Cities move 504 billion liters of water on a daily basis, enough to fill more than 200,000 Olympic-sized pools
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 today in Global Environmental Change, estimates that cities move 504 billion liters of water 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 researchers from nine institutions 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.
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 a 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 citizenry.”
The study finds that the top-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:
- 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 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.
- Land use in upstream contributing areas affects the raw water quality and quantity of surface water sources.
- We estimate that the roughly one-quarter of large cities in water stress contain $4.8 trillion of economic activity, or 22% of all global economic activity in large cities. This large amount of economic activity in large cities with insecure sources of water emphasizes the importance of sustainable management of these sources not just for the viability of individual cities but for the global economy.
“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.”
This research was supported by the National Socio-Environmental Synthesis Center (SESYNC) under funding received from the National Science Foundation DBI-1052875.
Geraldine Henrich-Koenis, The Nature Conservancy