ENGINEERING FEATURES
INGENIEUR
Turpan Karez Wells –
Ancient Underground Aqua
Channel System
By Ir. Tan Bee Hong
Turpan basin is located in the eastern region
of Xinjiang province of Western China. It lies
in the third deepest inland depression in the
world. Along the northern rim of the depression
is a range of high mountains with snow-capped
peaks called the Bogda Mountains.
Desert climate of Turpan region
The Turpan Depression Basin proper covers
an area of 28,600km 2 (11,000 sq. miles).
The Aydingkul Lake lies in the centre of the
basin which has an elevation of 154.5m (507ft)
below sea level, the sixth lowest exposed
elevation on the Earth’s land surface. It is the
lowest surface point in China. The Turpan Basin
has a harsh, continental desert climate in which
the precipitation is far less than the potential
evaporation.
Since temperatures tend to rise as elevation
goes down, the hottest spot occurs at the lowest
point of the Turpan Basin — Aydingkul Lake. This
lake used to cover 153km 2 but today it is almost
dried up. At 154.31m below sea level, the former
lake’s bed is the second lowest point in the world
after the Dead Sea at 422m below sea level.
During the day, the ground absorbs energy
from the sun, and this heat warms the ambient
air from the bottom up. In the Turpan Basin,
ground temperatures during summer often
exceed 75°. The Turpan Basin is not only the
hottest part of China, it is also the driest, with
an annual precipitation of less than 20mm. The
potential evapotranspiration reaches as high as
3,000mm/a in Turpan, so snowmelt water from
the high cold mountains becomes the main
water source for human use and natural needs.
The satellite photo in Figure 1 shows the steep
mountain range and the flat plain of the Turpan
region.
A lot of water is released as the snow melts
on the mountains and the locals had to think of a
way to bring it down to the basin without the water
evaporating or being lost by ground seepage. As a
result, they made underground tunnels. The locals
took advantage of the big difference in elevation to
make gently sloping underground tunnels (qanats
or karezes) to carry the water into the valleys
where they farm and live.
People in the oasis were entirely dependent on
irrigation in the past. Compared to surface irrigation,
karez wells or qanats can avoid high water losses
from evapotranspiration as well as seepage into the
very dry and coarse ground surfaces, so they can
transport water over greater distances.
During ancient times, when caravan traders
passed through the area as they went to or from
the Gansu Corridor to the east, the Turpan area
was a place to trade, get provisions, and rest.
There was a string of forts, cities, and small
kingdoms along the northern rim of the Turpan
Basin and the Taklamakan Desert.
During the Tang Dynasty (618–907), there were
two fortified cities called Jiaohe and Gaochang
in the Turpan area which were about 44km
(27 miles) apart. The caravans needed fodder for
the camels and other animals and food for the
guards, traders, and travellers. This meant there
was a high demand for water and food to support
the traffic through Turpan, and so the karezes
were used to supply some of the water.
74 VOL 81 JANUARY-MARCH 2020