the demands of their rapidly growing population.”

The study reveals that:

• Four in five (78%) urbanites in large cities, some 1.21 billion people, primarily depend on surface water sources. The remainder depend on groundwater (20%) or, rarely, desalination (2%).
• The urban water infrastructure of large cities cumulatively supplies 668 billion litres daily. Of this, 504 billion litres daily comes from surface sources, and that water is conveyed over a total distance of 27,000 km.
• 1 in 4 large cities is water stressed, containing almost 400 million people and accounting for $4.8 trillion in economic activity—more than 5 percent of global GDP
• 12% of large cities use inter-basin transfers for their water supplies, with cities in China, Central Asia and Mexico relying on sources as far as 100 km away.
• Cities with low per-capita income (<$1,035/person) are able to reach water suppliers on average 26 km away, while cities with high per-capita income (>$12,616/person) have sources that are on average 57 km away.

“The question of where cities get their water and whether they have enough to support residents’ needs and economic growth has major policy and security implications, which are exacerbated by increasing urbanization and — potentially — climate change,” said McDonald. “Accounting for urban water infrastructure is essential for accurately estimating the urban population in water stress and finding solutions to meet the ever increasing water demands.”

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