It’s official: For the first time in human history, more people are living in cities than rural areas — a trend that’s certain to accelerate. But with most freshwater systems already stressed, how will the world’s cities be able to supply water — and water that’s clean — to a projected 1.5 billion new residents over the course of the next 20 years?

That’s one of the questions Nature Conservancy scientist Rob McDonald set out to answer in a new study published in Ambio: A Journal of the Human Environment. For the study, McDonald and his co-authors developed a geography of urban water supplies according to three major constraints: water availability, water quality, and water delivery:

  • They calculated that water availability may be an issue for more than 500 million people worldwide living in arid areas of the world of the world likely to face water shortages, like the western United States, Australia, coastal Peru and Chile, North Africa, the Middle East, and central Asia. 
  • Poor water quality may be an issue for 890 million people, particularly in cities concentrated in major river basins like India’s Ganges and China’s Yellow River.
  • Finally, a whopping 1.3 billion people may be living by 2030 in cities with water delivery problems, particularly in fast-growing cities that lack resources and infrastructure.

So what’s the solution? McDonald and his co-authors note that the most common response to these challenges is to tap groundwater, a generally unsustainable approach. Other strategies include long-distance water transport, reservoir systems, rainwater harvesting, and purchase from private water sellers.

But there are strategies to use existing supplies more wisely, says McDonald, like using treated waste water, replacing old water intrastructure, and converting agriculture to less water-intensive use.

“The world is basically adding a city the size of Washington, D.C. to the planet every week,” says McDonald. “So that’s the real challenge if you’re an urban planner and trying to get water to your citizens.”

(Image: Reflections of New York City buildings in a puddle. Image credit: ShellyS’/Flickr through a Creative Commons license.)

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