The Silk Road town of Turpan is near the hottest, driest part of China - but it's famous for its grapes. How do they get water? Starting around 100 BC, they've dug wells and linked them with underground canals to collect runoff from the nearby Tian Shan mountains and carry it to their farms!
These canals take advantage of the steep land: while the mountains are quite high, the nearby Turpan Depression is the third lowest place on the Earth, 145 meters below sea level. Having canals underground helps reduce evaporation. They provide water year-round. They clearly work: there are over 1100 wells, and the canals have a total length of over 5000 kilometers!
Underground canal systems of this sort are common in the Middle East. Such a thing is often called a qanat - an Arabic word - but in Turpan they speak of the karez system, after a closely related Persian word, which has found its way into the local Uyghur language. For more, see:
http://en.wikipedia.org/wiki/Qanat
Sylvia Volk has a great page describing how qanats are built:
First, you need a man with money to invest. Qanats are a good, sound investment: a well-made qanat can be used by many families for many, many years. Two or three good qanats will supply all the irrigation water for a small village. And the man who builds the qanat owns the water that runs through it; the family who owns a qanat will be collecting revenue for several generations to come.
Next, you need a water-finding expert. The process by which they locate a water-source is apparently a jealously-guarded secret. But what is needed is a good well, about three hundred feet deep, with enough water that the bottom will accumulate a depth of two meters overnight. It has to be uphill of the area to be irrigated; that's very important. If it's not uphill, it's useless.
Surveyors follow the water-finding expert, and dig their well. This well is called 'the mother well'. The surveyors measure its depth with a rope. They calculate where the qanat will come out . . . and this must be on lower ground (naturally!) feasible for farming. (Most of the Asian desert is feasible for farming - just as long as you can get water to it.)
But this is how the surveyors find the spot where water will flow out: they set a pole upright, some distance downhill from the mother well. And stretch a string taut - from the well, to the pole. The string has to be leveled, and this they do by dripping water on it, and adjusting the tilt of the string until this water runs both ways (!!). Then they note the spot where this string meets the pole, and measure the height of the pole on the rope which measures the mother well's depth. They tie off a knot on the rope to mark it. Then they set out another pole downhill, mark the height, tie another knot, etc . . . and when the whole length of the string is knotted off, they have found the level at which the water must emerge. Theoretically. It seems like a hit-or-miss prospect to anyone raised with Western engineering; but this is stone-age technology, done with stone-age tools. And it does work: the proof is in the qanats.
The last task given to these surveyors is to dig shafts every three hundred yards along the course of the qanat, referring to the knotted string to dig to the correct depth.
Next, qanat experts called muqannis are called in. Wise men that they are, they start a little downhill from the emergence level indicated by the surveyors. The surveyors' shafts guide them, but they dig their own shafts every fifty feet or so, to remove material and for ventilation. Then they dig the qanat's channel, starting at the bottom and burrowing until they come to the top, and this is the channel down which the water will run. This is the business part of the qanat, and the muqannis dig the whole thing underground. It's a tunnel linking all these shafts together.
The channel is kept reasonably straight by placing two lamps in it, behind the diggers and several yards apart from each other. Then, the diggers can glance back over their shoulders as they work; if they see the two flames superimposed on each other, they know the way is straight. Nevertheless, there are usually sharp kinks in the channel just below each well, marking errors in the direction taken by the muqannis. Remember, these diggers start at the bottom and work uphill toward the mother well. When the muqannis reach water-bearing strata, the seepage runs away down the channel behind them.
Just before the channel breaks through to it, someone has to empty the mother well; otherwise, the diggers won't be happy men. (You could lose a lot of muqannis that way.) However, once the channel breaks through, the muqannis are finished. If need be, they brace the qanat walls with oval bricks. They leave the original shafts open, so that they can climb down into the qanats and clean them if necessary; for muqannis also hire themselves out as qanat maintenance men.
And there you have it: a working qanat. A great boon to the community. It will be honored with a name of its own, for qanats are always given names. On the surface, it will be visible from an airplane: it will be a line of perfectly round green spots running across barren ground, and these green spots are lush grass growing around each ventilation shaft. The shafts themselves are usually capped with wooden covers, but obviously the moisture escapes. The channels of the qanat will be about four feet wide, and muqannis will walk them regularly, cleaning them as sewers are cleaned - and ducking, for bats with eight-inch wingspans live in the qanat tunnels and fly in and out at every exit. At the qanat's outlet, canals will run in every direction, carrying the water to fields and gardens and houses. A single qanat in Iran usually run two miles or more, and irrigates an area two miles square.
http://www.iras.ucalgary.ca/~volk/sylvia/qanat.htm
#silkroad #sciencesunday
These canals take advantage of the steep land: while the mountains are quite high, the nearby Turpan Depression is the third lowest place on the Earth, 145 meters below sea level. Having canals underground helps reduce evaporation. They provide water year-round. They clearly work: there are over 1100 wells, and the canals have a total length of over 5000 kilometers!
Underground canal systems of this sort are common in the Middle East. Such a thing is often called a qanat - an Arabic word - but in Turpan they speak of the karez system, after a closely related Persian word, which has found its way into the local Uyghur language. For more, see:
http://en.wikipedia.org/wiki/Qanat
Sylvia Volk has a great page describing how qanats are built:
First, you need a man with money to invest. Qanats are a good, sound investment: a well-made qanat can be used by many families for many, many years. Two or three good qanats will supply all the irrigation water for a small village. And the man who builds the qanat owns the water that runs through it; the family who owns a qanat will be collecting revenue for several generations to come.
Next, you need a water-finding expert. The process by which they locate a water-source is apparently a jealously-guarded secret. But what is needed is a good well, about three hundred feet deep, with enough water that the bottom will accumulate a depth of two meters overnight. It has to be uphill of the area to be irrigated; that's very important. If it's not uphill, it's useless.
Surveyors follow the water-finding expert, and dig their well. This well is called 'the mother well'. The surveyors measure its depth with a rope. They calculate where the qanat will come out . . . and this must be on lower ground (naturally!) feasible for farming. (Most of the Asian desert is feasible for farming - just as long as you can get water to it.)
But this is how the surveyors find the spot where water will flow out: they set a pole upright, some distance downhill from the mother well. And stretch a string taut - from the well, to the pole. The string has to be leveled, and this they do by dripping water on it, and adjusting the tilt of the string until this water runs both ways (!!). Then they note the spot where this string meets the pole, and measure the height of the pole on the rope which measures the mother well's depth. They tie off a knot on the rope to mark it. Then they set out another pole downhill, mark the height, tie another knot, etc . . . and when the whole length of the string is knotted off, they have found the level at which the water must emerge. Theoretically. It seems like a hit-or-miss prospect to anyone raised with Western engineering; but this is stone-age technology, done with stone-age tools. And it does work: the proof is in the qanats.
The last task given to these surveyors is to dig shafts every three hundred yards along the course of the qanat, referring to the knotted string to dig to the correct depth.
Next, qanat experts called muqannis are called in. Wise men that they are, they start a little downhill from the emergence level indicated by the surveyors. The surveyors' shafts guide them, but they dig their own shafts every fifty feet or so, to remove material and for ventilation. Then they dig the qanat's channel, starting at the bottom and burrowing until they come to the top, and this is the channel down which the water will run. This is the business part of the qanat, and the muqannis dig the whole thing underground. It's a tunnel linking all these shafts together.
The channel is kept reasonably straight by placing two lamps in it, behind the diggers and several yards apart from each other. Then, the diggers can glance back over their shoulders as they work; if they see the two flames superimposed on each other, they know the way is straight. Nevertheless, there are usually sharp kinks in the channel just below each well, marking errors in the direction taken by the muqannis. Remember, these diggers start at the bottom and work uphill toward the mother well. When the muqannis reach water-bearing strata, the seepage runs away down the channel behind them.
Just before the channel breaks through to it, someone has to empty the mother well; otherwise, the diggers won't be happy men. (You could lose a lot of muqannis that way.) However, once the channel breaks through, the muqannis are finished. If need be, they brace the qanat walls with oval bricks. They leave the original shafts open, so that they can climb down into the qanats and clean them if necessary; for muqannis also hire themselves out as qanat maintenance men.
And there you have it: a working qanat. A great boon to the community. It will be honored with a name of its own, for qanats are always given names. On the surface, it will be visible from an airplane: it will be a line of perfectly round green spots running across barren ground, and these green spots are lush grass growing around each ventilation shaft. The shafts themselves are usually capped with wooden covers, but obviously the moisture escapes. The channels of the qanat will be about four feet wide, and muqannis will walk them regularly, cleaning them as sewers are cleaned - and ducking, for bats with eight-inch wingspans live in the qanat tunnels and fly in and out at every exit. At the qanat's outlet, canals will run in every direction, carrying the water to fields and gardens and houses. A single qanat in Iran usually run two miles or more, and irrigates an area two miles square.
http://www.iras.ucalgary.ca/~volk/sylvia/qanat.htm
#silkroad #sciencesunday
That looks interesting, but how does it work?
Is it actually somewhere in use?
Is it an old/ancient technique or a new developement?
Further pointers or links appreciated! :)Feb 18, 2013
Did you read my article? I said: in Turpan this system is in use now, but it goes back to 100 BC. The diagram shows how it works, and my article explains how these qanats are built. For more try the references I already gave:
http://en.wikipedia.org/wiki/Qanat
http://www.iras.ucalgary.ca/~volk/sylvia/qanat.htm
but also this:
http://www.waterhistory.org/histories/qanats/Feb 18, 2013- +John Baez I'm sorry, I was on my phone and g+ only showed me the picture without any additional info!
Now on my computer, with a web browser I can see it in detail. Of course I'll read your article. :)Feb 18, 2013 - Okay, now I understand! If you get interested in this stuff, there's quite a lot to read online by searching under 'qanat'.Feb 18, 2013
- Thanks! I'll definitely take a closer look. I like such simple, yet ingenious solutions.Feb 18, 2013
- Then you'll like how people combine a qanat with a 'windcatcher' to get a cooling system that works well in a desert and doesn't use any power:
http://math.ucr.edu/home/baez/diary/Wind-Tower-and-Qanat-Cooling.jpg
http://en.wikipedia.org/wiki/WindcatcherFeb 18, 2013 - That's pretty cool!
Not only that they used the venturi effect centuries before Venturi formalized it, nowadays they are constructing buildings like this again:
http://en.wikipedia.org/wiki/Passive_house
If I ever get to buy me a house I hope it'll feature this system for cooling/heating.Feb 19, 2013
