Secondly, Bogda Mounrtain, located north of the Turpan Basin is 5,440 meters above sea level, and the water levels of Aiding Lake, which is situated in the central area of the basin, are 156 meters under sea level. From the foot of Tianshan Mountain to Aiding Lake, the horizontal distance is only 60 kilometers, but the difference in height is 1,400 meters, which contributes to an average gradient of 1:40. The slope of the undercurrent is generally the same as the ground, which provides an indispensable component to digging a karez well.

Lastly, karez wells perfectly meet irrigation needs and play a vital role in local agriculture despite the dry and hot climate, large-scale vaporization and occasional sandstorms in windy seasons. When a sandstorm breaks out, sometimes the channels of an aboveground irrigation system can get filled up with sand and fail to work. The karez irrigation system, however, is immune to such problems since its channels run underground. Since karez water is primarily thawed snow and melted ice, the stream runs very steadily, and the changes in season do not influence it too much. Meanwhile, the underground system also contributes to a low evaporation rate.

A complete karez system is composed of four parts: perpendicular wells, subterranean channels, aboveground channels and laoba (small pool). To build a karez well, the first step is to find an undercurrent of thawed snow in the tall mountains and dig some perpendicular wells at certain intervals. The mouth of the well is usually one meter long and 0.7 meters wide. With the depth ranging from 10-20 meters, the purposes of the perpendicular wells are two-fold: ventilation and supporting the earth when digging or repairing the subterranean channels. Then bottoms of the wells are linked together by underground tunnels and water flows from one higher well to another lower one. When the water reaches the oasis, it comes out of the ground and flows into the aboveground channel, irrigating farmland. A laoba is usually built at the exits of the subterranean channel to hold some water. The length of each karez is about three or four kilometers long, but longer ones can extend as far as 20 kilometers.

Although the karez first appeared more than 2,000 years ago, it experienced a very slow development in history. Until the late Qing Dynasty (1368-1644) when Lin Zexu, the governor of Hunan and Guangdong provinces, who sought unsuccessfully to suppress the British-supported opium trade, was exiled to Xinjiang, he worked hard to construct new karez systems. The water in the karez is cool in the summer and warm in winter. Besides irrigation, the clear water from the karez is also ideal for drinking.