## 5 Discussion
### Newly Identified Subsidence Spots in Yunlin by InSAR
The rainfall in the dry season is the average rainfall from the previous October to the current April. Figure 15(a) shows that since 2013, the subsiding area increased and reached a maximum in 2015 (658.6 km\({}^{2}\)). Note that 2015 was the record drought year in Taiwan. The rainfall in the dry season of 2015 (October 2014 to April 2015) was only 105 mm, compared to 158 mm in the dry season of 2014. Figure 15(b) shows the Oceanic Nino Index (ONI) from April 2010 to April 2017 [34], indicating that 2015-2016 is an El Nino year (with ONI above 0.5 in the five consecutive overlapping three months) that may be associated with the droughs and floods in 2015 in the Pacific coastal regions [35], including Taiwan (low rainfall in the dry season, Figure 15(a)). It is likely that the continuous drought in 2015 not only increased the magnitude of subsidence, but also widened the area of significant subsidence. The rainfalls in Figure 15(a) suggest that, from 2011 to 2017, the areas of significant subsidence were negatively correlated with the dry-season rainfall (the correlation coefficient was -0.85). For example, in 2011, 2014, and 2017, the dry-season rainfalls were small, causing relatively large areas of significant subsidence (average: 360 km\({}^{2}\)). By comparison, in 2012, 2013, and 2016, the dry-season rainfalls were larger and the areas of significant land subsidence were smaller (average: 122 km\({}^{2}\)). Both InSAR and leveling detected almost the same areas of significant subsidence in 2017 (Figure 15(a); the difference was about 18 km\({}^{2}\)). Table 3 shows the areas with subsidence rates \(>\)30 mm/a in Yunlin. Table 3 shows the area of significant subsidence from InSAR (215.5625 km\({}^{2}\)) was close to the area from leveling (221.3325 km\({}^{2}\)). The two differ by only 2.6%, suggesting that InSAR is reliable for detecting the area of subsidence. Figure 17 compares the areas of significant subsidence from InSAR and from leveling. Both results showed that Tuku, Yuanchang, and their nearby townships, experienced significant subsidence. The InSAR velocity map showed significant subsidence near Western Mailiao areas that were not seen in the leveling velocity map (see also Section 5.2). However, InSAR did not detect the full spatial extent of significant land subsidence in Southwestern Yunlin near Sihu Township, because this area is largely covered by rice fields (Figure 16(a) vs. Figure 16(b)). As stated in Section 1, recent climate change has led to irregularities in the frequency, intensity, and magnitude of rainfall [2]. Such irregularities across seasons and across years (Figure 16) make it difficult to predict the amount of available surface water and short-term trends of land subsidence. Nevertheless, it is clear from the seasonal analysis in Section 5.1 that the subsidence in the dry season contributed the most to the annual subsidence (i.e., total subsidence in a year). Since Hushan Reservoir can supply surface water to counter the shortage of rain during dry seasons and droughts, it can potentially reduce the unpredictability of land subsidence in Yunlin. Figure 16: (**a**) Areas of significant subsidence from leveling (blue bars) from 2011 to 2017 [3] and from InSAR (green bar) in 2017 (this study), overlapped with annual rainfalls for the whole year (blue line) and the dry season (red line) in Yunlin [29]. (**b**) The ONI from April 2010 to April 2017 [34], the red-shaded period corresponds to the 2015 El Niño event and the blue-shaded periods show periods with ONI below \(-0.5\). Figure 17: Areas of significant subsidence (rate >30 mm/a) from (**a**) leveling and (**b**) InSAR. The dashed circles show significant subsidence in (**a**) Sihu Township (strong signal from leveling, but weak signal from InSAR) and (**b**) Mailiao Township (detected by InSAR, but not by leveling).