Under this irrigation system, there are two crop periods each year. Considering the available water resources, rice cultivation in Yunlin County should be confined to the second crop. Since that crop coincides with the rainy season, there should be no water shortage. Why, then, do we still have this problem of excessive groundwater extraction? The trouble is that, in order to make more money, farmers plant rice in the first crop period, from February to June. Since there is no surface water available at that time, the only way farmers can irrigate their paddy fields is by using groundwater drawn from wells that they bore themselves.This is illustrated by the debate over the subsidence issue and the HSR. One simulation estimated that only 60 wells need be shut down to reduce the problem to non-worrisome levels. Nevertheless....
Turning to the third point, there are more than 100,000 wells in Yunlin County, more than 90 percent of which were dug without confirming water rights or applying for permission to build hydraulic facilities, as required by the Irrigation Act. If things were run in accordance with the law, the authorities would clamp down and stop illegal extraction of groundwater.
The current reality, however, is that out of consideration for farmers’ livelihoods and to avoid clashes, illegal wells can only be dealt with when complementary measures are in place....
The ground where the elevated railway passes over Provincial Expressway No. 78 has sunk 55cm over the past seven years, according to data from the line’s operator, Taiwan High Speed Rail Corp (THSRC), raising potential safety concerns.Note that the law is made by the Central government but enforcement is handled at the local level. This means that local politicians would have to shut down their neighbors' -- and voters -- wells. Like that will ever happen.
The Water Resources Agency (WRA) issued a directive recently to seal off 1,115 shallow wells near the problematic areas to limit subsidence, but it was rejected by the Yunlin County Government, which said it would hurt farmers’ interests.
Yunlin County Commissioner Su Chih-fen (蘇治芬), who refused to follow the WRA’s directive without suitable complementary measures, said the measure would do little to mitigate the problem because the subsidence was mainly caused by deep wells that had all been sealed off years ago.
Chinese Nationalist Party (KMT) Legislator Lin Hong-chi (林鴻池) agreed, saying that well closings and high-speed railway safety involve many complex issues that require the collaboration of various government agencies.
in this article, in cm/year. Scary -- near the coast the land is subsiding at nearly 1 meter every five years. And sea levels are rising.....
How subsidence occurs is described in this piece about a similar problem in the Philippines:
How excess groundwater use causes land subsidence has been known for a long time (Terzaghi, 1925; Tolman and Poland, 1940), and the theory is summarised admirably by Galloway et al. (2001). In river deltas, groundwater is stored in and recovered from sandy and gravelly aquifer (‘water bearer’) layers. Aquifers are contained by interbedded aquitards, layers of clayey sediment that are much more porous and contain significantly more water, but, being of a very fine grain, have a great deal of grain surface to offer frictional resistance and retard the through-flow of water—hence their name.The reference I append at the end has a nice picture of the aquifers underlying the Choshui River's alluvial fan -- four in all, interlaced with aquitards. The Choshui is the large river bordering Yunlin on the north.
Deltaic sediment columns are supported in part by the fluid pressure of their pore waters. When water is extracted from an aquifer, support is transferred from its fluid pressure to the sediment grains comprising its granular skeleton, which is somewhat compressed, commonly causing the ground to subside a few centimetres. If groundwater extraction is not excessive, that compression and subsidence may be fully reversed when precipitation recharges the aquifer.
In addition to the farming practices outlined in the TT commentary above, many sources identify water withdrawn for fish farming as a major source of subsidence. As Taiwan Review noted in an excellent article on fish farming last year, at its peak in the 1990s the aquaculture industry covered 1.5% of the island's land area. It exports plenty of lucrative high value fish, but the staple export is the drab Tilapia (wu guo yu). This study in fact finds that salinization of the shallow aquifer along the coast in Yunlin is due to sea water used in fish ponds seeping back into the aquifer....
The determined local hydrogeologic setting suggests that the shallow aquifer may be connected to the sea water, resulting in salt water intrusion as a large amount of shallow ground water is withdrawn. The percent contributions of sea water intrusion, percolation through wells, and infiltration of water from fish ponds, to the salinization of the shallow aquifer at Ko-Hu in the Yun-Lin coastal area are approximately 27 percent, less than 1 percent and 73 percent, respectively. The results suggest that the vertical infiltration of salt water from fish ponds is the major cause of shallow ground water salinization in the coastal area of Yun-Lin.This report identifies Tuku as the center of the "basin-like" subsidence and says that compaction is greatest at depths greater than 200 m. Deep wells -- thousands of them -- are sprinkled throughout the area.
How is the Yunlin government dealing with the issue? The central government ordered a halt to pumping around the HSR but as noted above that is not a realistic possibility. Instead the county government has proposed a PV industrial district for the affected area:
Of course, this all only a proposal at the moment. And we've seen in the Central Taiwan Science Park land cases how compensation is often miserly and poorly handled....
The plan was put forward in an attempt to override the decision made by the Water Resource Agency of the Ministry of Economic Affairs to designate the subsidence area on the intersection of the rail lines by No. 78 highway as a model district subject to a ban on pumping underground water to irrigate the land.
The county government contended that the decision would take a serious toll on living of farmers depending on this piece of land, which is now a rice paddy. The county government suggests PV manufacturers on the planned industrial zone pay land owners NT$26,000 per hectare as monthly rental. Total rental for the 385-hectare land is estimated at NT$92.4 million (US$2.8 million at US$1:NT$32) a year, much higher than the NT$100,000 (US$3,125) average that Taiwan's rice farmers earn a year on each hectare.
It is understood that many PV manufacturers and the ministry have voiced support for the industrial park plan.
REF: Charts, cores, maps in this scholarly article on clay and subsidence in Yunlin.
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