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...
