Rapid population growth over the past 60 years has led to accelerated global urbanization. With more than half of the world's inhabitants now residing in cities, the human geography of the planet has shifted, condensing nearly 3.6 billion people into <1% of the total global land area (Schneider et al 2009, United Nations 2012). These urbanized areas represent hotspots of localized, concentrated development that place enormous pressure on natural resources, particularly freshwater supply. Globally, urban water use has increased five-fold since 1950, reflecting greater demands as more people move to urban centers and increase their per capita use, commensurate with higher standards of living (Richter et al 2013). Combined with water demands for agricultural irrigation, industrial production, and water to sustain environmental flows, water scarcity has become a global problem.

The impact of urban demands on freshwater resources not only alters the hydrologic balances in the basins that supply and receive water for urban areas, but can devastate the integrity of a watershed tapped for supply (Miltner et al 2004). An historical example is the city of Los Angeles, US, which in the early 1900s commandeered nearly the entire flow of the Owens Valley and Mono Lake basins, severely damaging the existing natural and human systems within each. More modern examples include the dramatic decline in the water quantity and quality of Lake Chapala through the early 2000s caused in part by the city of Guadalajara, Mexico drawing increasingly larger volumes of water for urban supply (Webster et al 2009). The Krishna basin in India suffers 'significant degradation of various ecosystems' due to widespread over-allocation across all sectors, including large withdrawals for the cities of Hyderbad, Pune, and water exports to Chennai (Venot 2009). On the other hand, places like New York City, US have instead promoted source watershed protection measures that leave supply basins with functional ecosystems, despite providing water to one of the largest cities in the world.

Freshwater vulnerability is only partially a reflection of growing urban demands. Although water for municipal use has become significantly more important over the past century, agricultural irrigation is by far the dominant user of water around the world (Gleick 1993). Since the 1950s, the volume of water used for irrigation purposes has nearly tripled (Molden 2007). However, historical trends suggest that as urbanization intensifies, a larger portion of available freshwater resources will be channeled to the urban sector (Oki and Kanae 2006). The options for closing the growing urban water deficit are building non-traditional, expensive sources (e.g., recycling, desalination), developing reallocation schemes, such as urban-rural partnerships (Gober 2010, Richter et al 2013), or by reducing water allocated for environmental flows (Rogers 1993).

Until recently, urban impacts on supply vulnerability were either performed for individual cities, regionally, e.g., in the US (Averyt et al 2013, Padowski and Jawitz 2012), or by assuming the basins within which urban areas reside are those supporting urban water demands (Jenerette and Larsen 2006, McDonald et al 2011). Currently, only the City Water Map Initiative has presented a global assessment of urban water stress that directly incorporates information about urban water sources (McDonald et al 2014).

The study presented here examines baseline and future vulnerability scenarios of cities under normal climate conditions and quantifies the magnitude of reallocation needed to reduce future vulnerability. To accomplish this, our global analysis evaluates the vulnerability of surface water supply basins serving large cities that lack source diversity, relying on either direct river withdrawals or releases from reservoir storage and asks: (1) which large urban areas are most vulnerable to demands for freshwater now and in the near future? (2) Of those vulnerable large cities, what is the magnitude of water reallocation needed to assure urban freshwater supplies?