For a place where nature didn’t intend lettuce to grow, the southwest corner of Arizona has built a spectacular record as “America’s salad bowl.” Thanks to copious irrigation and decades of public investment, Yuma and the bordering Imperial Valley of California supply as much as 90 percent of the nation’s salad greens during the winter, making the area pivotal to the debate over the future of American agriculture in an era of oppressive weather made worse by the changing climate.