On his first visit to Melbourne in 2009, Stanley Grant, a drought expert and professor of civil and environmental engineering at the University of California, Irvine, had a question for his taxi driver. “How’s the drought?” he asked. “It’s about 28 percent,” came the reply. Grant was puzzled. But shortly afterward, they drove past an electronic road sign announcing that the city’s reservoirs were indeed at just 28 percent of capacity. The taxi driver knew the state of the reservoirs exactly.