Water on an urban planet: urbanization, drinking water, and source watershed conservation

Background/Question/Methods

Urban growth is increasing the demand for freshwater resources, yet surprisingly the water sources of the world’s large cities have never been globally assessed, hampering efforts to assess the distribution and causes of urban water stress. I led the first global survey of the water sources of large cities (population > 750,000), surveying the 50 largest cities and a representative sample of more than a hundred other large cities. I then integrated our survey results with global hydrologic models to make statistical estimates of water stress for all large cities on Earth, paying particular attention to the financial and geographical limitations that leave some cities stuck with water stressed sources. Finally, I used ecosystem service models to quantify the value of forest and other natural habitats to maintaining raw water quality of urban water sources, comparing the output of ecosystem services models with actual operations and maintenance (O+M) and capital costs of water treatment plants.

Results/Conclusions

Previous global hydrologic models that ignored urban water infrastructure significantly overestimated urban water stress. Large cities obtain 78±3% of their water from surface sources, some of which are far away: cumulatively, large cities moved 504 billion liters a day a distance of 27,000±3800 km, and the upstream contributing area of urban water sources is 41% of the global land surface. Despite this infrastructure, one in four cities, containing $4.8±0.7 trillion in economic activity, remain water stressed due to geographical and financial limitations. The strategic management of these cities’ water sources is therefore important for the future of the global economy. Cities vary greatly in their dependence on the natural world for the maintenance of raw water quality, depending on the ecological processes of the natural habitat, the hydrology of the basin, and the treatment technologies in use at the water treatment plant. We show that the maintenance of natural habitat is empirically correlated with better raw water quality and reduced O+M and capital costs.