Hotels in India’s southern city of Chennai are rationing water for guests amid searing heat while companies limit showers as the city of 4.6 million faces its worst shortage in years. All four reservoirs that supply Chennai, known as the Detroit of south Asia for its flourishing automobile industry, have run dry this summer, largely because of poor monsoon rains last year. Chennai is one of 21 cities that a government think-tank warned last year could run out of ground water by 2020. This year’s monsoon is delayed, further compounding problems across a swath of western and central India. Employees in Chennai-based companies said they had been asked to cut back on water use in canteens and restrooms.