Study area
The Choshui River alluvial fan is located in the mid-western coast of
Taiwan and includes the counties of Changhua and Yunlin. The Choshui
River runs from east to west through the study area (Fig. 1a), which
covers approximately 1800 km2, and bisects the two
counties. The alluvial plain is surrounded by natural geographical
boundaries that include the Taiwan Strait to the west, the Central
Mountain Ridge to the east, the Wu River to the north, and the Beigang
River to the south (Yu and Chu, 2010). The measured groundwater levels
of the aquifers indicate that the system has two major flow directions,
one from the eastern mountain area to the northwest in Changhua and the
other in a southwest direction in Yunlin.
Fig. 2 shows a hydrogeological profile of the Choshui River alluvial fan
that comprises Aquifers I, II-1, II-2, III, and IV (F1–F4 in Fig. 2)
measured from the land surface downward. Aquitards (T1–T3) separate the
aquifers. Aquitards are most relevant in the distal-fan and mid-fan
areas and gradually diminish in thickness toward the east. The
proximal-fan represents the major recharge area of the aquifer system
(Jang et al., 2008; Yu and Chu, 2010).The distribution of sediments in
the alluvial fan transitions from largely gravel, to sand, and then to
clay from the proximal fan to the distal fan. Moreover, Aquifer II is
the major aquifer of the Choshui alluvial plain because of its
large spatial extent and acceptable depth for groundwater extraction
(Liu et al., 2004; Yu and Chu, 2010).
Given the insufficient surface water supply in the alluvial fan,
residents extract groundwater to supplement their demands for
irrigation, aquaculture, and household domestic use, especially in dry
seasons. Groundwater is the major
source of water for civilian and agricultural use and represents
approximately 240,000
m3/day in the
study area. The majority of water
is used in agriculture, such as rice paddies, with 1.8 and 1.5 billion
cubic meters per year on average for Changhua and Yunlin, respectively
(Lee et al., 2018). Groundwater overdraft causes serious land subsidence
in the area, especially during droughts. An area of approximately 300
km2 experiences significant subsidence (>
3 cm/year in 2015), despite the enforced restrictions on groundwater
exploitation. Fig. 1b shows spatial pattern of observed land subsidence.