MEXICO CITY — As the end of the four-month rainy season approaches here in Mexico City, it has finally begun to rain. But the daily downpours, which have overwhelmed the city’s drainage network and flooded subway stations, arrived too late. The Cutzamala reservoir system was depleted this spring. Mexico is enduring its worst drought in six decades. Crops are drying up in the fields and water is being rationed in the capital. Residents of poor neighborhoods have hijacked water trucks, ...