## 4 Results and Discussion
### Time Series Analysis of Surface Deformation and Groundwater Level  
JC01 is in a region with severe land subsidence, where long-term overexploitation of groundwater has contributed to high subsidence risk. Its monitoring data is representative of ground deformation in high-risk zones. In contrast, JC02 is situated in a region with relatively minor subsidence, providing a comparative dataset that reflects more stable groundwater level conditions. Both the groundwater level and cumulative deformation time series exhibit significant variability and are non-stationary (Figure 7). Long-term monitoring data indicate that the groundwater level in the JC01 area fluctuates significantly, particularly within the shallow groundwater range of 5.0 to 50.0 m. Due to excessive groundwater extraction, the groundwater level has notably declined, leading to the drainage of moisture from the soil, a decrease in pore pressure, and subsequent soil compaction and surface subsidence. In contrast, the region near JC02 experiences relatively mild subsidence, with smaller variations in groundwater levels and more stable surface deformation rates. Groundwater replenishment in this area is relatively sufficient, and the hydrological conditions are stable, preventing severe soil compaction and subsidence. As shown in Table 2, there is a strong correlation between the surface deformation and groundwater level fluctuation series at both JC01 and JC02 monitoring sites, with correlation coefficients exceeding 0.5. The cumulative deformation series for both stations reaches its maximum rebound around May each year. The time-lagged cross-correlation analysis reveals maximum lag times of 2 months and 3 months for JC01 and JC02, respectively. This indicates that the cyclical fluctuations in surface deformation at both sites are synchronized with groundwater level changes, exhibiting significant resonance characteristics and a clear time-lag effect. Further verification of the identified lag times was conducted using groundwater level and surface deformation data from 2021 to 2022 (Figure 8). At JC02, groundwater level recovery occurred during January-March and June-September of 2021, and June-July of 2022. Corresponding surface deformation rebound events were observed in April-May, September-November of 2021, and September-October of 2022, indicating an approximate 3-month response lag. At JC01, groundwater level recovery occurred during February-March and May-September of 2021, and again in June-July of 2022. The asso  
Figure 7: Surface deformation and groundwater level variation in hydrological sites from 2018 to 2023: (**a**) JC01: groundwater level vs. cumulative deformation; (**b**) JC02: groundwater level vs. cumulative deformation. ciated deformation rebound phases were observed in April-May, July-October of 2021, and August-September of 2022, suggesting a shorter lag time of about 2 months. These observations are consistent with the lag times derived from the TLCC analysis. A comparison of the two regions reveals distinct deformation responses to groundwater dynamics. In JC01, the deformation amplitude is generally higher, and the response lag is shorter, suggesting a more direct and sensitive coupling between groundwater level decline and soil compaction. This may be attributed to more compressible soil layers, long-term depletion of groundwater, and reduced soil elasticity due to previous consolidation [31]. On the other hand, the longer response lag and lower deformation amplitude observed in JC02 reflect a more buffered hydro-mechanical system, likely supported by better aquifer recharge conditions, thicker unsaturated zones, and relatively stiffer geological formations [32]. These contrasting behaviors highlight the spatial heterogeneity of land subsidence mechanisms even within the same region, underscoring the importance of localized monitoring strategies and tailored mitigation approaches.