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Urban water systems are impacted by land use within their source watersheds, as it affects raw water quality and thus the costs of water treatment. However, global estimates of the effect of land cover change on urban water-treatment costs have been hampered by a lack of global information on urban source watersheds. Here, we use a unique map of the urban source watersheds for 309 large cities (population > 750,000), combined with long-term data on anthropogenic land-use change in their source watersheds and data on water-treatment costs. We show that anthropogenic activity is highly correlated with sediment and nutrient pollution levels, which is in turn highly correlated with treatment costs. Over our study period , median population density has increased by a factor of 5.4 in urban source watersheds, whereas ranching and cropland use have increased by a factor of 3.4 and 2.0, respectively. Nearly all (90%) of urban source watersheds have had some level of watershed degradation, with the average pollutant yield of urban source watersheds increasing by 40% for sediment, 47% for phosphorus, and 119% for nitrogen. We estimate the degradation of watersheds over our study period has impacted treatment costs for 29% of cities globally, with operation and maintenance costs for impacted cities increasing on average by 53 ± 5% and replacement capital costs increasing by 44 ± 14%. We discuss why this widespread degradation might be occurring, and strategies cities have used to slow natural land cover loss.ecosystem services | History Database of the Global Environment | operations and maintenance H umanity is experiencing the fastest rate of urbanization in history. Over the 20th century, the urban population increased from 220 million to 2.9 billion, and by 2050, another 3.4-billion increase is expected (1, 2). One of the most fundamental requirements of urban existence is a source of clean, sufficient water (3, 4). Urban water supply systems are often complex, drawing water from multiple locations, some surface and some groundwater, some close and some far from the city center (5, 6). Seventy-eight percent of large cities rely on surface water sources (6), and their urban supply systems create teleconnections (7,8) between source watersheds and the urban users who depend on them. The world's largest cities (>750,000 population), the focus of this paper, occupy less than 1% of the Earth's land surface (9) but their source watersheds occupy 41% of its surface (6).Natural land cover in urban source watersheds provides important ecosystem services that help maintain the water quality at the city's water source, so-called raw water that will then be treated and distributed to urban residents (10, 11). Natural land cover stabilizes soil, minimizing erosion and sediment loading (12, 13). Natural land cover also has lower loading than most human land-uses of excess nutrients, such as nitrogen (N) and phosphorus (P), and other pollutants (14, 15). When humans convert natural land cover to other uses such as agriculture or hous...

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Governments, development banks, corporations, and nonprofits are increasingly considering the potential contribution of watershed conservation activities to secure clean water for cities and to reduce flood risk. These organizations, however, often lack decision-relevant, initial screening information across multiple cities to identify which specific city-watershed combinations present not only water-related risks but also potentially attractive opportunities for mitigation via natural infrastructure approaches. To address this need, this paper presents a novel methodology for a continental assessment of the potential for watershed conservation activities to improve surface drinking water quality and mitigate riverine and stormwater flood risks in 70 major cities across Latin America. We used publicly available geospatial data to analyze 887 associated watersheds. Water quality metrics assessed the potential for agricultural practices, afforestation, riparian buffers, and forest conservation to mitigate sediment and phosphorus loads. Flood reduction metrics analyzed the role of increasing infiltration, restoring riparian wetlands, and reducing connected impervious surface to mitigate riverine and stormwater floods for exposed urban populations. Cities were then categorized based on relative opportunity potential to reduce identified risks through watershed conservation activities. We find high opportunities for watershed activities to mitigate at least one of the risks in 42 cities, potentially benefiting 96 million people or around 60% of the urbanites living in the 70 largest cities in Latin America. We estimate water quality could be improved for 72 million people in 27 cities, riverine flood risk mitigated for 5 million people in 13 cities, and stormwater flooding mitigated for 44 million people in 14 cities. We identified five cities with the potential to simultaneously enhance water quality and mitigate flood risks, and in contrast, six cities where conservation efforts are unlikely to meaningfully mitigate either risk. Institutions investing in natural infrastructure to improve water security in Latin America can maximize their impact by focusing on specific watershed conservation activities either for cleaner drinking water or flood mitigation in cities identified in our analysis where these interventions are most likely to reduce risk.