¹ Director General, Water Resources Bureau, MOEA, Taiwan, China

12F, 41-3, Sec. 3, Hsin-I Rd., Taipei, Taiwan 106

² Section Chief, Water Resources Bureau, MOEA, Taiwan, China

9-12F, 41-3, Sec. 3, Hsin-I Rd., Taipei, Taiwan 106

³ Post Doctoral Fellow, Hydraulic Technical Laboratory, NTU,

Taiwan 5F, 41-2, Sec. 3, Hsin-I Rd., Taipei, Taiwan 106

Tel:886-2-27545940, Fax:886-2-27545943, E-mail:lhcheng@wrb.gov.tw

Abstract: Water resources utilization has brought prosperous economic growth in many industries and regional developments in Taiwan. In 1995, 6.2 billion tons of groundwater was withdrawn which is about 33% of total water consumption in Taiwan area. However, due to the overdraft of groundwater as compared with the natural recharge of 4 billion tons of groundwater which incurs social problems such as land subsidence, seawater intrusion and flooding. To alleviate these problems, the two master plans entitled the “Groundwater Monitoring Network Plan in Taiwan” and “Land Subsidence Prevention and Reclamation Plan” have activated to protect groundwater resources in Taiwan. The first plan is a 17-year project beginning from 1992 to 2008 for the purpose of improving groundwater monitoring in Taiwan area. The first stage (1992 to 1998) of the plan completed 141 hydrogeological survey stations and 332 groundwater monitoring wells at Choshui River Alluvial Fan, northern part of Chianan Plain and Pingtung Plain where serious land subsidence occurred. The second plan has been completed by mid 2000 for preventing further land subsidence by reducing over-pumping of groundwater and by better utilization and management of land use and water resources. A second-stage plan for another five years (2001–2005) has proposed. This paper presents the preliminary results and accomplishments of the above two plans.

Keywords: taiwan, groundwater monitoring, subsidence, network plan

The rapid population growth and the development of aquaculture and industry during the past three decades have increased water demand from 10 billion m³ in 1961 to 15 billion m³ in 1971, and to 18 billion m³ in 1992 (Wu l994). A water demand of 21 billion m³ is expected for the year of 2010, if the projected agricultural, industrial, and domestic requirements are correct. In addition, due to the demand for water supplies has risen, the advantages of lower costs, easier accessibility, and more stable quality of groundwater have encouraged people to use it. The annual groundwater withdrawal in Taiwan was estimated to be 3.2 billion m³ in 1975, 4.1 billion m³ in 1983, 6.3 billion m³ in 1988, and 7.1 billion m³ in 1991 (Wu l992). The annual groundwater recharge, however, was estimated to be only 4 billion m³. Over-pumping of groundwater by various water users, such as the extensively developed aquacultural industry in the coastal area of southern Taiwan, has lowered the water table quickly in this area. Consequently, the declining water level has resulted in the consolidation of alluvial sediments and caused land subsidence. However, the lack of reliable hydrogeologic data makes it difficult to quantify and regulate the groundwater use and control land subsidence. In 1992, the Department of Water Resources (DWR) was requested to initiate a plan entitled “Groundwater Monitoring Network Plan in Taiwan,” which is described in detail in DWR (1996), Hsu et al. (1996) and Hsu (1998). Since then, the DWR has taken the responsibility for planning, supervising, and raising funds for this plan. The main objective of this plan is to collect data at regional scale that would be ultimately used for the planning and management of groundwater resources in Taiwan. In 1993, in certain areas, land subsidence only became worse. In Nov. 1995, the “Land Subsidence Prevention and Reclamation Plan (LSPRP) was approved by Executive Yuan (MOEA and CAPD, 1995).

The Groundwater Monitoring Network Plan in Taiwan covers nine groundwater basins as well as The Penghu Island and Hengchun plain areas. The calls for 517 hydrogeologic survey stations and 990 groundwater monitoring wells. Table 1 shows the number of stations and wells per stage of each groundwater basin. Because of financial constraints, this plan is to be accomplished in three stages over a period of 17 years. The total estimated implementation cost was NT$ 6.4 billion(around US$ 200 million). The current plan aims at developing a reliable groundwater monitoring network in Taiwan to collect long-term groundwater data. The scope of the current plan (DWR 1996) consists of six items. The Groundwater Monitoring Network Plan in Taiwan has been carrying on since 1992. By the end of July, 1998, 145 hydrogeological survey stations and 315 monitoring wells were constructed. The plan is focused on the Choshui River alluvial fan and Pingtung plain during the first stage. The preliminary hydrogeologic systems, the occurence of aquifers and the locations of recharge areas have been identified. The groundwater hydrology, aquifer characterization, and groundwater flow pattern were analyzed (WRB 1999). The results are used for the comprehensive planning and management of the water resources in these two basins.

The Choshui River alluvial fan, the largest in Taiwan, is located at the west coast of central Taiwan. It is triangular in shape with the apex at Linnei and divided into north and south parts by Choshui river. The area of the fan is 1,800 km ². In the fan area, the overall water consumption is 3.1 billion m³, of which 0.9 billion m³ is from groundwater. The distribution of groundwater monitoring wells in the Choshui River alluvial fan area are shown in Figure 1. Geological drilling and stratigraphic analysis were conducted to interpret the subsurface geology in Choshui River alluvial fan. The groundwater basin is composed of three aquifers and two aquitards, as shown in Figure 2. Aquifer II is the most important water bearing formation of the fan area. Its large grain size suggests that it has high hydraulic conductivity. Aquifers I, II, and III probably pinch out in the Taiwan Strait, as shown in Figure 2, but it requires further verification. The groundwater stored in the fan area is generally confined in nature throughout most of the area, except at the upper part near the apex of the fan. The aquitards consist of clay and silt in variable thickness. From pumping test analysis, the transmissivities and the hydraulic conductivities of the aquifers range from 0.01 to 4.19 m²/min and 10^-3 to 10^-5 m/sec, respectively. The specific yields range from 0.18 to 0.29 for unconfined aquifer. The storage coefficients range from 10^-5 to 10^-3 for confined aquifer.

The apex of the fan can be conserved for groundwater storage and the groundwater should be extracted out during the dry season only from the middle and fringes of the fan. From long-term monitoring, the groundwater level had gradually declined since 1960, but yet rebounded after 1990. Aquifer II has been over withdrawal all the time. The water table fluctuates by as much as 15 m during a given month and 5 m during a given day. The average amount of groundwater recharge is 1.024 billion m³/yr. According to the study of numerical modeling, the safe yield, which is equivalent to the natural recharge is estimated to be 0.818 billion m ³/yr. The routine analysis of groundwater quality has reached the conclusion that there is no seawater intrusion in the Choshui alluvial fan.

The Pintung plain is located at the most southwestern part of Taiwan. It is rectangular in shape with 55 km long and 22 km wide, and encompasses an area over 1,200 km². The Kaoping creek is the largest river in Taiwan with an annual runoff of 8.5 billion m³. The overall water consumption is 1.47 billion m³ per year, of which 1.04 billion m³/yr are from groundwater. The locations of groundwater monitoring wells in the Pingtung plain are shown in Figure 3. A same investigation and study program was also conducted in Pingtung plain. The groundwater basin is represented by three aquifers and two aquitards, as shown in Figure 3. Aquifer III is the most important water-bearing formation. Aquifers I, II, and III have cropped out in the Manila trench a few kilometers off the coastal line, as shown in Figure 4. The aquitards are composed of clay, leading to a low hydraulic conductivity of the strata. From pumping test analysis, the hydraulic conductivities of aquifers range from 10^-3 to 10^-5 m/s, and decrease from the apex to the fringes. Transmissivities range from 0.00003 to 15.1 m²/min. The specific yields range from 0.01 to 3.2 for unconfined aquifers. The storage coefficients range from 10^-3 to 10^-5 for the confined aquifers. The discharge per unit drawdown ranges from 0.004 to 654.5 m³/m/h. The groundwater recharge areas can be delineated which include the proximal parts of Laonungchi fan, Ailiaochi fan, Linpienchi fan and Lilychi fan.

Groundwater level had been gradually declined since 1979, but yet rebounded after 1989. Aquifers I and II have been over pumped for a long time. The water table fluctuates by as much as 21 m during a given month and 5 m during a given day. The average amount of recharge is 1.11 billion m³/yr, and the safe yield is 1.0 billion m ³/yr. The seawater intrusion in the aquifers has been studies in terms of hydrogeology, groundwater level, groundwater salinity, and resistivity log. The result shows that the ease of seawater intrusion has benefited from the long extension of aquifer toward the Manila trench.

The LSPRP has two major goals: (1) To prevent over-pumping of groundwater, thereby alleviating land subsidence, and (2) To make reasonable use of land and water resources, thereby reducing physical damage and social costs. To evaluate the success of the Plan, five quantitative objectives are proposed. They are:

(1) The total aquacultural area will be reduced from 52,000 hectares to 22,000 hectares.

(2) The number of illegally constructed wells are to be reduced by 50 percent in the areas, where during the past three years, the cumulative land subsidence has exceeded one meter and in the areas where the average rate of subsidence has exceeded 15 centimeters per year.

(3) Groundwater extraction will be reduced to the safe yield level.

(4) There will be no further increase in land areas, where the groundwater table is below the mean sea level, and to reduce such land areas by 50 percent.

(5) Land subsidence in 50 percent of the affected areas will cease.

Major achievements of each work item are listed in Table 2. Based upon actual accomplishments of the five quantitative objectives, it is obvious that the LSPRP has not been fully successful. In fact, many obstacles have been encountered during the execution, which have slowed down the progress and hindered the achievements. Among the obstacles, the following four have been identified as the most serious: (1) Transforming of the subsidence area from aquacultural use to industrial use has not been successful due to the high cost and the fact that the natural and social-economic conditions are not suitable for industrial use. (2) The county-level governments have been facing difficulties in enforcing the groundwater control regulations due to lack of manpower. (3) A small number of new illegal wells has been detected during the execution of the LSPRP. Furthermore, it has been realized that, because of the persistence effect, land subsidence will continue regardless.

The “Groundwater Monitoring Network Plan in Taiwan,” which was initiated in 1992 is geared to define the goals and implementation procedures of the effective management of groundwater for Taiwan. The preliminary results of the studies have shown that the conjunctive use of surface-water and groundwater is the best managing strategy for the Choshui River alluvial fan and Pingtung plain. The apex areas should be conserved for groundwater recharge, and the other areas of the fan will provide groundwater during dry seasons.

The comprehensive “Land Subsidence Prevention and Reclamation Plan” is Taiwan’s first attempt to remediate the extremely serious land subsidence problem with all related agencies participating. After the execution of the Plan (by mid 2000) the specified five quantitative objectives have been achieved to some degree. Although some obstacles have been encountered so that the expected goals are not achieved; however, without question, the implementation of the LSPRP has prevented the land subsidence problem from worsening. By assessing the results obtained to date and the obstacles encountered, the Water Resources Bureau has proposed a second-stage plan for another five years (2001–2005). It was expected that there would be difficulties in reaching the quantitative objectives at the end of the second-stage plan. Nevertheless, it is also expected that, after the second-stage plan is fully implemented, land subsidence in Taiwan will be under proper control.

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MOEA (Ministry of Economic Affairs) and CAPD (Council for Agricultural Planning and Development) (1995) Land subsidence prevention and reclamation plan. Taipei, Taiwan, 22p (in Chinese).

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Table 1  Planned hydrogeologic survey stations and groundwater monitoring wells to be installed at different stages

Table 2  Major work items and corresponding achievements of LSPRP by mid 2000

Fig. 1  Locations of existing hydrogeological survey stations and observation wells in the Choshui River alluvial fan. (After WRB 1999)