Scarcity and quality risks for future global urban water supply

Abstract Context Supply of freshwater to the world’s cities is increasingly affected by human pressures and climate change. Understanding the effects of human pressures and climate change on global urban water scarcity and quality risks in an integrated way is important. Objectives The objective of this study is to assess the scarcity and quality risks to water security for 304 large cities (population > 1 million) across the world for 2015 and 2050. Methods We assessed the water scarcity according to water demand and availability, and evaluated the quality of water supply in terms of the population density, cropland fertilization, and landscape patterns in source watersheds. In addition, the impacts of human pressures and climate change on urban water risks were quantified using contribution analysis. Results We found that about 90% of these cities faced water risks in 2015. The number of cities facing quality risk was about three times the number of cities facing scarcity risk, and nearly a quarter faced dual risks. From 2015 to 2050, 88.8–99.7% of cities were projected to face rising water risks with about one-third facing dual risks by 2050. Increase in water demand was the main cause of rising scarcity risk; growth in population and crop fertilization in source watersheds were the main reasons for rising quality risk. Conclusions There is an urgent need to promote landscape conservation of urban water source areas, implement sustainable urban water planning and governance, improve water supply infrastructure, and refine ecological compensation regimes to achieve global urban water security.