Large cities occupy only 1 percent of global land surface, but draw water from almost half of that surface
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.
In fact, the first global database of urban water sources and stress, published recently in Global Environmental Change, 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. Laid end to end, all those canals and pipes would stretch halfway around the world. While large cities only occupy 1% of the Earth's land surface, their source watersheds cover 41% of that surface, so the raw water quality of large cities depends on the land-use in this much larger area.
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 world’s 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 water to fill 16,800 Olympic size pools.
There’s good news here. Many cities are not as water stressed as previously thought. Earlier estimates put 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,” says 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 ten 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:
“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.”
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.
Through its Securing Water Program, the Conservancy is helping cities provide safer, more sustainable and reliable water services through investments in natural infrastructure and sound resource management. This includes the creation of more than 20 Water Funds in 7 countries, which transfer funding from downstream water users to upstream communities to pay for watershed conservation and protection and create governance platforms that develop joint visions and plans for watershed investments.
SOURCE: The Nature Conservancy