Preserving the bright moon and clear spring in the Dunhuang desert

When you think of the desert, the first thing that comes to mind is the yellow sand everywhere. But if you travel to Mingsha Mountain in the south of Dunhuang, Gansu, you will see a clear spring in the desert. It is like a large piece of jade embedded in the vast sea of ​​sand, and like a clear-cut bright moon, so it is named "Crescent Moon Spring". For thousands of years, the mountains and springs coexist, and the sand and water coexist, which is recognized as a geographical wonder.

The wonder of desert and spring coexisting

Just like there are many rivers on the surface of the earth, there is also groundwater in the strata beneath our feet. Springs are formed when groundwater naturally emerges from the ground. The water source of Crescent Moon Spring comes from underground. There is a Dang River in the Dunhuang area, which has been replenishing water to the ground near the river throughout history. When the groundwater level reaches a certain height, it seeps out of the soil surface from a higher position, forming the beautiful Crescent Moon Spring in the low-lying area.

The most peculiar thing about Crescent Lake is that it is surrounded by sand hills. The quicksand and the spring water are only a few dozen meters apart. In addition, there are strong winds and flying dust in the desert all year round, but the clear water of the spring is neither buried by the quicksand nor evaporated and dried up due to water circulation. How is this possible? It turns out that Crescent Lake is surrounded by towering sand hills, which form a circular high mountain depression. When the strong wind in the desert blows into the spring water area located in the center of the depression and with the lowest terrain, it will form an upward whirlwind, rolling a large amount of quicksand around Crescent Lake to the top of the mountain or throwing it to the other side of the mountain. In this way, there is no need to worry about the quicksand swallowing up the spring water.

Therefore, it is the special geographical environment that creates the magic of harmonious coexistence of Crescent Spring and the desert.

The drop in water level puts Crescent Lake in danger

With the population growth and the need for economic and agricultural development in Dunhuang, reservoirs and canals were gradually built on the nearby Shule River and Dang River. In addition, local farmers used wells to extract groundwater for irrigation. Since 1975, the groundwater level in Dunhuang has dropped sharply, and the area of ​​Crescent Lake has continued to shrink. In 1960, the depth of Crescent Lake was 7.5 meters; by 1985, its average depth was only 0.7 meters. Due to the lack of water, the spring dried up and could no longer form the beautiful crescent shape of the past. This year, Crescent Lake, which has a 2,000-year history of written records, was in danger.

The rapid drop in the water level of Crescent Lake has caused great concern among the public and has also attracted great attention from government departments, so they have organized investigations, research and exploration tests many times to find a good solution. Dunhuang has tried to build an artificial lake to fill Crescent Lake with water, build a large pool upstream of Crescent Lake to fill it with water, and use surface water to replenish the water source for Crescent Lake, but the results are minimal, and these methods are difficult to maintain in the long term. Therefore, what method can be used to replenish water for Crescent Lake in an effective and sustainable way needs to be further explored.

Follow the natural way to replenish water and preserve the "first spring in the desert"

Finally, wise water conservancy workers creatively built the Crescent Lake Water Replenishment Project in the Dang River. It followed the natural principle that the Crescent Lake relied on groundwater supply to bring the Crescent Lake back to life by replenishing the water in the underground runoff near the spring area.

The Crescent Lake Restoration and Replenishment Project started in October 2016 and was completed three years later. It divides the Dang River into two vertically by building a dike in the middle of the Dang River bed. The west side is a flood channel similar to the common rivers we see, which can ensure the natural flow of the river and the smooth discharge of flood water in case of floods; the east side is the trick of the Crescent Lake Restoration and Replenishment Project - the riverbed is made into a natural seepage field, and 11 retaining dams and overflow weirs are used to divide the river channel between 4.5 and 8.25 kilometers away from Crescent Lake and 5.8 kilometers long from north to south to form 12 interconnected seepage pools.

Crescent Lake has resumed its water replenishment project, with the seepage pool on the right. Image source: Xinhua News Agency

Because the retaining dam has a certain height, after the river water flows into the seepage field, each seepage pool can store a certain level of river water. It is the river water that has been in the reservoir for a long time that replenishes water to the ground through natural infiltration and accumulation over time, and the groundwater level near Crescent Lake slowly rises. At the same time, once the river water in the seepage pool exceeds the set height, it will flow through the overflow weir to the next reservoir and continue to flow forward, which ensures that the river water will not overflow the river channel and also ensures the safety of the areas around the river section of the seepage field.

The Crescent Lake Restoration and Replenishment Project has extended the time for water to be replenished underground by building a seepage field for long-term water storage, restored the original groundwater replenishment environment of Crescent Lake, and improved the self-repair ability of Crescent Lake. After the project was put into operation, the average water depth of Crescent Lake has steadily increased, and the area of ​​the crescent has also been significantly improved. Finally, by 2020, the water depth of Crescent Lake has returned to more than 2 meters, meeting the requirements for restoring the natural ecological landscape.

Of course, in addition to restoring the water to Crescent Lake by building projects that follow the natural water replenishment method, Dunhuang City has also taken a series of protection and management measures, such as restricting the exploitation of groundwater in the upper reaches of Crescent Lake and surrounding areas, vigorously developing water-saving agriculture, and adopting comprehensive management methods to improve the ecological environment, in order to preserve the beauty of the "First Spring in the Desert."