Overview of subsidence and drawdown
Fig. 3 displays the correlation between subsidence and variations in groundwater level. The subsidence–drawdown relation is nonlinear (correlation coefficient = 0.27, 0.54, 0.57, and 0.54, respectively, in each of the four aquifer layers). Groundwater drawdowns in Layers 2–4 are similar (correlation coefficient = 0.84 in Layers 2 and 3; 0.78 in Layers 2 and 4). Drawdowns in Layer 1 (unconfined aquifer) vary spatially over shorter distances compared to those of the confined aquifer (Layers 2–4). Figs. 1b and 4 show the spatial distribution of annual observed subsidence and accumulated drawdown, respectively, for 2015, when the maximum annual permanent subsidence is approximately 7 cm (Fig. 1b). The spatial distribution of subsidence reveals a bowl in the central area of Yunlin (Hwang et al., 2008). Within the study area, the maximum accumulated drawdown in Layers 1–4 ranges from 15–20 m (Fig. 4). The cumulative drawdowns implies that similar cones of depression are located in Layers 2–4 in the southwest distal fan area, near the coast. Drawdown cones are located along the distal fan and mountain areas in Layer 1. Compared with the four-layer drawdown pattern, the drawdown is greatest in Layer 2 in the study area. In addition, drawdowns in Layers 2–4 are highly correlated (Fig. 3), while Layer 3 is most closely correlated to the subsidence of the four-layer drawdown model (Fig. 3).
The compression behavior of clay agrees with Terzaghi’s consolidation theory (Liu et al., 2004). Land subsidence with inelastic compression continues as water levels continuously decline. The inelastic compression of fine-grained aquifer interbeds is likely responsible for the vast majority of subsidence problems, based on the compressibility and total thickness of fine-grained deposits throughout the region (Galloway et al., 1998b; Galloway and Sneed, 2013). The cone of maximum drawdown occurs in the distal fan area, whereas the current major subsidence bowl occurs inland. Consequently, the cone of maximum drawdown fails to fully coincide with the location of the subsidence bowl (Erban et al., 2014). In the coastal area or distal fan area, the rate of pumping is proposed to decrease to stabilize or reduce water level declines. This subsequently reduces land subsidence, given that the largest occurrence in the inland area induced by heavy withdrawal of groundwater. This is a sensitive environmental concern because the Taiwan High Speed Rail is constructed through the central subsidence area, which might pose a serious threat to its operation (Hwang et al., 2008; Tung and Hu, 2012).