Our results show the highly varied geography of urban water provisioning resulting from the overlay of water availability, water quality, and water delivery. Different issues matter in different places, with some unfortunate cities suffering from two or three of these management challenges. While perhaps obvious, an appreciation of this varied geography is important because international policy discussions often focus on only one of the three axes. For example, climate change discussions tend to focus on implications for water availability, ignoring issues of water quality and delivery which our results suggest affect more urban dwellers. Moreover, the response to an issue on one axis, like climate change’s impact on water availability, will necessarily be governed by the social and economic situation of the city, and any response will likely also affect the other two axes. We hope this article motivates a broader discussion among policymakers of the complex interplay among the three axes of urban water provision.
Regardless of which management challenge a city faces, the first response of many cities seems to be increased use of groundwater. Groundwater use reduces immediate concerns about water availability and sometimes water quality, and because groundwater pumping is cheap it is affordable in most economic contexts. Given this heavy reliance on groundwater, it is surprising that more global datasets have not augmented the World Bank’s review (Foster et al. 1998
). More information is needed about which cities rely on groundwater for their municipal supply as well as the water budgets and recharge rates of their aquifers. Without better information on groundwater use, it is hard to know how many millions of people are in cities that are essentially mining groundwater in an unsustainable fashion. If even a fraction of cities facing the availability management challenge unsustainably pump groundwater, hundreds of millions of urban dwellers may have an unsustainable source of drinking water.
A second major trend is the importance of private sector solutions where local governments cannot meet demand for clean water. While some of these solutions are complementary to public strategies (e.g., rainwater harvesting, household water treatment), others may conflict with public projects, such as illegal water withdrawals from water supply systems and the drilling of private boreholes which can lead to groundwater table declines and land subsidence.
We identified two major categories of strategies that cities can use, increasing water supply or using existing supply more wisely. The latter has the potential to be used more widely than it is currently, and it seems likely to have less of a negative impact on freshwater ecosystems, already one of the most damaged habitat types on Earth. However, the scarcity of information on the use of strategies that seek to use existing supply more wisely makes it difficult to evaluate the frequency of such strategies or their cost relative to infrastructure-based solutions. While renewed interest in environmentally friendly solutions to problems of water provision has resulted in a number of excellent case studies, most cities do not document the extent to which, for example, they are dependent for forests or wetlands for water filtration. In contrast, detailed engineering data is usually available for infrastructure that increases water supply.
Regardless of whether cities are investing in infrastructure to increase water supply or trying to use existing supplies more wisely, it is clear that substantial financial resources will be required to address these management challenges in the future. One study estimated that from 2003 to 2025 necessary annual investments would exceed $180 billion per year (World Panel on Financing Water Infrastructure 2003
). While our study cannot shed light on the cost of investment, we estimate that a significant number of people in cities are already facing water provisioning challenges, in the hundreds of millions for water availability and water delivery and more than a billion for water quality. Furthermore, our analysis of delivery challenges suggests that many cities will be unable to finance water delivery projects themselves, highlighting the need for substantial international funding. While plenty of possible solutions to water quantity and quality problems exist, including some that are relatively less harmful to the environment, they all take money and time to implement. For more than a billion people in cities facing water delivery challenges, both are in short supply.