Wednesday, November 2, 2011
The Qanat - Persian Water Systems of Old
Construction was basically by hand with a few basic tools - plumb bobs, leather buckets, shovels, hatchets, ropes, reels and lights. The first step was to find the water source - usually associated with (upgradient from) an alluvial fan meeting the mountain or foothill. A test well was dug, which if successful, became the mother well. This information on water supply elevation, yield and quality were needed to design the rest of the system. The main channel could not be too flat (reduced yield) or too steep (erosional problems). A typical gradient would be 1 foot in 1000 feet of distance, but the longer qanats would require less gradient than that. In some cases original qanats were expanded with side channels. They were at times re-routed to provide cooling for ice houses and residences. Even retrofitted with power devices. They were community systems in every respect of the word.
The vertical shafts were multipurposed. They provided air, a transport avenue for the cuttings and maintenance access over time. They were typically spaced every 100 feet or so, but this distance would vary depending on the depth of the main channel and the material being excavated. Typically work began at the bottom (where the water was to be delivered) and worked back to the mother well, but in the longer systems, work could be started from both ends toward the middle. One needed very accurate plumb bobs in these cases.
The typical qanat would be on the order of two miles or so in length, but some have been measured to be 35 miles long. The shafts would range from 50 feet to in some cases 600 feet deep. Quite a construction job in that day and time. In fact, typically making 100 to 150 feet per day on the main channel, many qanats took years to build.
In antiquity, the job was hired out to specialized workers called muqannis, who not only built the systems, but maintained them too. These were important jobs - usually handed down from father to son - which paid reasonably well. It was also a dangerous job. Air quality and cave-ins were the main hazards. The entire community was responsible for the qanat's O&M - whether that be a do-it-yourself job or hire the muqannis.
How many qanats are there? It's hard to say, but it is estimated that as many as 50,000 were at one time in use in Iran alone. Today, only half of those are still in use. UNESCO reports 3,000 currently in use in Oman. One of the oldest and largest qanats is in the Iranian city of Gonabad. It is 2,700 years old and still provides water to 40,000 people - both domestic and limited irrigation. It's about 25 miles long, and the mother well at the end is just over 1,100 feet deep.
There is no question as to the importance of these early watering systems. I'm amazed at how many there were, how big they can be, how sophisticated they are and how long they have been in use. This had to be difficult work, but as has been said many times before, there is no substitute for water.