Beside the winding curves of the Colorado River, the Palo Verde Valley spreads out in a lush plain in the middle of the desert, a farming oasis filled with canals and fields of hay. For 12 years, the valley’s farmers have been participating in a program that pays them to leave some of their lands unplanted and fallow, helping to slake the thirst of Los Angeles and cities across Southern California. The arrangement has been widely praised as a model of how cities and farming areas can work together to stretch water supplies further while keeping agriculture alive.