The water wheels were introduced into Lanzhou from Yunnan Province by a local people, who used to work in Yunnan in Ming Dynasty (1368 - 1644), the first water wheel was successfully built on the north bank of the Yellow River in 1556. The large water wheels have a diameter of about 20 meters, small ones around 10 meters, therefore, through the irrigation device, the water can be lifted about 15-18 meters high. Generallywheels can irrigate 600-700 Mu of land (1 Mu=666.6 square meter), andone about 100-200 Mu. In the ancient time, water wheels were very advanced and convenient irrigation tool.