Does water storage necessarily have to be visible? Reservoirs do not necessarily have to be constructed on high ground among hills and mountains.  Both Taiwan’s and Asia’s first large-scale man-made lake with artificially recharged groundwater has been constructed in southern Taiwan’s Pingtung County. The facility, which redirects floodwater to replenish groundwater, completely reshapes old notions that reservoirs can only be built above ground. Rather, this is a reservoir that is not in danger of silting up: it stores water underground.

Arriving at the Great Chaozhou Artificial Lake, instead of the expected beautiful lake among the hills, one’s eyes are met instead by a giant, visibly dry pit.

Ting Che-Shih, dean of the Pingtung University of Science and Technology’s College of Engineering, a hydraulic engineer by trade trained in the Netherlands, says, “The Great Chaozhou Artificial Lake is not meant for water storage, but to redirect flood waters from typhoons and other heavy rainfalls from the Linbian Creek into the manmade lake to rapidly seep under the surface and become subterranean water, and further, to use the alluvial plains on both sides of the Linbian Creek as aquifers.”

An important condition for the manmade lake is that redirected water must be able to penetrate rapidly into the subterranean water table. Accordingly, the top of the Linbian alluvial plain delta, located where the runoff comes down from the mountains, was selected for this purpose. Here, the pores in gravel and pebbles are large in diameter, facilitating the rapid underground penetration of floodwater. This is why seeing a dry manmade “lake” is completely normal - because its function was never to store water above ground, but to rapidly transport water to the water table below the ground.

This is not a new concept. Ting studied in the Netherlands, a prominent country in the utilization of underground water resources. The Netherlands has redirected water from the Rhine River below sand bars along the shoreline, using soil to filter and improve water quality prior to extraction.

“The underground realm is really a gigantic reservoir. In fact, UNESCO’s research institute believes that this is the new twenty-first century thinking about aquifer space. And the Netherlands has over a century of history developing underground aquifers as subterranean reservoirs, so the technology and theory is actually quite mature,” says Ting.

For many years, clutching The Complete Works of Li Yizhi (1882–1938), known as the father of modern China’s hydraulic engineering, Ting has preached the gospel of “storing water underground, and collecting flood water in canals.” Ting has stressed that storing its ample annual rainfall under ground could give Taiwan greater flexibility in the use of its water resources.

What about Subsidence?

The excessive extraction of underground water along Taiwan’s western coastal corridor has led to land subsidence and soil salinization. Consequently, talk of underground water extraction makes people blanche, and the concept of underground reservoirs has gained a bad reputation.

If underground space can be effectively utilized, it would enable the storage of a staggering volume of water. By Ting’s reckoning, water has been extracted from up to 200 meters below the surface of the Pingtung Plain, covering 1,220 square kilometers. Multiplied by a 0.1 soil porosity (water is contained in pores in the soil), the entire Pingtung Plain can hold and modulate over 20 billion tons of water, or four times the combined capacity of all of Taiwan’s reservoirs. At 7.5 billion tons of water storage capacity, the area of the Linbian Creek alluvial plain alone can hold more water than all of Taiwan’s reservoirs combined.

Underground aquifer promotion in Pingtung County did not get off to a good start. Only after Tsao Chi-hung, a native of Linbian Township with a deep appreciation of the ravages of coastal land subsidence in the area, became county magistrate and strongly supported the utilization of underground water as a resource, was Linbian Creek floodwater redirected underground. This helped raise the water table, alleviating land subsidence and slowing down soil salinization from seawater seepage.

An underground reservoir is by definition intended to extract water, but does that mean that there are no concerns about land subsidence? Tsai Chang-chan, director of the Water Resources Bureau explains: “The government has spent over NT$10 billion on subsidence prevention efforts, monitoring underground water levels to measure underground capacity and how much water has been directed underground. The guiding concept of the Great Chaozhou Man-made Lake is correct, to first recharge groundwater, and extract it for use once the water reaches a certain level.”

Phase one of the Greater Chaozhou Artificial Lake Project has been allocated a budget of NT$1.4 billion. Over the past two years, trials have been run to replenish groundwater under the Pingtung Plain, and the manmade lake can be recharged with an estimated 150 million tons of underground water per year. After 10 years of replenishment, it is hoped that half of the water volume (75 million tons) can be extracted and used each year.

In addition to measuring groundwater, the recharging process also includes other challenges. Huang Shih-wei, director of the Southern Region Water Resources Office under the Water Resources Administration of the Ministry of Economic Affairs, relates that over time, silt can plug up pores and cracks in the man-made river, so that if the bottom sediment is not removed, it loses its exchange functions. Consequently, facilities must be installed upstream to reduce the flow of silt into the man-made lake, and the bottom must be dredged regularly to remove silt.

In the effort to maximize recharging of the Great Chaozhou Manmade Lake, Ting Che-Shih conducted four years of on-site experiments, and studied the methods for channeling water and redirecting silt used by the Dujiangyan Canal in China to reduce the inflow of silt accompanying flood water into the manmade lake. Further, he conducted simulations of various origins and causes of pore and crack blockage, finding solutions for each issue. “Still, eventually, dredging out the bottom layer of sediment is unavoidable in order to maintain normal penetration,” says Ting.

Illegal Dumping, Fighting Sedimentation

The third challenge is underground refuse. In past decades, Taiwan’s western plains have been plagued by unscrupulous operators who replaced illegally mined gravel and replaced it with refuse, or who even buried animal carcasses underground to fill in the gaps. In order to safely utilize groundwater, apart from measuring the water table, such contaminated sources of water must be avoided.

According to Yu Ching-yun, assistant professor in the hydraulic engineering division of National Taiwan University’s Department of Civil Engineering,

“We’ve been taught to believe that extracting groundwater is bad, but the truth is that some places in Taiwan have an overabundance of groundwater. For instance, both Taipei and Taichung have seen excessively high water tables, and it is both possible and necessary for Taiwan to extract and use groundwater in an appropriate fashion.”

From a sustainability standpoint, artificial recharging is not as desirable as natural recharging. “In recent years, a large amount of agricultural land has been repurposed for development in Taiwan. And the transformation of earth into concrete eliminates the land’s natural ability to replenish (groundwater). These cities claim to be ‘sponge cities’ capable of containing water resources, but underneath them is actually still just concrete. For instance, underneath Da-An Forest Park is all concrete, without any capacity for recharging groundwater,” asserts Thomas Chan, deputy administrator of the Environmental Protection Administration.

The critical mission of underground reservoirs is to alleviate the replacement of earth and soil with concrete, cultivate water content resources, and utilize appropriate methods in appropriate places to replenish groundwater.

One such example is the artificial lake atop the alluvial plain. At present, regular groundwater extraction is not prudent; subterranean water should first be preserved and stored so that it can be extracted and used as potentially life-saving water in the event of a drought.

Translated from the Chinese article by David Toman

Additional Reading

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♦ Taiwan: The Water-starved Island
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