A watermill is a structure constructed to grind grain. Its main parts are a water house, two millstones, rotor blades and an axis which connects the rotor blades and upper millstone vertically. The operation of the watermill is based on the potential energy of water due to the height of water house: The deeper the water house, the more energy is generated. Sometimes the depth of a water house reaches 10 meters below the depth of the qanat, in order to increase the water pressure.
Figure1 - Watermill
In fact water house is a shaft well that receives and accumulate water. When the qanat water reaches the water house, it pours down the shaft well and gushes out from a tiny nozzle at the bottom of the well and hits the blades, making the blades rotate, and imparting energy to the rotor which then rotate the upper millstone. The lower millstone is motionless. The friction between the upper and lower millstones grinds the wheat into flour. In some places, several watermills were operated by the water of only one qanat.