###### Abstract  
Extracting groundwater for agricultural, aquacultural, and industrial use in central Taiwan has caused large-scale land subsidence that poses a threat to the operation of the Taiwan High Speed Railway near Yunlin County. We detected Yunlin subsidence using the Sentinel-1A Synthetic Aperture Radar (SAR) by the Small Baseline Subset (SBAS) method from April 2016 to April 2017. We calibrated the initial InSAR-derived displacement rates using GPS measurements and reduced the velocity difference between the two sensors from 15.0 to 8.5 mm/a. In Yunlin's severe subsidence regions, cumulative displacements from InSAR and GPS showed that the dry-season subsidence contributed 60%-74% of the annual subsidence. The InSAR-derived vertical velocities matched the velocities from leveling to better than 10 mm/a. In regions with few leveling measurements, InSAR increased the spatial resolution of the vertical velocity field and identified two previously unknown subsidence spots over an industrial zone and steel factory. Annual significant subsidence areas (subsidence rate > 30 mm/a) from leveling from 2011 to 2017 increased with the declining dry-season rainfalls, suggesting that the dry-season rainfall was the deciding factor for land subsidence. A severe drought in 2015 (an El Nino year) dramatically increased the significant subsidence area to 659 km\({}^{2}\). Both InSAR and leveling detected similarly significant subsidence areas in 2017, showing that InSAR was an effective technique for assessing whether a subsidence mitigation measure worked. The newly opened Hushan Reservoir can supply surface water during dry seasons and droughts to counter rain shortage and can thereby potentially reduce land subsidence caused by groundwater extraction. H. Reservoir; land subsidence; Sentinel-1; SBAS; Yunlin 2019 11 2  
in Taiwan. Multiple techniques have been used for monitoring, including leveling, the use of global positioning system (GPS), multi-layer compaction monitoring well (MLCW), and interferometric synthetic aperture radar (InSAR) [4]. For long-term monitoring of subsidence, leveling and GPS are typically used. Leveling is accurate but time-consuming, and it can only provide time-lapsed displacement measurements in a network that has a limited number of benchmarks. GPS can measure three dimensional point displacements, but the precision of the vertical displacement from GPS is poor compared to that from leveling. A MLCW can detect compactions at different depths, which are important for understanding the mechanisms of land subsidence. As MLCWs are costly, only few are installed in Taiwan. These shortcomings in spatial and temporal resolutions and cost can be mitigated by InSAR, which has been widely used for monitoring land subsidence around the world. Many techniques have been used for the InSAR detection of land subsidence in Taiwan, e.g., Differential InSAR (DInSAR) [4; 5], Persistent Scatterer Interferometry (PSI) [5; 6; 7], Temporarily Coherent Point SAR Interferometry (TCPInSAR) [4], and Small BAseline Subset (SBAS) [8]. Yunlin (see Figure 1 for Yunlin's location in Taiwan and a hydrogeological profile) is a coastal county in central Taiwan and is the focus area for this paper on land subsidence. Yunlin is situated over the Chuoshui River Alluvial Fan (CRAF) in central Taiwan that has long experienced significant land subsidence [6] (significant subsidence is defined as the case in which a subsidence rate is greater than 30 mm/a). The Taiwan High Speed Railway (THSR) passes through several subsidence-stricken townships of Yunlin, causing a major concern regarding the risk of THSR operation [9]. According to a recent land subsidence report [3], Yunlin's coastal townships, such as Mailiao Township, experienced severe land subsidence from 1992 to 1999; however, after 1999, the subsidence rates in coastal areas have been significantly reduced. In contrast, from 1999 to 2017, Yunlin's inland areas, such as Huwei, Tuku, Yuanchang, and Baozhong Townships, experienced increasingly severe land subsidence [3].