Land subsidence analysis in Taipei Basin, Taiwan, integrating Sentinel-1 InSAR, groundwater and rainfall data

Excessive groundwater extraction in the Taipei Basin, Taiwan, has resulted in significant land subsidence in the past.
In the 1950s, the Taipei basin experienced strong subsidence rates due to excessive groundwater pumping and regulations were necessary to control them.  The geodetic monitoring of ground deformation in the basin started in 1948 when the government established levelling routes to monitor the land subsidence impact. From 1975 to 1989 subsidence rates decreased and the aquifer exhibited signs of recovery turning into uplift due to elastic rebound from 1990s until early 2000s. Since then, the basin has experienced interchangeable periods of subsidence and uplift, showing the high variability and complexity with its geological setting.

In this study, we use the remote sensing technique of Differential Interferometric Synthetic Aperture Radar (DInSAR) to quantify contemporary deformation in the Taipei basin from October 2014 until October 2024 using the open access satellite images from Sentinel-1. Additionally, we have investigated the basin along the same time period from different sources of data as groundwater level, levelling data and a rainfall station in the center of Taipei.

We applied the Small BAseline Subset (SBAS) approach to retrieve the deformation time series by using multi-look and single-look interferograms. For the multi-look processing, we formed a network of interferograms with temporal baselines between 30 and 90 days with the open source software Miami InSAR time series in Python (MintPy), in order to minimize the impact of the phase bias. The single-look processing was performed by using a stack of coregistered SLC images to form a network of interferograms with a maximum temporal baseline of  120 days using the recently released open source software SARvey (Survey with SAR) for InSAR time series. The results show various subsidence deformation clusters in the basin with subsidence rates of 2-3 cm/yr, most of which also exhibit high seasonal deformations with an amplitude of 2 cm. Additionally, an uplifting signal was identified from late 2021, characterised by a  well-defined spatial boundary with a cumulative displacement of 3-4 cm. Comparison against groundwater level data suggests that this uplift signal in the center of the basin is associated with a rapid recovery going from -17 m in mid 2021 until -2 m by 2024 with a net increase of approximately -15 m. This might indicate an recharge event, however, no significant changes were identified in the rainfall data during this period, suggesting that there is reduction in the groundwater extraction activities.