We develop a shrinkage-free approach for fusing GRACE-based total water storage changes with models using wavelet multiresolution analysis. Here, we present a fusion approach based on wavelet multiresolution analysis to combine long-wavelength GRACE TWSC observations with short-wavelength measurements of TWSC from the GLDAS Noah model in the contiguous United States (CONUS). We decompose TWSC maps into building blocks at different spatial wavelengths, examine their statistical characteristics, and combine complementary components at wavelet coefficient levels. The fused TWSC is then obtained by inverse wavelet transform. A spectral analysis indicates that the fused TWSC comprises frequency content that balances spectral characteristics of both input datasets. The fused TWSC dataset possesses enhanced details in the spatial domain, while it accurately quantifies the water budget and its long-term spatial trend at basin scale during 2003–2015, showing a good agreement with GRACE estimates. Independent validation against elastic hydrologic loading deformation measured at ~2000 GNSS stations across CONUS shows similar overall performance for GRACE and fused datasets, while it outperforms GRACE in modeling ground- water storage changes in CONUS when compared to CLSM-DA.

Groundwater withdrawal can cause localized and rapid poroelastic subsidence, spatially broad elastic uplift of low amplitude, and changes in the gravity field. Constraining groundwater loss in Mexico City, we analyze data from the Gravity Recovery and Climate Experiment and its follow-on mission (GRACE/FO) and Synthetic Aperture Radar (SAR) Sentinel-1A/B images between 2014 and 2021. GRACE/FO observations yield a groundwater loss of 0.85–3.87 km3/yr for a region of ∼300 × 600 km surrounding Mexico City. Using the high-resolution interferometric SAR data set, we measure >35 cm/yr subsidence within the city and up to 2 cm/yr of uplift in nearby areas. Attributing the long-term subsidence to poroelastic aquifer compaction and the long-term uplift to elastic unloading, we apply respective models informed by local geology, yielding groundwater loss of 0.86–12.57 km3/yr. Our results suggest Mexico City aquifers have been depleting at faster rates since 2015, exacerbating the socioeconomic and health impacts of long-term groundwater overdrafts.

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. We aim to utilize the regional sensitivity to ∆TWS provided by GRACE mascon solutions with higher spatial resolution provided by GNSS observations. We focus our study in California, USA, which has a dense GNSS network and where recurrent, intense droughts put pressure on freshwater supplies. We find that our joint inversion framework results in a solution that is regionally consistent with the GRACE ∆TWS solutions at different temporal scales but has an increased spatial resolution that allows us to differentiate between regions of high and low mass change better than using GRACE alone.

We show that GPS‐derived elastic load models may not fully capture the contribution of groundwater to terrestrial water loading. Measured GPS vertical displacement rate compared to the predicted vertical displacement rate from the elastic response to groundwater loss in the Central Valley as given by a 1D-poroelastic model applying InSAR deformation rates. Blue dots show forward‐modeled vertical velocity of elastic groundwater unloading. Red stars show measured GPS vertical velocity from Nevada Geodetic Laboratory for each station. Purple dots show predicted vertical velocity using the 300 km (orange), meant to show similar resolution to what is achievable using GRACE, and Orange dots show predicted vertical velocity using the 50‐km Gaussian filtered data set. Dashed black line shows mean GPS vertical velocity error of the stations included. All rates are given in mm/year.

Frequent droughts and growing population in the Southwest US stress water resources and cause groundwater overdraft. Following droughts, the land subsidence might continue for years, even though different components of the hydrological cycle show a recovery, which is evident from groundwater levels, precipitation anomalies, and TWS variations. Residual compaction is variable depending on local hydrogeology and pumping history. This phenomenon has to be considered for integration of deformation and gravity data. 

Here we show a compilation of data sets collected for the San Joaquin Valley in California. a) Total water storage change for the GRACE data frame, interpolated from scaled and unscaled GRACE JPL mascons, and smoothed monthly precipitation anomalies (black) derived from TRMM for the period 2002 and 2017. For San Joaquin Valley, b) InSAR vertical velocity map from January 2015 to October 2017 across the San Joaquin valley. c) Time-series of groundwater level changes obtained from the USGS groundwater level network for the period 2002–2017. f) Time-series of surface deformation at selected locations (panel b).