## 5 Discussion
### Newly Identified Subsidence Spots in Yunlin by InSAR  
The next subject of discussion is the ability of InSAR to identify spots of subsidence not detected by leveling, thanks to InSAR's much higher sampling density compared to leveling (Figure 2). Here, we focused this ability of InSAR on the places that probably experience severe land subsidence, but have only few leveling benchmarks. The red rectangular box in Figure 14 shows one of the townships with the most severe land subsidence is Baozhong Township, and the zoom-in map is shown in Figure 15. The village of Shinhu in Baozhong Township shows a spot of potential large land subsidence. Note that the largest subsidence rate around Baozhong was 67.6 mm/a. We conducted an in-situ investigation in Shinhu Village and conclude that, although Shinhu is largely a residential and farming area, it houses a steel factory with a location that is consistent with the location of large land subsidence seen in Figure 15 (within the dashed circle). Thus, we suspected that the factory was the source of the land subsidence. It was suspected that the loading of steel and unauthorized groundwater pumping inside the factory contributed to soil compaction around the factory. Thus, it is recommended that a densified network of leveling benchmarks be deployed here to detect land subsidence for further actions. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Township**} & \multicolumn{2}{c}{**Wet Season**} & \multicolumn{2}{c}{**Dry Season**} \\ \cline{2-5}  & **Cumulative** & **Ratio (\%)** & **Cumulative** & **Ratio (\%)** \\  & **Displacements (mm)** & **Displacements (mm)** & **Ratio (\%)** \\ \hline Yuanchang & \(-\)16.97 & 27.52 & \(-\)44.70 & 72.48 \\ Tuku & \(-\)17.68 & 26.25 & \(-\)49.67 & 73.75 \\ Huwei & \(-\)18.44 & 37.22 & \(-\)31.11 & 62.78 \\ Baozhong & \(-\)18.12 & 39.23 & \(-\)28.07 & 60.77 \\ Mailiao & \(-\)44.26 & 80.94 & \(-\)10.42 & 19.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The contributions of land subsidence in the wet and dry seasons in the most severe land subsidence locations of the five townships. the InSAR result (Figure 14) shows large, evenly distributed subsidence rates in this industrial zone, with a maximum rate of about 82.3 mm/a. Area of Significant Subsidence and Rainfall: The Potential of Hushan Reservoir in Reducing Land Subsidence  
Here, our final discussion focuses on whether the InSAR technique has the potential to identify areas with significant subsidence and the relationship between land subsidence and rainfall. In Taiwan, the area of significant land subsidence (subsidence rate \(>\) 30 mm/a) is an indicator to judge whether a land subsidence prevention policy is successful. In order to reduce the use of groundwater and to mitigate land subsidence in Yunlin, the Taiwan government encourages farmers to use surface water supplied through waterways, and to grow crops less dependent on water [32]. In central Taiwan, the storage capacities of reservoirs have been substantially reduced by siltation in the reservoirs, resulting in decreased surface water supplies. To compensate for the reduction of surface water supply and shortages in the dry season, a new reservoir in Yunlin, called Hushan Reservoir, was opened in July 2016, and started to supply water in January 2019. Hushan Reservoir cost 0.66 billion USD to construct and its storage capacity is about 51,390,000 m\({}^{2}\)[33]. First, we examined the areas of significant subsidence in recent years using leveling data in order to see whether the government's past policies could mitigate land subsidence. Figure 15(a) shows the areas of significant land subsidence in Yunlin from 2011 to 2017 from leveling data [3] and annual rainfall and rainfall in the dry season from rain gauges [29]. As the leveling data were collected every April (once a year), an annual subsiding area represents the area of subsidence during the one year from the previous April to the current April.