The Persian Qanat

Most of Iran is high desert. Annual rainfall on the central plateau rarely exceeds 200 mm, yet for three thousand years cities of fifty- and hundred-thousand have flourished there — Yazd, Kerman, Nain, Kashan, Isfahan. The qanat is the reason. Iranian engineers discovered that a tunnel driven horizontally into a mountain aquifer would carry water entirely by gravity to a distant town, losing nothing to evaporation on the desert surface.

The earliest written record is the Assyrian king Sargon II's account of capturing the Urartian city of Ulhu in 714 BCE, where he found "a hidden river led from the depths of the mountains to fertilise the fields". By the Achaemenid period the technology was imperial policy: Polybius reports that any Persian who brought new land under qanat irrigation enjoyed five generations of tax relief.

"The Persians, when they ruled Asia, granted the produce of the land they irrigated by underground channels to its discoverers and their descendants for five generations."

At the start of the 20th century Iran had an estimated 50,000 working qanats supplying 75 percent of the country's water. Some are tens of kilometres long; the Gonabad qanat reaches its mother well 300 metres below the surface, the deepest known. The Zarch qanat in Yazd province is over 100 km long.

The qanat was the centre of a complete desert-water ecosystem. The āb-anbār stored its water in domed underground cisterns; the bādgir wind tower cooled them by drawing desert air across the water surface; the yakhchāl ice pit froze winter qanat-water into summer ice. Together these inventions made cities like Yazd and Kerman possible — and Yazd, inscribed in 2017, became the first UNESCO World Heritage city built entirely on qanat water.