Author(s): Patrick W. Keys 1,2,*, Lan Wang-Erlandsson 2,3,4, Line J. Gordon 2
Urban environments are home to the most rapidly growing human populations on the planet . Water is just as important in cities as it is elsewhere, but the concentration of people within cities makes the sustainable supply of clean freshwater a particularly urgent priority for science and policy . Globally, cities rely on a wide range of water sources, including groundwater, lakes, reservoirs, and in some cases desalinated ocean water . Research on urban water supplies often focuses on how surface and groundwater can be managed. However, water supply is also affected by precipitation systems including the evaporation from land uses far away from cities . This connection in the atmospheric water cycle, where evaporation from one location travels through the atmosphere to fall as precipitation in another location, is called moisture recycling.
Urban water only comes from a few different types of sources. Surface water generally comes from either direct rain runoff, or from snow or glacial melt, and is overwhelmingly the most common source of water for urban areas globally . Groundwater can come either in the form of fossil aquifers or actively recharged aquifers . Desalination, though not as widely used as surface or groundwater, is concentrated in arid areas, particularly in the oil-exporting nations of the Arabian Gulf . The conventional spatial unit for surface water is the watershed, and a watershed is delineated by topography, with water flowing downhill to the lowest point, most commonly the ocean . Likewise, the surface watershed is also an appropriate boundary for considering the water that is actively recharging groundwater aquifers that are near the surface .
The water that forms this surface runoff in the watershed originates as precipitation (i.e., rain, drizzle, snow, sleet, graupel, or hail). Previous work enables the analysis of the sources of the precipitation that falls in a given location, including in a watershed [9,10]. Advances in global modeling enable the tracking of atmospheric moisture flowing around the planet, and furthermore can identify the specific locations where moisture enters the atmosphere as evaporation, and where it falls out as precipitation. In many parts of the world, evaporation from land later returns to land as precipitation. On average, 40% of precipitation on land originates from evaporation that came from land, but this can be substantially higher in some regions and during certain seasons .
In an effort to link the management of surface water with the sources of precipitation for a given location (e.g. a watershed), Keys et al.  introduced the concept of the precipitationshed, which defines a spatial boundary enclosing upwind evaporative sources of downwind precipitation (Fig 1). In the precipitationshed, a precipitation 'sink region' receives precipitation from upwind 'source regions', as well as from within the sink itself (Fig 1). Other work has found that precipitationsheds are spatially consistent among years, though the magnitude of contribution from different parts of the precipitationshed does fluctuate .
Fig 1. Conceptual diagram...