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Iran HydrologyIran's land surface covers 165 million hectares, more than half of which is uncultivable. A total of 11.5 million hectares is under cultivation at any time, of which 3.5 million hectares were irrigated in 1987, and the rest watered by rain. Only 10 percent of the country receives adequate rainfall for agriculture; most of this area is in western Iran. The water shortage is intensified by seasonal rainfalls. The rainy season occurs between October and March, leaving the land parched for the remainder of the year. Immense seasonal variations in flow characterize Iran's rivers. 

The Karun River and other rivers passing through Khuzestan (in the southwest at the head of the Gulf) carry water during periods of maximum flow that is ten times the amount borne in dry periods. Several of the government's dam projects are on these rivers. In numerous localities, there may be no precipitation until sudden storms, accompanied by heavy rains, dump almost the entire year's rainfall in a few days. Often causing floods and local damage, the runoffs are so rapid that they cannot be used for agricultural purposes. 

Water shortages are compounded by the unequal distribution of water. Near the Caspian Sea, rainfall averages about 128 centimeters per year, but in the Central Plateau and in the lowlands to the south it seldom exceeds 10 to 12 centimeters, far below the 26 to 31 centimeters usually required for dry farming. 

Scarcity of water and of the means for making use of it has constrained agriculture since ancient times. To make use of the limited amounts of water, the Iranians centuries ago developed man-made underground water channels called qanats that were still in use in 1987. They usually are located at the foot of a mountain and are limited to land with a slope. A qanat taps water that has seeped into the ground and channels it via straight tunnels to the land surface. The qanats are designed to surface in proximity to village crops. 

The chief advantage of the qanat is that its underground location prevents most of the evaporation to which water carried in surface channels is subject. In addition, the qanat is preferable to the modern power-operated deep well because it draws upon underground water located far from the villages. The chief disadvantages of the qanat's are the costs of construction and maintenance and a lack of flexibility; the flow cannot be controlled, and water is lost when it is not being used to irrigate crops.

  In the late 1980s, an estimated 60,000 qanats were in use, and new units were still being dug (although not in western Iran, where rainfall is adequate). Qanat water is distributed in various ways: by turn, over specified periods; by division into shares; by damming; and by the opening of outlets through which the water flows to each plot of land. So important is the qanat system to the agricultural economy and so complex is the procedure for allocating water rights (which are inherited), that a large number of court cases regularly deal with adjudication of conflicting claims.

Construction of large reservoir dams since World War II has made a major contribution to water management for both irrigation and industrial purposes. Dam construction has centered in the province of Khuzestan in the southwest as a result of the configuration of its rivers flowing from the Zagros Mountains. The upper courses flow in parallel stretches before cutting through the surrounding mountains in extremely narrow gorges called tangs. The terrain in Khuzestan provides good dam sites. The first dams were built on the Karaj, Safid, and Dez rivers. 

The first of the major dams had a significant impact on the Iranian economy. Completed in 1962.

http://countrystudies.us/iran/74.htm

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