Phoenix gathers its water from several places. It relies on melting snow in the north to feed the rivers that supply its water system: the Salt and the Verde, which begin and end in Arizona, and the overstretched Colorado, which slices the Southwest. It pumps from aquifers, strained by development over time, and then works to replenish them whenever water is in surplus, which happens occasionally.

To irrigate its many golf courses, it reuses most of the water drained from bathroom faucets and washing machines. It uses treated wastewater to cool a nuclear power generating station and to feed a man-made wetland complex known as Tres Rios, home to more than 150 species of birds.

A system of canals crisscrosses the city and stretches beyond its boundaries, a legacy of the prehistoric Hohokam Indians that allowed fertile farms to flourish in the desert. To this day, half of all the water used in the Sun Corridor, the area from Phoenix to Tucson, goes to agriculture, according to a 2011 report by the Morrison Institute for Public Policy at Arizona State University. Steadily, though, much of the farmland has given way to development.

Figuring out how water will be used here is like solving a puzzle speckled with blank pieces, in which the unknowns are the housing market and climate change.

Water managers weigh wet and dry cycles over the past 100 years against climate change models designed in the previous year and demographic projections. They also analyze the way parcels of land are zoned to make assumptions about how water will be used.