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These efforts have shrunk Albuquerque's domestic per capita water use from 140 gallons a day to around 80. The city "anticipates another 50 years of water, economically and sustainably supplied, even with a growing population," says Yuhas. After that there's the option to desalinate brackish water nearby and new technologies such as dual plumbing: one set of pipes to deliver highly treated potable water and another to recycle less treated water for flushing toilets, watering lawns, and other nonpotable uses. Albuquerque already uses wastewater—from treatment plants and from industry—to irrigate golf courses and parks. Other municipalities have gone a step further and collect wastewater—yes, from toilets—filter and disinfect it to the nth degree, then pump it back into the local aquifer for drinking. There are similar schemes worldwide: Beijing reportedly aims to reuse 100 percent of its wastewater by 2013.

Industry, too, is adapting to less certain water supplies. Frito-Lay will soon recycle almost all its water at its plant in Casa Grande, Arizona; Gatorade and Coca-Cola remove the dust and carton lint from beverage containers using air instead of water; and Google recycles its own water to cool its giant data centers.

This is all reassuring—until you remember that irrigated agriculture accounts for 70 percent of the fresh water used by humans. Given this outsize proportion, it seems obvious that farmers have the greatest potential to conserve water.

Standing on the banks of a trickling ditch, Don Bustos—sunbaked and thickly bearded—demonstrates how he irrigates 130,000 dollars' worth of produce on 3.5 acres north of Santa Fe. "I lift this board"—he points to a plank that forms a gate in the ditch—"and I shove in a stick to hold it up." Gravity does the rest.

For 400 years farmers in the arid Southwest have relied on such acequias—networks of community-operated ditches—to irrigate their crops. The acequia diverts water from a main stream, then further apportions the flow through sluiceways into smaller streams and onto fields. "Without the acequia, there would be no farm," Bustos says. He's also built a water tank with drip-irrigation hoses that feed some of the acequia water directly to the plant roots—and cut his water use by two-thirds as a consequence.

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