Geodetic Assessment of Drought Intensity and Hydrological Dynamics in the Cantareira System, Southeastern Brazil

Hydrological drought, expressed as anomalously low water availability in rivers, aquifers, and reservoirs, poses a growing threat to water security in densely populated regions such as the Metropolitan Region of Sao Paulo (MRSP), Brazil. Characterizing how drought propagates into large-scale terrestrial water storage (TWS) deficits, and how these deficits translate into measurable surface deformation, remains challenging in complex, human-managed hydrological systems such as the Cantareira Water Supply System. This study conducts an integrated, multi-sensor geodetic assessment of drought-related hydrological dynamics in the Cantareira System, combining vertical/LOS ground displacement derived from continuous GPS observations and Sentinel-1 InSAR time series with terrestrial water storage anomalies (TWSAs) from GRACE/GRACE-FO, groundwater level records, reservoir storage, and meteorological drought indicators. Cross-correlation analysis reveals a strong and statistically significant coupling between GRACE-TWSA and GPS-derived vertical displacement, with correlation coefficients of r=−0.8 (Upper Tietê basin) and r=−0.6 (PCJ basin), consistent with an elastic crustal response to hydrological loading. Reservoir storage in the PCJ basin is similarly correlated with regional TWSA (up to r=0.7 for the Jaguari–Jacareí reservoir), reinforcing GRACE’s sensitivity to the main upstream storage component of the Cantareira System. Empirical Orthogonal Function (EOF) decomposition of the InSAR deformation fields, retaining the first four modes (72% of variance in Upper Tietê, 66% in PCJ), further demonstrates that deformation patterns are strongly influenced by hydrogeological controls, with distinct spatial responses between the PCJ basin, where hydroclimatic forcing is more pronounced, and the more urbanized and anthropogenically influenced Upper Tietê basin. The integrated dataset captures major drought episodes between January 2020 and Dicember 2025, demonstrating the capability of combining GPS, InSAR, and GRACE observations to quantitatively link climatic drought forcing, terrestrial water storage deficits, and surface deformation, thereby providing a robust approach for monitoring water storage changes and supporting water resource management in densely populated regions.