Maria Celia Navarro smiles wistfully as she remembers the now-unthinkable luxury of taking a shower in her home before a water shortage struck one of Mexico’s wealthiest cities. Nestled in mountains a few hours’ drive from the US border, industrial powerhouse Monterrey boasts living standards that many Mexicans could only dream of. Residents of the northern city, whose metropolitan area is home to around five million people, have in general been spared the chronic lack of services that plagues many poorer areas of the country. But for several weeks, a heat wave and dearth of rain means that Mo…