過去三十年來臺灣中西部因抽水導致地層下陷，且造成了不少民生公共設施危害，使得此議題開始受到關注。為了阻止日益嚴重且持續的地層下陷，政府施行了不少防禦措施並制定法規，然而，地層下陷還是不斷的擴張，尤其在乾季為甚。為了瞭解地層下陷的成因和分佈，我們利用歐洲太空總署Sentinel-1衛星的漸進式掃描地表觀測雷達進行合成孔徑雷達干涉，推估臺灣中西部下陷影響的區域，透過結合臺灣密集的GPS測站網，增加地表變形觀測的空間及時間解析度。首先將衛星所測量到在視衛星(line-of-sight, LOS)方向的地表變形量利用GPS資料校正，能夠將合成孔徑雷達干涉的時間序列資料的誤差從22.14公釐降至8.17公釐。從觀測到的資料顯示，從2015年11月至2017年11月，三個主要地層區的下陷量分別為雲林(125 mm)，彰化(50~70 mm)以及嘉義(40~70 mm)。由於Sentinel-1高頻率的拍攝，我們得以從合成孔徑雷達的時間序列中萃取出長期項以及季節項。在雲林縣水林鄉觀測到最大約兩公分的季節性振幅。我們也比較雲林縣的季節性地表變形及地下水變動的相位延遲，發現淺層地下水的回補明顯受到濁水溪和濁水溪沖積扇上游的補注影響。;Land subsidence induced by over-pumping of groundwater has been noticed in west-central Taiwan over the past three decades. The government has promulgated prevention measures and rules to slow down the deterioration in surface deformation. However, ground subsidence continues as a result of periodic demands of water resources, particularly in dry seasons. To understand the cause and distribution of subsiding areas due to environmental parameters, we take advantages of the TOPS mode (Terrain Observation with Progressive Scan) of Sentinel-1 interferometry to map the affected zones in west-central Taiwan. Along with precise surface displacement provided by a dense GPS network, the detailed of spatiotemporal variation of land subsidence can be investigated. We tie the satellite-measured land motion in line-of-sight direction with land-based GPS stations to improve the accuracy of the InSAR measurement. The root-mean-square error between the InSAR and GPS time series improves from 22.14 mm to 8.17 mm. Between November 2015 and November 2017, the main subsidence area is located in Yunlin (125 mm), Changhua (50~70mm) and Chiayi (40~70mm). Thanks to the high temporal revisit frequency of Sentinel-1 satellite, we are capable of extracting both the linear term and seasonal term from InSAR time series. The largest seasonal displacement of ~20 mm has been observed in Shuilin Township in Yunlin County. We also compare the phase delay between seasonal displacement and groundwater fluctuation across the Yunlin County. The recharge of shallow groundwater is significant from both Choushui River and proximal-fan of Choushui River Alluvial Fan.
