Correlation Between Groundwater Level And Altitude Variations in Land Subsidence Area of The Choshuichi Alluvial Fan, Taiwan Chieh-Hung Chen, Chung-Ho.

Correlation Between Groundwater Level And Correlation Between Groundwater Level And Altitude Variations in Land SubsidenceAltitude Variations in Land SubsidenceArea of The Choshuichi Alluvial Fan, TaiwanArea of The Choshuichi Alluvial Fan, Taiwan

Chieh-Hung Chen , Chung-Ho Wang,

Ya-Ju Hsu, Shui-Beih Yu, Long-Chen KuoInstitute of Earth Sciences, Academia Sinica, Taipei 115, Taiwan

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報告人 : 蕭惠如

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In Taiwan, groundwater resources have been depleted in the western and southwestern regions in the past decades due to excessive extraction and caused extensive land subsidence along coastal areas.

The most notorious land subsidence region is located at the Choshuichi Alluvial Fan of central Taiwan , with an active subsiding area of over 600 km2 and a maximum subsiding rate up to 10 cm/yr. In short, the groundwater level dropped from a value close to sea-level down to −30 m from 1974 to 2006

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Fig. 1. Locations of the groundwater monitoring wells (open circles) and GPS sites (solid dots) in the Choshuichi Alluvial Fan of western Taiwan. The Choshui River flows through the middle of the fan and separates two sections: northern Changhua and southern Yunlin Counties. Groundwater flow directions are expressed in gray dashed arrows. The severe land subsidence is located in the southern area of the Yunlin County. Stations which are taken as examples and discussed in Figs. 5–7 are marked with open squares.

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County Station Code Longitude Latitude Aquifer 1 Aquifer 2 Aquifer 3 Aquifer 4Changhua Chaochia CC 120.3872 23.9393 O OChanghua Hsikang CG 120.2813 23.8625 O O O OChanghua Chuanhsin CH 120.5043 24.1738 O O O OChanghua Chutang CT 120.4202 23.8617 O OChanghua Hsihu CU 120.4708 23.9517 O OChanghua Hsichou CZ 120.4931 23.8569 O OChanghua Ershui ES 120.6098 23.8135 OChanghua Fangyuan FY 120.3123 23.9256 O OChanghua Hanbao HB 120.3442 24.0088 O O OChanghua Hohsin HN 120.4500 23.8959 O O OChanghua Haoshiu HO 120.4501 24.0087 O O OChanghua Hsienhsi HS 120.4595 24.1340 O O OChanghua Huatang HT 120.5352 24.0285 O O OChanghua Kanyuan JY 120.5255 23.8248 O OChanghua Kuoshen KS 120.5610 24.0945 O O OChanghua Lochin LT 120.4220 24.0562 O O OChanghua Hsiantien ST 120.3689 23.8757 O OChanghua Tanchien TC 120.3394 23.8374 O OChanghua Tungfang TF 120.5078 24.0646 O OChanghua Tienwei TW 120.5192 23.8932 O OChanghua Tienchung TZ 120.5787 23.8564 O OChanghua Wenchang WC 120.4114 24.0100 O O OChanghua Yuanlin YL 120.5666 23.9534 O O OChiayi Anho AH 120.3045 23.5166 O O OChiayi Sanho SH 120.4798 23.6070 O OChiayi Tungjung TR 120.4274 23.5594 O O OChiayi Tungshi TS 120.1464 23.4622 O O OYunlin An-nan AN 120.2407 23.7058 O OYunlin Paotze BT 120.1434 23.6353 O O OYunlin Chiungpu CP 120.1992 23.5202 OYunlin Fengjung FG 120.3028 23.7925 O O OYunlin Fangtsao FT 120.3659 23.7202 O OYunlin Chiuchuang GC 120.3925 23.6362 O O O OYunlin Chiahsin GH 120.4514 23.6500 O OYunlin Hou-An HA 120.2267 23.7910 OYunlin Huwei HE 120.4242 23.7160 OYunlin Haifeng HF 120.2178 23.7667 O OYunlin Hofeng HG 120.2153 23.7409 O OYunlin Huhsi HH 120.5030 23.7240 O O OYunlin Hsilo HL 120.4592 23.7977 O OYunlin Honglung HR 120.3399 23.6884 O OYunlin Hsinhua HU 120.2808 23.7620 O OYunlin Haiyuan HY 120.1709 23.7226 O O O OYunlin I-wu IW 120.1802 23.5431 O O O OYunlin Chiulung JL 120.4229 23.7529 O O OYunlin Kanchiao KC 120.5298 23.6142 O OYunlin Kinghu KH 120.1452 23.5751 O OYunlin Kukeng KK 120.5587 23.6464 OYunlin Kanghou KU 120.3839 23.7983 O O OYunlin Liuho LH 120.5544 23.7708 O OYunlin Lungtze LZ 120.3467 23.6095 OYunlin Minte MT 120.1911 23.6547 O O OYunlin Peikang PK 120.2938 23.5807 O OYunlin Shuilin SN 120.2380 23.5749 O OYunlin Shiliu SO 120.5777 23.7225 O OYunlin Takuo TG 120.2025 23.5680 O OYunlin Tungho TH 120.5612 23.6877 O O OYunlin Tungkuang TK 120.2639 23.6537 O O O OYunlin Tzetung TN 120.4887 23.7586 O OYunlin Tsaitso TT 120.2111 23.6141 O OYunlin Tienyang TY 120.3009 23.7272 O O OYunlin Wentso WR 120.5040 23.6596 O OYunlin Yuanchang YC 120.3019 23.6547 O

Table 1. The locations and observation aquifers of Choshuichi alluvial fan used in this study.

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Because the Choshuichi Alluvial Fan can be divided into three aquifers for a depth of 250 m according to subsurface hydrogeology each station may have one to five screens situated in different wells for fully observing changes from shallow to deep aquifers. The groundwater levels of these aquifers indicate two major flow directions: northwest in Changhua county and southwest in Yunglin county

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Fig. 2. Contours of groundwater level aquifer 1 from years of 1994–2005 versus 2006

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In this study, we (Chen et al.) examine the correlation between the land subsidence (deduced from GPS data) and the groundwater level variations of monitoring wells in the period between 1994 and 2006.

Our (Chen et al.) aim is to quantitatively describe the relationship between vertical displacement on surface and groundwater level variation in identifying the distinctive effects among aquifers and derive the long-term trend for the land subsidence area.

The behavior of aquifers is vital to the understanding of land subsidence process. The long-term trend is very valuable in developing an effective and appropriate remediation strategy for the land and water resources management in a large scale.

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Taiwan GPS Network was firstly established by the Institute of Earth Sciences, Academia Sinica in 1989 . The number of continuous GPS sites had been rapidly increased to 320 after the 1999 Taiwan Chi-Chi earthquake.

In this study, we use GPS vertical displacements from two continuous sites, PKGM (RGPSPKGM; 23.5799°N, 120.3055°E) and S103(RGPSS103;23.5644°N, 120.4752°E) to analyze variations of vertical motions associated with groundwater levels from 1994 to 2006.

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The groundwater-levels in the aquifers are recorded digitally every hour by piezometers. For a better and consistent comparison, records of the groundwater level and vertical displacement of GPS are both transferred as monthly data, RAaqST and RGPSST, where aq and ST denote the aqth aquifer and the station, respectively.

AaqST and GPSST, are respectively calculated as the yearly changes of the RAaqST and RGPSST with a step of one month.

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The linear relationship between the AaqST and GPSST can be written as:

(1)

where xaq and i denote the coefficients of the AaqST and the sequence numbers of monitor aquifers, respectively

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Because responses of land subsidence caused by excessive

extraction in groundwater are generally not constant, for simplification in analysis, the unknown long term trend is expressed by a temporal function of 4 orders since 1974, and is added into Eq. (1). Thus, the linear relationship between GPS and groundwater level can be rewritten as:

(2)

where y is the observation year and xj is the coefficient of the long term subsidence

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Since recording the temporal period of the AaqST exceeds the unknown elements xaq and xj, the traditional least squares method is employed:

(3)

Here, A is the AaqST in a particular year (y−1974)j (y=1994 to

2006). B is the GPSST, and x represents the xaq and xj of Eq. (2).

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When we solve the linear relationship, the synthetic surface variations (Sv) can be simultaneously given by A multiplied by x; and the obtained correlation coefficient (C.C.) serves as an index which expresses the strength and direction of a linear relationship between the GPSST and Sv . In general, when the C.C. is larger than 0.5, the relationship is mainly a positive correlation and the GPSST can be roughly estimated by the Sv.

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Fig. 2. Contours of groundwater level aquifer 1 from years of 1994–2005 versus 2006Fig. 2. Contours of groundwater level aquifer 1 from years of 1994–2005 versus 2006

aquifer 1aquifer 1

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Fig. 3. Contours of groundwater level aquifer 2 from years of 1994–2005 versus 2006 Fig. 3. Contours of groundwater level aquifer 2 from years of 1994–2005 versus 2006

aquifer 2aquifer 2

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aquifer 3aquifer 3

Fig. 4. Contours of groundwater level aquifer 3 from years of 1994–2005 versus 2006 Fig. 4. Contours of groundwater level aquifer 3 from years of 1994–2005 versus 2006 17

To explore the relationship between the land subsidence and the groundwater level changes, records of two GPS observations, PKGM in the severe land subsidence area and S103 in a normal stable place, are compared with the groundwater variations of Peikang (PK) and Tungjung (TR) wells,

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(a) Time-series variations of the raw GPS, groundwater data of aquifers 2 and 3.

(b) Time-series variations of the raw GPS (shadow line) and groundwater data of aquifers 2 and 3 without seasnal effect and the correlations between them. The synthetic vertical changes are expressed as lines of solid dots without long term subsidence accounted) and open triangles (with long term subsidence accounted), respectively.

(c) The deduced long term trend of the land subsidence relative to 1974. The shadow zone represents study data covering the Chi-Chi

earthquake.

Relations between groundwater level variations and GPSPKGMvertical changes at the Paikang (PK) site.

Relations between groundwater level variations and GPSPKGMvertical changes at the Paikang (PK) site.

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Relations between groundwater level variations and GPSs103 vertical changes at the Tungjung (TR) site.

Relations between groundwater level variations and GPSs103 vertical changes at the Tungjung (TR) site.(a)Time-series variations of the

raw GPS s103 and groundwater data of aquifers 1, 2 and 3.

(b) Time-series variations of the raw GPS (shadow line) and groundwater data of aquifers 1, 2 and 3 without seasonal effect, and the correlation between them. Line with solid dots shows the synthetic vertical changes without long term subsidence estimation. The shadow zone represents study data covering the Chi-Chi earthquak

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Relations between groundwater level variations at the Tungkuang (TK) site and GPSPKGM vertical changes.

Relations between groundwater level variations at the Tungkuang (TK) site and GPSPKGM vertical changes.(a) Time-series variations ofthe raw GPS (shadow line), groundwater data of aquifers 2 and 3 without seasonal effect and the correlations between them. The synthetic vertical changes are expressed as lines of solid dots (without long term subsidence accounted) and open triangles (with long term subsidence accounted), respectively.(b) The deduced long term trend of the land subsidence at the TK site. The shadow zone exhibits study data covering the Chi-Chi earthquake

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Overdraft of groundwater in the Choshuichi Alluvial Fan has been the major mechanism for a negative impact of land subsidence.

The elevation changes in the subsidence area are primarily affected by two factors: (1)the current groundwater level variations and (2)a long term trend caused by the past excessive extraction in aquifers. The two factors can be separated and estimated by a linear relationship and temporal functions.

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In addition, the correlation coefficient between the synthetic and observed elevation changes can be served as an effective and quantitative indicator in differentiating the normal and/or subsidence area and weighting factor for various aquifers.

The results of this study can provide a useful reference of remediation strategy for the land and water resources management in active subsiding areas.

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