The qanat water reservoir is an underground structure, constructed to store freshwater for domestic use. The reservoir was fed from a nearby shallow qanat. All of the water reservoirs had a storage tank whose dimensions depended on the amount of qanat discharge and the demand for water. Most of the storage tanks were made of "sarooj". The different parts of a water reservoir were the storage tank, the roof of the storage tank, wind tower, stairway and ornamental portal.
The storage tank is of variable dimensions and its plan can be square, octagonal or a circle. The whole body of the water reservoir, or the most parts of it is constructed under the ground.
There were one or more faucets in the reservoir wall at 0.5 - 1.5 meters from the bottom. These faucets were used to transfer water from the storage tank to the "pa shir" area, where people took the water through these faucets. The reasons why the faucets were set above the base are: to fill people's containers with water easily; to prevent the impurities deposited at the bottom of the storage tank, getting out of the faucets; and to have a high discharge because of the hydrostatic head in the tank.
Figure 1- Water reservoir in Yazd, Iran
The water storage tank's roof can be flat, but most of them are dome-shaped or conic. The conic roofs are two types; flat or step wise.
The wind tower or "badgir" is one of the traditional structures, locatedin the water reservoir compound. Badgirs cause the air to circulate in the water storage area, evaporating the water and making it cool. The number of badgirs in a water reservoir ranges from 1 to 6.
One had to go down a stairway to reach a landing named "pa shir" to tap the water. The number of steps depended on water reservoir's depth. The pa shir was constructed at the same level as the bottom of the water storage tank, or a little lower. There was a drain at the bottom of the pashir to discharge the waste water to a canal named "rah ab" and this water was then directed to a nearby qanat. Sometimes the water in the reservoir is polluted and can no longer be used, so they have to dispose of it and fill the reservoir again.
The ornamental portal of the water reservoir usually shows the wisdom and talent of Iranian architecture. We can see varieties of inscriptions, decorative masonry, etc. on these portals.
Water reservoirs were filled in the wet season with the water of a nearby shallow qanat and in the dry season could provide people with cool and fresh water. In order to prevent sedimentation of the storage tank, a small pool was constructed near the water reservoir and a canal conveyed the qanat's water to the pool. Then after the sediment settled, the clean water in the pool was directed to the storage tank of water reservoir.
Figure 13 – Profile and plan of a typical water reservoir