Have you ever wondered how cities in the deserts get water? Maybe you have or haven’t but I sure have, and during my trip to Iran last spring I finally got an answer. Large parts of Iran are semi-arid or arid deserts, and these areas receive very little rainfall and there are also very few sources of surface water. Instead, people in these areas have had to rely on groundwater that comes from the surrounding mountains. In order to access the groundwater a technology needed to be developed, and the result is an elegant network of underground aqueducts, the qanat system.
The qanat technology originated in Iran nearly 3000 years ago, and over time the technology has spread to many other arid countries. These qanats have not only made human settlements in the deserts possible by providing water for agriculture, they have also made sure that water is not overexploited. Unlike wells which are vertical and can be dug deeper and deeper, the qanats are horizontal and can only access groundwater at a certain level. The nature of the qanat thus allows the natural groundwater level to recharge every year, ensuring that water is available for the future generations.
How the qanat is built
The water of a qanat comes from a natural aquifer located deep in the ground at the side of a mountain. The qanat is basically a horizontal underground tunnel dug into the aquifer. If the water level of the aquifer drops, the qanat will become dry until the aquifer is recharged again through the natural hydrological cycle. The qanat tunnel has a very shallow slope, around 0.5%. If the slope is too steep the water will erode the bottom of the tunnel, and if it is too shallow then the water might not flow and there will be more sedimentation which causes extra maintenance work. Since the qanat is underground, there is much less evaporation of water than if the water channel was above ground.
The first step in qanat building is to dig a deep well, called the mother well, that reaches the aquifer and the water table which is the depth where groundwater is located. Finding a good spot requires some trial and error, because the well needs to be built at a location where the water table is at a height that allows for a good slope from the well to the desired outlet of the qanat. Next the exact outlet point of the qanat is determined and this depends on where the water is required and on the resulting slope from the mother well. After this the construction of the tunnel begins, starting from the outlet which is at ground level. The tunnel is dug horizontally, and every 20-50 meters a vertical shaft well is dug, and these shaft wells are used to remove the earth from the dig and also to ventilate the tunnel. Later the shaft wells can be used to access the qanat for cleaning and maintenance.
The tunnels were dug by hand and are just large enough to fit a person. Building a qanat requires a lot of hard labour and time and can also be dangerous. While the shortest qanats are just a few hundred meters, the longest once are nearly 100 kilometers. The longest qanats took decades to build and they were done with a long-term perspective in mind. The qanat builders knew that once the qanat was finished it would have a very long lifespan and would help generations of people.
Access to water
The main driving force behind qanat building traditionally was that it provided irrigation water for agriculture. The outlet of the qanat was thus located close to farmlands, and the farms were located at a lower elevation than the outlet so that water could be diverted to farms using ditches and gravity. Water use from the outlet was well monitored so that each farmer received the same share of water. The ditches that were above ground were shaded with trees in order to keep the water cooler and to prevent evaporation.
Qanats also provided water for household use and drinking. Before reaching the outlet, the qanat can pass through several cities, and inside cities the qanat is divided into several branches. There were public underground access points to the qanat in each part of the city, but some houses also had their own access point in the basement. Thanks to the cooling effect of the water and the underground location, these basements were very cool during summer and they provided a place to escape from the heat. Sometimes the qanats were used together with windcatchers to provide cooling for the whole house. Cities also had water reservoirs that ensured that there would be water even during the dry season.
Qanats require cleaning and maintenance to keep the water flowing, but today many qanats are being neglected because of social and technological changes. Over the last few decades qanats have been rapidly replaced by deep wells. Wells are quicker to build and it is also much easier to extend the well to dig further and further into the groundwater, making it easier to take water for granted and to exploit it. By making the wells deeper and deeper, more water is used than what the natural hydrological cycle can replenish. This is causing the groundwater level to drop which is not only alarming because it increases water insecurity, but it can also intensify desertification. The resulting lower water table has also caused a lot of qanats to dry up.
And yet there are still thousands, even tens of thousands, of active qanats in Iran and elsewhere. With the help of modern technology qanats could easily be economically competitive with wells. Wells may be quicker to build but they also have a shorter lifespan. Qanats also save a lot of energy since the water is transported with the help of gravity and doesn’t need to be pumped. Qanats could potentially also be used to generate energy, and watermills connected to qanats have been traditionally used to grind wheat. As water levels in many parts of the world are dropping due to overexploitation and climate change, a revival of qanats could provide a more sustainable alternative that assures that human life in these dry regions of the world can continue.