Safe and sustainable water in cities

The visible crisisThroughout history, most cities were established in areas with ample freshwater to meet their residents' needs.Continued city growth, exacerbated by climate change, is making urban water management harder, however.Cities are confronting unprecedented hydrologic changes due to global climate change.This affects not only the spatial and temporal availability of water but can also cause damage to existing urban water infrastructure, deteriorate surface water quality, and pose serious threats to current operating models of urban water management [1].By one estimate, between one third and half of the world's urban population will face water scarcity in the year 2050 [2].Cities' water sources are also threatened by contamination with solid waste and domestic wastewater generated by households [3].Currently, only about half of all urban wastewater flows are collected and safely treated before being discharged to the environment.A host of other contaminants, including industrial chemicals, agricultural fertilizers, microplastics, and sediments are discharged to urban water bodies through multiple contamination pathways, impairing water quality significantly more than previously estimated [4].Exposure to pollutants is highest, and access to water and sanitation services lowest, among populations who are marginalised because of income, disability, gender, religion and other characteristics.These inequities in turn lead to disparities in health, economic and social outcomes [5].Such challenges confront cities in every world region and across the entire economic spectrum.This set of PLOS Water articles-and others that will be added to the collection in coming months-helps to deepen our collective understanding of those challenges.It also provides an opportunity to learn from novel approaches and lighthouse initiatives around the world that are transforming planning, management and monitoring of both watersheds and water and sanitation service delivery systems in urban areas.From the diverse array of topics addressed by the papers' authors, a handful of themes emerged as worthy research priorities for the urban water community.