Most rivers in Iran are seasonal and have traditionally not been able to supply the needs of urban settlements. Major rivers like the Arvand, Aras, Zayandeh, Sefid and Atrak were few and far between in Persia.
With the growth of urban settlements during the ages, locally dug deep wells (up to 100 meters deep) could no longer keep up with the demand, leading to the systematic digging of a specialized network of canals known as Qanat.
Qanat and Kariz
Persia's Qanat system dates back many centuries, and thousands of years old. The city Zarch in central Iran has the oldest and longest Qanat (over 3000 years and 71 km long) and other 3000 years old Qanats have been found in northern Iran. The Qanats mostly came in from higher elevations, and were split into a distributing network of smaller underground canals called kariz when reaching the city. Like Qanats, these smaller canals were below ground (~20 steps), and were built such that they were very difficult to contaminate. These underground aqueducts, built thousands of years ago suffer no evaporation loss and are ideally suited for drinking water since there is no pollution danger.
But with the further growth of the city in Persian lands, even the Qanats could not respond to the needs of residents. That is when some wealthy inhabitants started building private reservoirs called ab anbar (Persian آب انبار).
In the middle of the twentieth century, it is estimated that approximately 50,000 qanats were in use in Iran, each commissioned and maintained by local users. Of these only 25,000 remain in use as of 1980.
One of the oldest and largest known qanats is in the Iranian city of Gonabad which after 2700 years still provides drinking and agricultural water to nearly 40,000 people. Its main well is more than 360 meters deep and the qanat is 45 kilometers long. Yazd, Khorasan and Kerman are the known zones for their dependence with an extensive system of qanats.
In traditional Persian architecture, a Kariz (کاریز) is a small Qanat, usually within a network inside an urban setting. Kariz is what distributes the Qanat into its final destinations.
Kariz in Persia
Qanats of Gonabad also is called kariz Kai Khosrow is one of the oldest and largest qanats in the world built between 700 BC to 500 BC. It is located at Gonabad, Razavi Khorasan Province, Iran. This property contains 427 water wells with total length of 33113 meters. This site were first added to the UNESCO's list of tentative World Heritage Sites in 2007, then officially inscribed in 2016 with several other quants under the World Heritage Site name of "The Persian Qanet".
According to Callisthenes, the Persians were using Water clock in 328 BCE to ensure a just and exact distribution of water from qanats to their shareholders for agricultural irrigation. The use of water clocks in Iran, especially in Qanats of Gonabad and kariz Zibad, dates back to 500BCE. Later they were also used to determine the exact holy days of pre-Islamic religions, such as the Nowruz, Chelah, or Yaldā – the shortest, longest, and equal-length days and nights of the years. The Water clock , or Fenjaan, was the most accurate and commonly used timekeeping device for calculating the amount or the time that a farmer must take water from Qanats of Gonabad until it was replaced by more accurate current clocks. Many of the Iranian qanats bear some characteristics which allow us to call them feat of engineering, considering the intricate techniques used in their construction. The eastern and central regions of Iran hold the most qanats due to low precipitation and lack of permanent surface streams, whereas a small number of qanats can be found in the northern and western parts which receive more rainfall and enjoy some permanent rivers. Respectively the provinces Khorasan Razavi, Southern Khorasan, Isfahan and Yazd accommodate the most qanats, but from the viewpoint of water discharge the provinces Isfahan, Khorasan Razavi, Fars and Kerman are ranked first to forth.
It is Henry Goblot who explores the genesis of qanat for the first time. He argues in his book entitled “Qanats; a Technique for Obtaining Water” the ancient Iranians made use of the water that the miners wished to get rid of it, and founded a basic system named qanat or Kariz to supply the required water to their farm lands. According to Goblot, this innovation took place in the northwest of the present Iran somewhere bordering Turkey and later was introduced to the neighboring Zagros Mountains.