Cities across time have stretched to secure water. The Romans built aqueducts, the Mayans constructed underground storage chambers, and Hohokam farmers dug more than 500 miles of canals in what is now the U.S. Southwest.

Today’s cities use portfolios of technologies to conserve supply — everything from 60-story dams and chemicals to centrifugal pumps and special toilets. And yet, the cities of tomorrow will have to do more.