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November 18, 2018

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Home » Sunday » Now and Then

Karez’s ‘Great Wall’ water setup

By Zhang Qian | 00:00 UTC+8 November 18, 2018 | Print Edition

THERE are two “Great Walls” in ancient China. The one on the ground defends the ancient Chinese borders in the north, and the other underground nurtures the land in the Xinjiang Uygur Autonomous Region with precious water.

Karez, which literally means “well” in Uygur language, was the life source in Turpan and Hami of Xinjiang. That particular water system, with more than 2,000 years of history, played a crucial role in the region’s development as an important oasis stopover on the ancient Silk Road, skirting the barren and hostile Takalamakan Desert. Today, there are over 1,000 karez systems exceeding 5,000 kilometers in the Turpan Basin, which gained its nickname as “the underground great wall.”

There were records of karez as early as in “Shi Ji” (Historical Record) by Sima Qian of the Western Han Dynasty (206 BC-AD 25). In the reign of Emperor Wu of the Western Han Dynasty, migration, making canals and opening wasteland were taken as important parts for defending the border from the Huns, which paved way for the innovative water irrigation development there.

A karez water system is composed of vertical wells, underground canals, above-ground canals and small reservoirs.

The bottoms of the wells are linked by an underground water canal. The canal collects water from the watershed surface runoff from the nearby snow mountains, and then channels the water to the surface, taking advantage of the current provided by gravity of the downward slope of the Turpan Basin.

The canals are mostly underground to reduce water evaporation. The ground canals and reservoirs help hold the waters for irrigation.

Vertical wells

The vertical wells are for ventilation, digging and maintenance of the karez. They are usually set every 20 to 70 meters, from the water source to the irrigation land. Their depths descend as their distance to the water source grows.

The vertical wells near the water source may be 100 meters deep, while further downstream they are less than 10 meters deep. And the number of wells can vary from a dozen to more than 100 for one water system, transporting water underground from 3 to 30 kilometers long.

If you see a series of orderly-set mounds down a slope to an oasis, it is very likely to be part of a karez water system.

The underground canal is the most important part of the water system, which helps collect and transport the water to targeted land. An underground canal is usually 1.7 meters high, 1.2 meters wide, and extended at most 30 kilometers long. It was a very difficult job to make such long canals completely underground.

To make sure of the direction, ancient workers hung a wood stick, sharpened at one end, in each of two neighbor wells. The line connected the two sharpened head pointing at each other makes the shortest cut for the underground canal direction.

Also to keep the right direction underground, some workers hung an oil lamp at the central line of the well.

They stood between the lamp and the direction they were going, and then dug their own shadow. They had a basket with them to help measure the proper depth. They needed to dig until the underground water covered the basket brim.

Digging calcareous clay in the limited underground space was a huge job. The workers often needed to kneel in icy water while working; most of them died before they were 30 years old.

The melted snow water traveled through the underground canal will be naturally filtered and became mineral water with rich microelement. There are quite a number of local elderly aged more than 100 years old, gaining Turpan also a credit of “longevity village.”

The number of functioning karez has been shrinking throughout the years since the 1950s. There is said to be about 1,700 karez water systems in the Xinjiang Uygur Autonomous Region back in the 1950s, but only about 860 in the 1980s. The number of karez systems in Turpan alone dropped from more 1,273 to 725.

Global warming which led to ice cap melting has been one of the major reasons. And regional ecological damages also posed great threats to the karez water resources.

That includes desertification, wide use of motor-pumped wells and the great water consumption by oil exploitation.

Now and Then


 

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