Karez systems are the crystallization of ancient people's diligence and intelligence. At its peak, the karez irrigation system exceeded 5,000 km (3,106 miles) and was also referred to as 'the underground Great Wall'. It is among the three ancient major irrigation projects with the other two being Ling Canal and Dujiangyan Irrigation Project.
Karez systems are the life source of Turpan. In a sense, without the karez, there would be no Turpan culture. According to records, the history of the karez in Xinjiang dates back to 103B.C. Karezes have been found in Iran, the Sahara, etc., but the ones in Turpan are the most complete. Currently there are still over 400 systems.
Karez systems are very delicate irrigation systems made up of vertical wells, underground canals, above-ground canals and small reservoirs. Generally, a karez is 3 km (1.9 miles) with the longest being 20 to 30 km (12-19 miles) with several dozen vertical wells. Sometimes the number of vertical wells exceeds 300. Until today, the shortest karez found is only 30 meters (98 feet) long. The vertical wells are for ventilation, digging and maintenance of the karez. The bottoms of all the vertical wells are connected so that water can pass through. The underground canal is about 2 meters (6.5 feet) high and covered with earth to resist the heat. The surface canals, connected to the underground ones, are not more than 1 meter (3.2 feet) wide with trees planted on both sides to prevent evaporation.
Melting snow from the Tianshan Mountain is the water source of the karez. Water is collected by vertical wells and conducted by the underground canals to the oasis, where the water is held in the ground canals for irrigation. The vertical wells near the water source may be 100 meters (328 feet) deep while further downstream they are less than 10 meters (32.8 feet) deep.
A karez reduces evaporation, avoids getting polluted and needs no other power equipment; it runs from high to low ground owing to gravity alone. Turban is the hottest place in China and the arid climate makes water all the more precious. A karez system helps to ideally solve this problem.