<p>Phase information from vertical land motion observed by the Global Positioning System (GPS) and predicted by Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) has untapped potential for advancing hydrological and geophysical studies. After correcting for effects of glacial isostatic adjustment, non-tidal oceanic and atmospheric loading using existing models, GPS observations reflect both elastic deformation due to hydrological loading and poroelastic deformation in some aquifer regions. In contrast, GRACE(FO) data are sensitive only to mass changes and thus provide a good reference of elastic loading deformation. Over aquifer systems, discrepancies in the annual phase of vertical displacement between GPS and GRACE(FO) can therefore reveal where deformation is not explained by elastic loading alone but must also be influenced by poroelastic effects. In this study, we explore this phase difference as a diagnostic tool to identify GPS stations influenced by poroelastic deformation. By comparing the annual phases of GPS observation and GRACE(FO)-predicted elastic loading deformation, we establish a threshold-based criterion for identifying poroelastically influenced stations. We applied this method to the Central Valley of California and validated the results using in-situ well observations and a terrestrial water storage inversion based on GPS data. This phase-based identification method not only provides a simple, robust, and physically interpretable method for identifying GPS stations affected by poroelastic deformation but also highlights the broader potential of exploiting GPS and GRACE(FO) phase information in hydrological and geophysical studies.</p>