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.