New study provides first global look of Earth's water-stressed cities

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ARLINGTON, VA, June 5, 2014 -- According to recent statistics, a number of cities throughout the globe are experiencing increased water stress and looking for additional water supplies to support their continued growth, as more people move to urban areas.

The first global database of urban water sources and stress, published in Global Environmental Change on Tuesday, June 3, estimates that cities move 504 billion liters of water -- enough to fill 200,000 Olympic swimming pools -- a distance of 27,000 kilometers every day. While large cities only occupy 1 percent of the Earth's land surface, their source watersheds cover 41 percent of that surface, so the raw water quality of large cities depends on the land-use in this much larger area.

Accordingly, a team of scientists working at the Socio-environmental Synthesis Center, led by Rob McDonald, senior scientist with the Nature Conservancy, surveyed and mapped the water sources of more than 500 cities globally, providing the first global look at the water infrastructure that serves the worlds large cities.

They 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. 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. In fact, the 20 largest inter-basin transfers in 2010 totaled over 42 billion liters of water per day -- enough to fill 16,800 Olympic size pools.

However, many cities are not as water stressed as previously thought. Earlier estimates placed approximately 40 percent of cities into the water-stressed category. This analysis has the number at 25 percent, containing almost 400 million people.

The study also makes clear the extent to which financial resources and water resources are intertwined. It 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.

"Cities, like deep rooted plants, can reach quite a long distance to acquire the water they need," said McDonald. "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 finds that the 10 largest cities under water stress are Tokyo, Delhi, Mexico City, Shanghai, Beijing, Kolkata, Karachi, Los Angeles, Rio de Janeiro, and Moscow. The study also reveals that:

  • Four in five (78 percent) urbanites in large cities, some 1.21 billion people, primarily depend on surface water sources. The remainder depend on groundwater (20 percent) or, rarely, desalination (2 percent).
  • The urban water infrastructure of large cities cumulatively supplies 668 billion liters daily. Of this, 504 billion liters daily comes from surface sources, and that water is conveyed over a total distance of 27,000 km.
  • One 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.
  • Twelve percent 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."

See also:

"Water scarcity poses risks for major U.S. cities, businesses, finds report"

"Watershed management website to be launched by Nature Conservancy, IBM"


About the Nature Conservancy

The Nature Conservancy is a leading conservation organization working around the world to conserve the lands and waters on which all life depends. The Conservancy and its more than 1 million members have protected nearly 120 million acres worldwide. For more information, visit www.nature.org.

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