By summer, four million people in the city of Cape Town—one of Africa's most affluent metropolises—may have to stand in line surrounded by armed guards to collect rations of the region's most precious commodity: drinking water.

Population growth and a record drought, perhaps exacerbated by climate change, is sparking one of the world's most dramatic urban water crises, as South African leaders warn that residents are increasingly likely to face "Day Zero." That's the day, previously projected for mid-April but now mid-July, when the city says it will be forced to shut off taps to homes and businesses because reservoirs have gotten perilously low—a possibility officials now consider almost inevitable.

"The question that dominates my waking hours now is: When