Taras Grescoe at the Long Now:
The principle behind the qanat is simple. A horizontal tunnel, up to ten feet in height and four feet in width, is dug by hand through bedrock or subsoil, until it reaches a vertical “mother well,” which has been sunk into an aquifer. At regular intervals, smaller well shafts are dug from the surface to provide light and ventilation. Though the site has to be chosen with care, a qanat can be dug anywhere the water table is higher than the farms or settlements to be irrigated. Water from the saturated soil naturally collects in the tunnel. The slope is engineered with exacting care: if it’s too great, the flowing water will erode the tunnel floor; if the gradient is too slight, sediment will build up, blocking the flow. Built correctly, though, a well-maintained qanat can provide a reliable stream of fresh water for centuries.
“Qanats flow continuously yet do not diminish the aquifer,” notes Lightfoot. “They offer a low-cost, practical solution to the problems of water supply, especially in arid regions with high evaporation rates, because water in a qanat is channeled underground and evaporation is much reduced compared to water flowing in surface canals.”
more here.
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