Hidden waters

UNESCO Courier, Jan, 1994 by Daniel Balland

For centuries arid land in many parts of the world has been made fertile by tapping underground mountain water through tunnels--often several kilometres long--that are masterpieces of hydraulic engineering

The world's biggest and oldest oases are, to adapt Herodotus's celebrated phrase about Egypt, the "gifts" of great rivers such as the Nile, the Tigris, the Euphrates, the Helmand, the Indus, the Amu Darya and the Syr Darya, the Tarim and others. However, it is in the extraction of "hidden" water from underground aquifers that oasis-dwellers have shown their genius for hydraulic engineering. An extraordinary range of techniques have been used for this purpose. Most of them share a common characteristic: the use of an external energy-source to raise the water from the aquifer to the fields that need irrigating.

There is, however, a remarkable exception to this rule. The drainage tunnels known as qanats in Arabic and karez in Persian are driven into hillsides virtually horizontally, with just enough slope to allow water tapped from an underground source to flow off into the open air by the force of gravity alone. A regular 0.1 or 0.2% gradient (which is to say an incline of a metre or two per kilometre) is about right: less than that, and the slope encourages alluviation, so the tunnels need to be cleaned out more often; more, and the flow of water causes excessive erosion.

Each tunnel divides into a tapping section penetrating into the aquifer and a downstream section that carries the water out to the fields. The amount of water extracted, and therefore the surface area irrigated, depends on the length of the former. The tunnel must be big enough for a man to move about and work in. Typically, it may be 1.3 metres high by 80 centimetres wide. Lengths vary greatly, from a few hundred metres to several dozen kilometres; the average seems to be about three kilometres.

On the surface the only sign that there is a tunnel is a line of air-vents sunk every 20 metres or so. These vertical shafts, which provided access when the tunnel was being dug, are used subsequently for maintenance work. The further up the hillside they are, the deeper they are. Shafts several dozen metres deep are common; some may be far deeper than that. Each one is surrounded by a ring of rubble, like some giant molehill pierced in the middle by the vent itself.

At the point where the tunnel emerges into the open air, there is always something magical about the sight of fresh water flowing naturally all the year round into a pool or irrigation channel. Women and children come to fetch unpolluted water, and the menfolk like to gather here after the day's work. This is a focal point of social life in the oasis.

Some small oases are irrigated by a single tunnel, but more commonly several--sometimes even several dozen--form an organized network. In some cases the system is added on to a network of channels diverted from a river. When building an underground infrastructure of this type, whether simple or complex, specialists have to decide on three essential matters: the location and depth of the aquifer that is to be tapped; the area to be irrigated; and the direction and slope of the tunnel needed to connect the two. Building the tunnel is a long and dangerous job. It is only really feasible when labour is cheap and plentiful, typically under some form of agrarian capitalism based on slave-owning or a feudal system. Most of the tunnels currently in use are, therefore, survivals from the past, living fossils that are kept in working order with varying degrees of success. Clearing them out is a relatively straightforward job, but rebuilding them when they collapse or lengthening them to compensate for a lowering of the water table are difficult and expensive operations.

It is estimated that some 30,000 tunnels are today in use in different parts of the world. Placed end to end, they would stretch for more than 100,000 kilometres, or over two and a half times round the Earth.

Oriental satrapies and New World colonies

Although some scholars have tried hard to prove that this irrigation system originated in their own part of the world, it seems to have spread as a result of a fairly straightforward process of diffusion. The most comprehensive treatise on the art of digging a karez, the Kitab inbat al-miyah al-Khafiyya, or "Book of the Extraction of Hidden Waters", was written in Arabic in about 1019 A.D. by Mohammed al-Karadji. As his name suggests, the author came from Karadj, a town that once stood on the slopes of the Zagros Mountains, 100 kilometres southeast of Hamadan. Now Hamadan, as it happens, is the ancient Ecbatana, imperial capital of the Medes and summer residence of the first Achaemenids--in other words, the centre of the very region that was the cradle and principal diffusion point of this irrigation technique.

The construction of karez in fact originated in the kingdom of Urartu, in the district surrounding Lake Urmia, early in the last millennium of the pre-Christian era. It probably developed as a simple dewatering technique designed to run off seepage threatening to inundate mines that penetrated a subterranean aquifer. The genius of the people of Urartu lay in transforming it into a technique to tap water for irrigation.