Residents of this coastal city of 4 million have begun stockpiling bottled water and lining up at natural springs. The reason? Cape Town, a global tourist destination and South Africa’s second-largest city, may soon switch off its taps. Dams are running low in the midst of an extreme three-year drought, one that has been compounded by extended delays in new infrastructure investments. “Day Zero” — the date when officials plan to shut down the municipal supply and start dispensing rations from some 200 collection points — is now expected to arrive in June, a month after the winter rains usually begin.