When Reina Cervantes Trejo heard the truck, gears grinding as it climbed the street to her house, she rushed outside.

Ubiquitous water tankers are a lifeline as the 22 million people in this metropolitan area wait for rain and a little relief.

Water sources are depleted around the world," said Victoria Beard, a professor of city and regional planning at Cornell University. “Every year, more cities will face ‘Day Zero,’ with no water in their piped systems.”