## 5 Conclusions  
This study integrated SBAS-InSAR technology, groundwater monitoring, and borehole-based distributed fiber-optic strain sensing to systematically investigate the spatiotemporal evolution of land subsidence in Dangshan County from 2018 to 2024, as well as its time-lagged response to groundwater level changes and subsurface strain. The main conclusions are as follows.  
Land subsidence was primarily concentrated in urbanized areas, with peak deformation rates exceeding 80 mm/year. STL decomposition revealed that the subsidence process was dominated by annual cycles, with maximum amplitudes reaching 22 mm. The timing of seasonal rebound exhibited spatial differences, occurring around mid-February in urban zones and mid-May in agricultural areas, corresponding to seasonal precipitation and irrigation activities. TLCC analysis indicated a lag of 2 to 3 months between surface deformation and groundwater level fluctuations. In contrast, subsurface strain signals at depths of 5-50 m were found to precede surface subsidence by approximately 3 months, with a peak Spearman correlation coefficient of \(\rho=0.858\). These findings demonstrate that distributed fiber-optic sensing is capable of detecting early-warning strain signals prior to measurable subsidence. The integration of InSAR, groundwater, and geotechnical data provides a robust basis for understanding subsidence mechanisms and offers valuable support for early warning, monitoring, and disaster prevention in regions affected by land subsidence.  
Conceptualization, Z.Z. and Q.H.; methodology, H.L.; software, H.L. and H.S.; validation, Q.H., J.D., and L.W.; formal analysis, Q.H. and H.L.; investigation, H.L., H.S., and J.D.; resources, L.W. and H.S.; data curation, H.L. and J.D.; writing--original draft preparation, Q.H. and H.L.; writing--review and editing, Z.Z., H.L., L.W., and J.D.; visualization, H.L. and H.S.; supervision, Z.Z.; project administration, Z.Z. and L.W.; funding acquisition, Z.Z. and L.W. All authors have read and agreed to the published version of the manuscript.  
Figure 10: Time-lagged cross-correlation analysis between optical fiber strain and cumulative subsidence (2021–2022).  
**Funding:** This research was funded by the Strategic Research and Consulting Project of Anhui Research Institute of Engineering and Technological Development Strategy, Chinese Academy of Engineering (2023-02). National Natural Science Foundation of China (Grant no. 42071085).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data underlying the conclusions of this article will be made available by the authors upon request.  
**Acknowledgments:** The authors gratefully acknowledge the European Space Agency (ESA) for providing free access to Sentinel-1 SAR data, the United States Geological Survey (USGS) for the SRTM DEM data, and the developers of the ENVI SARScape software used for SBAS-InSAR processing. The authors appreciate the anonymous reviewers for their valuable suggestions that helped improve the quality of this paper.  
**Conflicts of Interest:** The authors declare no conflicts of interest.