## 5 Discussion  
### Land Subsidence in the Wet and Dry Seasons  
The first discussion is about the InSAR-detected subsidence in the wet and dry seasons and its implication. First, we assessed the InSAR displacements in the two seasons by GPS measurements. Figure 12a shows the locations of five evenly distributed GPS stations for this assessment. These stations were located in Tuku, Yuanchang, Huwei, Baozhong, and Mailiao Townships, where the subsidence rates were relatively large. Figure 12b-f show the time series from InSAR and GPS. The overall patterns of the InSAR and GPS time series were similar, but some of the InSAR displacements were anomalous. At GPS stations FRES, GFES, MDES, and KTES (station names and townships are given in Figure 2), the time series showed sudden shifts with respect to the GPS time series on October 23, 2016 (roughly the transition time from the wet season to the dry season). However, in both the dry and wet season, the trends of subsidence from InSAR and GPS were similar. To show the consistency in the subsidence trend between the GPS time series and InSAR time series in different seasons, we shifted the time series of InSAR in the dry season in Figure 12b-f (red lines). The shifts in the InSAR time series in the dry season may have been caused by un-modeled, wet tropospheric delays induced by rain cells or by sudden, large increases of soil moisture that delayed radar signal propagation [30], unwrapping error [31], or due to other unknown error sources in our InSAR data processing. Figure 13a shows the cumulative areal land subsidence values derived from InSAR (starting from April 2016) in the five townships and the rainfalls (Figure 8) at about one-month intervals in Yunlin. Figure 13b shows time series of groundwater level changes in Yunlin in the five townships. In general, the lowest groundwater tables occurred in April, while the highest tables occurred in September. The range of groundwater level changes in Figure 13b varied from one location to another. The slight land rebounds in the wet season were mostly due to rising groundwater that was recharged by rain. The dry season began in October with gradually decreasing rainfalls that resulted in increased land subsidence. Table 2 shows the contributions of the land subsidence in the two seasons in the five townships. The ratios of contribution of land subsidence in different seasons are calculated as:  
\[Ratio_{\text{{wet}}}= \frac{Deformation_{\text{{wet}}}}{Deformation_{\text{{wet}}}+Deformation_{ \text{{dry}}}} \tag{5}\] \[Ratio_{\text{{dry}}}= \frac{Deformation_{\text{{dry}}}}{Deformation_{\text{{wet}}}+Deformation_{ \text{{dry}}}}\]  
Figure 11: (a) Differences between the vertical velocities from leveling and from InSAR at leveling benchmarks. (b) The scatter plot for velocities from leveling and InSAR. The gray line is the “ideal” line when the InSAR and leveling results are the same, and the cyan line shows the actual linear relationship from the least-squares fitting to the two velocity sets. where \(Ratio_{\textit{wet}}\) and \(Ratio_{\textit{dry}}\) are percentages of contributions of land subsidence in the wet and the dry seasons, respectively, and \(Deformation_{\textit{wet}}\) and \(Deformation_{\textit{dry}}\) are cumulative displacements in the two seasons. Figure 12: (**a**) The locations of five GPS stations where the time series of LOS displacements from GPS and InSAR are plotted to show subsidence contributions from the wet and dry seasons, (**b–**f**) LOS displacements from GPS (blue) and InSAR (orange) at FRES, GFES. LNJS, MDES, and KTES, overlapped with monthly rainfalls averaged from all rain gauge stations in Yunlin (Figure 8). The shifted time series (red lines, with shifted subsidence values) are given to show the consistency between the LOS displacements from InSAR and GPS. Figure 13: (**a**) Cumulative subsidence and monthly rainfalls in the most severe land subsidence locations of the five townships (see Figure 5), overlapped with the cumulative averaged rainfalls [29].