The history of our cities has been written in water.

In Mesopotamia, Egypt and the Indus valley, the first urban settlements were built around irrigated farmland. Flooded terrace fields for rice, corn and quinoa accompanied the spread of civilization in East Asia and the Americas. Without water, the megacities that will define the 21st century would wither and perish. 

That's looking like a risk factor in an increasing number of locations. Cape Town and Chennai in recent years endured punishing droughts that left them on the brink of crisis. Similar conditions afflicted the upwardly-mobile Indian cities of Bengaluru and Hyderabad last year. Now Tehran (biggest of all, with about 15 million people) is facing the same emergency.

Residents of Iran's capital are having their taps turned off for hours at a time to ration usage, amid a five-year drought and rainfall that's been running at 96 per cent below average levels. The city may have to be evacuated altogether if a current dry spell doesn't break soon, President Masoud Pezeshkian said last month. That underlines a grim fact of modern life: Even relatively affluent and sophisticated cities might be just a few dry years away from “day zero” — the point where life-giving water supplies run out.

Tehran's desperate situation has its origins in the same mania for........

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