Abstract
Interferometric Synthetic Aperture Radar (InSAR) products from the European Ground Motion Service (EGMS) are released through successive, partially overlapping temporal windows, offering the potential to construct ultra-long-term (ULT) deformation records, i.e., continuous records spanning multiple releases and exceeding the 5–6-year extent of any individual release. However, direct combination of independently processed releases may introduce artificial offsets and trend changes. This study presents an enhanced Temporal Alignment via Statistical Testing (TAST) procedure to harmonize four EGMS Ortho releases (R1–R4; January 2016–December 2024) over the Lecco–Como area of northern Italy in both Up–Down and East–West components. The method applies translation and linear velocity correction optimized through a joint Cramér–von Mises/RMSE (Root Mean Square Error) objective while preserving within-release deformation evolution. The resulting ULT records were segmented at three regional groundwater-level (GWL) turning points (TPs) and analyzed using the introduced Hydro-Deformation Regime Concordance (HDRC) metric. The results show that inter-release velocity discrepancies reached 7.12 mm/y in U-D and 11.21 mm/y in E-W, demonstrating their potential to substantially alter apparent long-term deformation regimes. Vertical deformation was predominantly anti-concordant with GWL turning behavior, with 59.7–69.9% negative HDRC, whereas E-W responses evolved from strong positive HDRC dominance to near-balanced conditions. HDRC exhibited significant spatial organization (Moran’s I up to 0.51) and geological association (Cramér’s up to 0.31), while geological-group response rankings remained persistent across successive hydro-regime transitions (Kendall’s W up to 0.88). A screening of potential alternative regional drivers identified no clear evidence that seismicity, capable faults, or isolated extreme precipitation events accounted for the observed multi-TP patterns. These results demonstrate that systematic multi-release EGMS harmonization enables temporally consistent long-term monitoring and reveals non-stationary, spatially structured, and geologically conditioned deformation responses associated with groundwater dynamics.