We focus our study in California, USA, which has a dense GNSS network and where recurrent, intense droughts put pressure on freshwater supplies. This allows us to preserve trends, annual, interannual, and multi‐year changes in TWS that were previously challenging to capture by satellite‐based measurement systems or hydrological models, alone. Our approach utilizes a continuous wavelet transform to decompose signals into their building blocks and separately invert for long‐term and short‐term mass variations. We aim to utilize the regional sensitivity to ∆TWS provided by GRACE mascon solutions with higher spatial resolution provided by GNSS observations. Here, we put forward the mathematical framework for a joint inversion of GNSS vertical displacement time series with GRACE ∆TWS to produce more accurate spatiotemporal maps of ∆TWS, accounting for the observation errors, data gaps, and nonhydrologic signals. The satellites provide highly accurate ∆TWS estimates with global coverage but have a low spatial resolution of ∼400 km. The Gravity Recovery and Climate Experiment (GRACE) is another satellite‐based measurement system that can be used to measure regional TWS fluctuations. However, these estimates require a high observation station density and minimal contamination by nonhydrologic deformation sources. Global Navigation Satellite System (GNSS) vertical displacements measuring the elastic response of Earth's crust to changes in hydrologic mass have been used to produce terrestrial water storage change (∆TWS) estimates for studying both annual ∆TWS as well as multi‐year trends. Peltier at the University of Toronto ( ) for the GIA model ICE6G_D and from the Earth‐System‐Modelling Group at GFZ for the NTAL ( ) and NTOL ( ) corrections. Corrections applied to the GNSS timeseries are from W.R. Hannes Müller Schmied ( at the University of Frankfurt provided global ∆TWS maps from the WGHM model version 2.2d. Data from the GLDASCLSM hydrological model is downloaded from. GRACE COST‐G solutions are downloaded from. GRACE CSR mascon products are downloaded from.

GNSS time series are downloaded from the Nevada Geodetic Laboratory ( ). GRACE JPL mascon data products are downloaded from.

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