The Qanat System of Iran and the Maghreb
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Environmental Science. Please check back later for the full article.
A qanat, a kind of subterranean and subhorizontal tunnel, is usually excavated in soft sediments and conducts groundwater to the surface at its emerging point. In addition to the tunnel, each qanat contains several to hundreds of vertical wells for removal of dig materials and ventilation of the tunnel. The depth of these wells increases towards the last one, which is the mother well. According to the literature, the qanat was invented around 800 to 1,000 years bce, northwest of Iran and, afterwards, was utilized in many other countries in Asia, Africa, southern Europe and even in America. The areas utilizing the qanat have three characteristics in common: the shortage of surficial water (streams), indicating an arid or semiarid climate; suitable topographical slopes that help to conduct groundwater to the surface for a distance by a gently sloping tunnel (qanat); and the presence of unconsolidated sediments (usually alluvial), which act as reservoirs and as diggable material using primitive tools. In other words, dry areas with mountain-plain topography and alluvial fans and dry stream beds are conducive to qanats. Major parts of Iran and some parts of the Maghreb have such conditions. This is why these two regions have been somewhat dependent on qanats as a means of water supply. Although the invention of qanats helped human settlement and welfare in parts of dry countries, it had some negative impacts. First, the presence of humans due to qanats directly affected the wildlife and vegetation cover of those areas. Second, in some cases, changes in the groundwater regime caused wilting and drying because of already limited water resources for plants and wildlife. It seems that qanats had three major stages of development in Iran and the Maghreb, as well as in many other countries containing them. The first stage supposedly encompasses about 1,000 to 2,000 years after invention and includes the rapid spread and increase in their number due to the transfer of technology to new areas. During the second stage, the spread of qanats was halted, as they had been constructed in almost all suitable areas. This second stage continued until about 100 years ago when, as a result of the relative merits of wells, costliness of qanat maintenance, and some other factors, qanats gradually were abandoned and dried up. Thus, in the last (third) stage, the number of active qanats and their role as a kind of groundwater harvest decreased. This stage continues at varying rates in different countries. Unfortunately, the rate of decrease in Iran, the home country of qanats, is more than many other places, mainly because of mismanagement, decreasing from 38,000 qanats in 1966 to 18,000 in 2010.