Tehran’s climatic condition has no significant negative impact on the City’s development and its inhabitants. The contrast between mountain and desert, and even between north and south of the City, prevents weather crises from taking shape in any part of the year. When heavy snow obstructs mountain roads, plains in the rim of the desert (Dasht-e Kavir) have often a mild sunny weather. When it is too hot in the desert, one can refuge in the mountain just a few kilometers away. The significant change of weather in different seasons is ultimately pleasing and allows a great variety of activities, either agricultural or recreational, to take place.
Tehran region has a dry climate. Unlike many other cities of the Middle-East, however, Tehran does not have an acute shortage of water supply. Although the annual rate of rainfall is only 229mm at the downtown, 400mm in northern neighborhoods and less then 150mm in arable lands of Varâmin, heavy rainfalls in Alborz mountains, mainly as snow, provide a fairly reliable source of water supply for Tehran’s residents and the City’s development.
Urban development has often relied on access to water sources. The wealthy have built their houses in higher elevations of the City in order to enjoy a cleaner and more copious water supply. This water flows through a number of rivers and flood canals toward the desert in the south and is swallowed up by the thick layers of alluvium. The traditional methods provide 20% of water mainly used for irrigating gardens, but the real source of Tehran’s water supply is a system of large dams built by Tehran’s Water Organization over Karaj River in 1956, over Jajrud river in 1968 and over Lâr river in 1981. There is also a new dam under construction over Tâleqân River.
Southern slopes of Alborz are dry (less than 500mm of rainfall in a year), while mountain tops and northern slopes are very humid and have an annual rainfall of more than 2000mm, mostly as snow. Thus, rivers are fed for a long time. Each valley of the southern slope defines the down stream town associated with it. Tehran, however, is an exception since no river passes through the City other than Kan floodway which is almost always dry. Eastward, Hableh rud River (with 225 million cubic meter of water per year) flows down Firuzkuh mountains and drains into Garmsâr plain and the desert. This large and deep valley has always been an important route connecting the Iranian plateau and the Caspian sea, as evidenced by remnants belonging to Timurid, Safavid and Qajâr eras. The old road of Safavid era is still being used by Iran’s railroad to pass over the Alborz Mountains, between Garmsâr and Sâri. Further west, is the small river of Kilân that irrigates Eyvânekey plain which is often dry in late summer.
Jâjrud (“the Furious River”, with 295/000/000m3 of water per year) flows down from Shemshak mountains, passes beyond Hezardarreh hills, east of Tehran, and irrigates the great arable plain of Varâmin. Since 1970, most of its flow has been deviated toward Tehran to fill the water reservoir behind Latyân dam, causing the level of underground water sources in arable plains to drop. To solve this problem, Tehran’s sewage system is directed toward these lands.
Westward, Karaj river has the most copious flow in this region (535/000/000m3 of water per year). A diversion canal was built in 1860 to irrigate the gardens of northern Tehran in the Qajâr era. This canal, known as “âb-e Karaj” is still there in now Keshâvarz Blvd. Amir Kabir Dam is the first modern dam constructed in Iran over this river. Down stream arable lands of Karaj, between Shahriâr and Eslâmshahr, is one of the largest and richest agricultural zones of the Metropolis. Sometimes, floods of Karaj River reach the Grand Salt Lake (Daryâcheh-e Namak) in the desert, north of Qom.
In the west of Tehran, Savojbolâgh plain, between Karaj and Qazvin is not irrigated by any major river and only the small Kordân river flows through it. Taleqân and Alamut rivers flow through mountains westward and join to from Shâhrud river, which joins with Qezel Uzen river to create Sefidrud River that irrigates Gilân’s plain, on the coasts of the Caspian sea. In Taleqân, in place of a small diversion dam built in 1970 to change the flow toward Qazvin’s plain, a big dam is now being constructed, to become operational in 2007. These new water resources provide for the development needs of Tehran Metropolis for at least the next two decades.
The main problem of Tehran in this regard is filtration of sewage. Since 1991, the project of constructing the main canals of the sewerage system for collection of the surface water has accelerated, although more work remains to be done. Tehran’s underground water tables are often contaminated by cesspools that exist in all buildings. Thus far, abundance and high quality of Tehran’s water supply had obviated the need for major investment in purification and recycling of water. Today, however, the only way to make sure that this metropolis can have a sustainable development is to think about water treatment and recycling.