Karez

The Karez, an irrigation system of wells connected by underground canals, is considered as one of the three great ancient projects in China, the other two being the Great Wall and the Grand Canal. It was invented and built by the ancient people of various ethnic groups in Xinjiang to adapt to the natural environment in the dry region, and by taking advantage of the mountain slopes.

In Xinjiang, the Karez wells are mainly distributed in the Hami,Turpan, and Mori areas, but they are mostly in the Turpan Basin, where, at its peak, there were altogether 1,100-odd Karez wells totaling 5,000 kilometers in length, hence the moniker of "the underground canal."

Why were the Karez wells built in great number in the Turpan Basin? The answer is due to the local natural geographical conditions. To the north of the Turpan Basin is the Bogeda Mountain and to its west, the Kalawucheng Mountain.

In summer, large amounts of melted snow water and rainwater flow down from the mountains into the valleys, then infiltrate into the ground, thereby making an underground current, leading to an adequate underground water source for the Karez wells. Besides, the earth in the Turpan Basin is solid and good for building wells and underground channels.

In the basin, it is very dry and hot in summer, with a high evaporation rate. In the windy season, the strong wind blows sand into every corner, often burying a lot of farmlands and water channels in sand. But the Karez, unaffected by the season and sand, can continue supplying water through underground channels. In addition, the Karez, with a very low evaporation rate and a stable supply of water, can irrigate farmlands all the year round.

The word "Karez" means well. The Karez works entirely by gravity, thus eliminating the need for pumps -- it runs from high to low ground due to gravity alone. It is a very delicate irrigation system composed of four parts: a vertical well, an underground canal, an aboveground canal, and small reservoirs. The vertical wells are for the 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 high and is covered with earth to resist the heat. The surface canals, connected to the underground ones, are not more than 1 meter wide, with trees planted on both sides to prevent evaporation.

The Karez is built this way: First, find water source in a place where there is an underground water current; secondly, dig out vertical wells with a certain space between them; then, build underground canals connecting these wells, to allow water to flow through the underground canals. The outlets of underground canals are connected to the aboveground canals, through which the underground water can flow out and irrigate farmlands.

As far back as the Han Dynasty(206BC-220AD), the Karez was recorded in Records of the Great Historian(Shi Ji). Most of the existing Karez wells in the Turpan area were built in the Qing Dynasty(1644-1911) and in later years. Nowadays, large stretches of fertile land are still irrigated by the Karez.

Generally, a Karez is 3 kilometers with the shortest being 100 meters or so, and the longest, 20 to 30 kilometers with several dozen vertical wells. Sometimes the number of vertical wells exceeds 300.

As the life source of the Turpan area, the Karez was an ingenious creation of the underground irrigation by the ancient people in Xinjiang, and was later exported to the Central Asia countries and Persia (today's Iran) via the ancient Silk Road.