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Atmospheric rivers (ARs) deliver significant and essential precipitation to the western United States (US) with consequential interannual variability. The intensity and frequency of ARs strongly influence reservoir levels, mountain snowpack, and groundwater recharge, which are key drivers of water‐resource availability and natural hazards. Between October 2022 and April 2023, western states experienced exceptionally heavy precipitation from several families of powerful ARs. Using observations of surface‐loading deformation from Global Navigation Satellite Systems, we find that terrestrial water‐storage gains exceeded 100% of normal within vital California watersheds. Independent water‐storage solutions derived from different data‐analysis and inversion methods provide an important measure of precision. The sustained storage increases, which we show are closely associated with ARs at daily‐to‐weekly timescales, alleviated both meteorological and hydrological drought conditions in the region, with a lag in hydrological‐drought improvements. Quantifying water‐storage recovery associated with extreme precipitation after drought advances understanding of an increasingly variable hydrologic cycle. Human communities in the western United States rely on seasonal mountain snowpack and precipitation for agriculture, industry, and the civilian water supply. Over the past two decades, increasingly severe drought conditions have plagued western states. In this study, we investigate gains in water storage related to recent atmospheric rivers, which are long and narrow corridors of atmospheric moisture associated with extreme precipitation, using an interdisciplinary approach called hydrogeodesy. Hydrogeodesy uses satellite‐ and ground‐based sensors to track subtle changes in the shape of the Earth and its gravity field caused by the redistribution of water through the Earth system. By treating the Earth as a natural scale, we can infer how much water is gained or lost from a particular area over time. Advancing our understanding of water‐resource availability, the Earth's water cycle, and the Earth's changing climate is critical for improving water‐resource management, sustaining natural ecosystems, and mitigating natural hazards. We use surface deformation to assess changes in terrestrial water storage (TWS) associated with families of strong atmospheric rivers (ARs) in 2022–2023Large jumps in mountain water storage from ARs spurred recoveries from major California droughts in 2012–2015 and 2019–2022Early‐season water‐storage gains, driven by ARs, nearly equal precipitation inputs, indicating extensive reservoir recharge We use surface deformation to assess changes in terrestrial water storage (TWS) associated with families of strong atmospheric rivers (ARs) in 2022–2023 Large jumps in mountain water storage from ARs spurred recoveries from major California droughts in 2012–2015 and 2019–2022 Early‐season water‐storage gains, driven by ARs, nearly equal precipitation inputs, indicating extensive reservoir recharge

Geodetic data show that the Los Angeles metropolitan area is undergoing 8–9 mm/year of north‐south tectonic shortening associated with the Big Bend of the San Andreas Fault. This shortening has been linked to multiple damaging twentieth century thrust earthquakes as well as possible Mw≥ 7.0 Holocene thrust events beneath central Los Angeles. To better characterize this seismic hazard, we assess how this shortening is being accommodated by interseismic strain accumulation on subsurface faults, incorporating detailed seismology‐ and geology‐based models of fault geometry and the low‐stiffness Los Angeles sedimentary basin. We find that strain accumulation on local strike‐slip faults likely contributes no more than 1–2 mm/year of the shortening. We formally invert the geodetic data for the pattern of interseismic strain accumulation on the north dipping Sierra Madre, Puente Hills, and Compton thrust faults and a master decollement. We explore the impact of the assumed material model, strain accumulation on faults to the west and east, and other model assumptions. We infer that the three faults slip at 3–4 mm/year over the long term and are currently partially or fully locked and accruing interseismic strain on their upper sections. This locking implies an annual deficit of seismic moment, 1.6 + 1.3/−0.5 × 1017Nm/year in total, which is presumably balanced over the long‐term average by the moment released in earthquakes. The depth distribution of moment deficit accumulation rate matches that of seismicity rates in Los Angeles to first order, in part, because the models incorporate the blind nature of the Puente Hills and Compton Faults. We build GPS‐based models of strain accumulation on faults beneath Los Angeles that incorporate the effect of the sedimentary basinWe infer that three major thrust faults slip at 3‐4 mm/year over the long term and are partially or fully locked on their upper sectionsThis locking corresponds to a total seismic moment deficit rate of 1.6 + 1.3/‐0.5 times 1017Nm/year, which is presumably released in earthquakes

The accelerated rate of decline in groundwater levels across California's Central Valley results from overdrafting and low rates of natural recharge and is exacerbated by droughts. The lack of observations with an adequate spatiotemporal resolution to constrain the evolution of groundwater resources poses severe challenges to water management efforts. Here we present SAR interferometric measurements of high‐resolution vertical land motion across the valley, revealing multiscale patterns of aquifer hydrogeological properties and groundwater storage change. Investigating the depletion and degradation of the aquifer‐system during 2007–2010, when the entire valley experienced a severe drought, we find that ~2% of total aquifer‐system storage was permanently lost, owing to irreversible compaction of the system. Over this period, the seasonal groundwater storage change amplitude of 10.11 ± 2.5 km3modulates a long‐term groundwater storage decline of 21.32 ± 7.2 km3. Estimates for subbasins show more complex patterns, most likely associated with local hydrogeology, recharge, demand, and underground flow. Presented measurements of aquifer‐system compaction provide a more complete understanding of groundwater dynamics and can potentially be used to improve water security. Time‐dependent vertical land motion in California's Central Valley reflects the evolution of groundwater stocks during 2007–2010Drought‐related dynamics of the aquifer‐system across the valley are investigated using deformation and groundwater level dataRegional‐scale distribution of mechanical properties of the aquifer‐system is resolved.

The most recently published model of the glacial isostatic adjustment process in the ICE-NG (VMX) sequence from the University of Toronto, denoted ICE-6G_C (VM5a), was originally developed to degree and order 256 in spherical harmonics and has been shown to provide accurate fits to a voluminous database of GPS observations from North America, Eurasia, and Antarctica, to time dependent gravity data being provided by the GRACE satellites, and to radiocarbon-dated relative sea level histories through the Holocene epoch. The authors of the Purcell et al. (2016, https://doi.org/10.1002/2015JB012742) paper have suggested this model to be flawed. We have produced a further version of our model, denoted ICE-6G_D (VM5a), by employing the same BEDMAP2 bathymetry for the Southern Ocean as employed in their analysis which has somewhat reduced the differences between our results. However, significant physically important differences remain, including the magnitude of present-day vertical crustal motion in the embayments and in the spectrum of Stokes coefficients for present-day geoid height time dependence which continues to “flatten” at high spherical harmonic degree. We explore the reasons for these differences and trace them to the use by Purcell et al. of a loading history for the embayments that differs significantly from that tabulated for both the original and modified versions of our model. The ICE-6G_C (VM5a) model of the glacial isostatic adjustment process includes a detailed description of the transition from thick-grounded ice to floating ice shelves in the Ross Sea and Weddell Sea embayments of Antarctica. It is unique in this sense among all such models. The model also reconciles virtually all available Global Positioning System measurements of vertical motion of the crust in both Northern and Southern Hemispheres. The ICE-6G_C (VM5a) Sea Level Equation solver employs a grounding line tracking algorithmThis supports thick grounded ice at LGM in the Ross and Weddell Sea embayments and on the continental shelf extending to the modern shelf breakStokes spectrum flattening at high degree is caused by interaction between grounding line tracking and rotational feedback in the embayments

We integrate Global Positioning System displacements, Gravity Recovery and Climate Experiment gravity data, reservoir water volumes, and snowpack to estimate change in subsurface water in California. We find 29% of precipitation infiltrates mountain soil and fractured bedrock each autumn and winter and is lost in the spring and summer by evapotranspiration and lateral subsurface flow either within mountain watersheds or into California's Central Valley. The Central Valley lost groundwater at 2.2 ± 0.7 km3/yr from 2006 to 2021, with 68% of the loss occurring in the southern third of the Valley. Water in Central Valley fluctuates each year by a mean of 10.7 ± 1.1 km3with maximum water in April (not August). A third of Central Valley groundwater lost during recent severe drought is recharged during subsequent years of heavy precipitation. Of the 50 km3of water entering Central Valley each year, 28 km3comes from rivers, 17 km3from precipitation, and 5 km3from mountain groundwater. We combine measurements from Global Positioning System positioning and Gravity Recovery and Climate Experiment gravity to infer change in water components in Central Valley and its source watershed, the Sacramento‐San Joaquin‐Tulare River basin better than possible with either technique separately. We find that the Central Valley has lost groundwater from 2006 to 2021 at 2.2 ± 0.7 km3/yr (95% confidence limits follow the “±” sign), with 2/3 of the groundwater loss occurring in the southern part of the Valley. We estimate the seasonal recharge and loss of subsurface water in the Central Valley. Comparison to a model accounting for precipitation, evapotranspiration, and river water entering and leaving the Central Valley suggests that deep groundwater may be flowing from the Sierra Nevada mountains into the Central Valley. The seasonal oscillation in mountain groundwater is 1/5 of yearly cumulative precipitation and 1/2 of the oscillation in total water storageThe southern Central Valley has lost groundwater at 1.5 ± 0.4 km3/yr since 2006, accounting for 68% of the total loss in the Valley50 km3of water enters the Central Valley each year; 56% is river water, 34% precipitation, and 10% groundwater (mountain‐block recharge) The seasonal oscillation in mountain groundwater is 1/5 of yearly cumulative precipitation and 1/2 of the oscillation in total water storage The southern Central Valley has lost groundwater at 1.5 ± 0.4 km3/yr since 2006, accounting for 68% of the total loss in the Valley 50 km3of water enters the Central Valley each year; 56% is river water, 34% precipitation, and 10% groundwater (mountain‐block recharge)

Globally, groundwater represents a critical natural resource that is affected by changes in natural supply and renewal, as well as by increasing human demand and consumption. However, despite its critical role, groundwater is difficult to accurately quantify as it is beneath the Earth surface. Here, we review several state‐of‐the‐art remote sensing techniques useful for local‐ to global‐scale groundwater monitoring and assessment, including proxies for groundwater extraction. These include inferring changes in subsurface water from mass changes using gravitational measurements, and analyzing changes in the Earth surface height using Interferometric Synthetic Aperture Radar, Light Detection and Ranging, Airborne Electromagnetic Systems, and satellite altimetry. Remote sensing information is often used in tandem with ground‐based observations such as hydraulic head in wells, Global Navigational Satellite System monitoring, and numerical modeling to complement the space‐based approaches. In the future, fusing different remote sensing techniques capable of operating in various environments will yield additional insight on the state and rate of use for groundwater across the globe. Accurately measuring and monitoring groundwater storage and fluxes is critical for water, food, and energy securityRemote sensing approaches such as gravitational measurements, Interferometric Synthetic Aperture Radar, Global Navigational Satellite System, lidar altimetry, and Airborne Electromagnetic Systems can yield indirect yet valuable information about groundwaterFusing multiple remotely sensed data sets or employing other tools such as numerical models increase the applicability of individual approaches Accurately measuring and monitoring groundwater storage and fluxes is critical for water, food, and energy security Remote sensing approaches such as gravitational measurements, Interferometric Synthetic Aperture Radar, Global Navigational Satellite System, lidar altimetry, and Airborne Electromagnetic Systems can yield indirect yet valuable information about groundwater Fusing multiple remotely sensed data sets or employing other tools such as numerical models increase the applicability of individual approaches

The Global Positioning System (GPS) measures elastic ground loading deformation in response to hydrological mass variations on or near Earth's surface. We present a time series of change in terrestrial water storage as a function of position in Washington and Oregon estimated using GPS measurements of vertical displacement of Earth's surface. The distribution of water variation inferred from GPS is highly correlated with physiographic provinces: the seasonal water is mostly located in the mountain areas, such as the Cascade Range and Olympic Mountains, and is much smaller in the basin and valley areas of the Columbia Basin and Harney Basin. GPS is proven to be an independent measurement to distinguish between hydrological models. The drought period of 2008–2010 (maximum in 2010) and the recovery period of 2011–2012 in the Cascade Range are well recovered with GPS‐determined time‐variable monthly water mass series. The GPS‐inferred water storage variation in the Cascade Range is consistent with that derived from JPL's GRACE monthly mass grid solutions. The percentage of RMS reduction is ~62% when we subtract GRACE water series from GPS‐derived results. GPS‐determined water storage variations can fill gaps in the current GRACE mission, also in the transition period from the current GRACE to the future GRACE Follow‐on missions. We demonstrate that the GPS‐inferred water storage variations can determine and verify local scaling factors for GRACE measurements; in the Cascade Range, the RMS reduction between GRACE series scaled by GPS and scaled by the hydrological model‐based GRACE Tellus gain factors is up to 90.5%. Dense GPS network can be an independent tool to estimate water storage changeTime‐variable water mass change of dry and wet periods can be recovered by GPSGPS can fill missing water mass measurement and provide scaling factor for GRACE

Hydrogeodesy, a relatively new field within the earth sciences, is the analysis of the distribution and movement of terrestrial water at Earth's surface using measurements of Earth's shape, orientation, and gravitational field. In this paper, we review the current state of hydrogeodesy with a specific focus on Global Navigation Satellite System (GNSS)/Global Positioning System measurements of hydrologic loading. As water cycles through the hydrosphere, GNSS stations anchored to Earth's crust measure the associated movement of the land surface under the weight of changing hydrologic loads. Recent advances in GNSS‐based hydrogeodesy have led to exciting applications of hydrologic loading and subsequent terrestrial water storage (TWS) estimates. We describe how GNSS position time series respond to climatic drivers, can be used to estimate TWS across temporal scales, and can improve drought characterization. We aim to facilitate hydrologists' use of GNSS‐observed surface deformation as an emerging tool for investigating and quantifying water resources, propose methods to further strengthen collaborative research and exchange between geodesists and hydrologists, and offer ideas about pressing questions in hydrology that GNSS may help to answer. Hydrogeodesy is a relatively new discipline within the earth sciences that analyzes the storage and movement of water at or near Earth's surface using measurements of Earth's shape, orientation, and gravitational field. In this paper, we review the current state of hydrogeodesy, focusing on the use of Global Positioning System and Global Navigation Satellite System (GNSS) to measure water masses as they change over time and in location. GNSS stations are anchored directly to Earth's crust and measure the movement of the land surface under the weight of changing (i.e., addition, loss, or movement) water masses. Herein, we describe how GNSS position time series respond to changing inputs of rain and snow and variable water loss, how they can be used to estimate the distribution of terrestrial water storage over time, and how they can improve drought characterization. We assert that GNSS‐based hydrogeodesy is an emerging tool that hydrologists can use to investigate and quantify water resources and we seek to explain and facilitate its use, propose methods to further strengthen collaborative research and exchange between geodesists and hydrologists, and offer ideas about pressing questions in hydrology that GNSS may help to answer. Hydrogeodesy analyzes the distribution and movement of terrestrial water using measurements of Earth's shape, orientation, and gravityWe review the current state of hydrogeodesy, focusing on Global Navigation Satellite System (GNSS) measurements of hydrologic loading and terrestrial water storageWe aim to facilitate the use of GNSS‐observed surface deformation as an emerging tool for investigating and quantifying water resources Hydrogeodesy analyzes the distribution and movement of terrestrial water using measurements of Earth's shape, orientation, and gravity We review the current state of hydrogeodesy, focusing on Global Navigation Satellite System (GNSS) measurements of hydrologic loading and terrestrial water storage We aim to facilitate the use of GNSS‐observed surface deformation as an emerging tool for investigating and quantifying water resources

We find seasonal horizontal crustal motions observed by GPS positioning in elastic response to heavy rainfall in the Amazon Basin and to monsoons in Southeast Asia to be consistent with those inferred from Gravity Recovery and Climate Experiment (GRACE) gravity observations of water mass loading. Solid Earth moves toward the Amazon during heavy spring rainfall and toward Southeast Asia during summer monsoons and back away from these areas 6 months later when the water load is minimum. Vertical oscillations observed by GPS and inferred from GRACE are 2 to 3 times larger than horizontal oscillation near the margins of the areas of large mass loading. Some discrepancies between GPS and GRACE are probably caused by local effects that influence GPS measurements, because the GPS sites that show significant discrepancies also do not match nearby GPS sites. However, when the load is short wavelength, the limited spatial resolution of GRACE can cause systematic misfits. Long‐wavelength hydrological mass variation causes horizontal crustal motionGPS and GRACE can resolve consistent horizontal seasonal loading deformationDiscrepancy is caused by local effects and limited GRACE's spatial resolution

We distinguish between two models of solid Earth's viscoelastic response to unloading of the Laurentide ice sheet over the past 26,000 years. The upper mantle viscosity in both models is 0.5 × 1021Pa s. The viscosity of the top 500 km of the lower mantle (670–1,170 km) in model L17 is 13 × 1021Pa s, eight times larger than the value of 1.6 × 1021Pa s in ICE‐6G_D (VM5a). In ICE‐6G_D (VM5a), viscous relaxation of solid Earth was rapid 8,000 years ago and is slow today, with present‐day uplift at the Laurentide ice center being 12 mm/yr. In L17, solid Earth relaxed more slowly 8,000 years ago but is faster today, with present uplift of the ice center occurring at 20 mm/yr. The significant difference is not due to different ice histories given that total ice loss in L17 is just 12% less than in ICE‐6G_D. We determine a comprehensive set of GPS uplift rates for North America that is more accurate than in prior studies due to (1) more sites and a longer data time history, (2) removal of elastic loading produced by increase in Great Lakes water, and (3) technical advances in GPS positioning that have significantly reduced the dispersion in position estimates. We find uplift at the ice center to be about 12 mm/yr, supporting the low value of the viscosity of the top 500 km of the lower mantle in ICE‐6G_D (VM5a), but ruling out the high value in L17. Laurentia is today rising at just ≈12 mm/yr, showing that solid Earth has to a large extent viscously relaxed in response to ice unloadingThe viscosity of the top third of the lower mantle is roughly 1.6 × 1021Pa s, not 10 × 1021Pa s or higherCorrecting Gravity Recovery and Climate Experiment data for glacial isostatic adjustment using the high viscosity value results in implausibly fast estimates of water loss Laurentia is today rising at just ≈12 mm/yr, showing that solid Earth has to a large extent viscously relaxed in response to ice unloading The viscosity of the top third of the lower mantle is roughly 1.6 × 1021Pa s, not 10 × 1021Pa s or higher Correcting Gravity Recovery and Climate Experiment data for glacial isostatic adjustment using the high viscosity value results in implausibly fast estimates of water loss

Great Lakes water levels rose 0.7–1.5 m from 2013 to 2019, increasing surface water volume by 285 km3. Solid Earth's elastic response to the increased mass load is nearly known: The Great Lakes floor fell 8–23 mm, and the adjacent land fell 3–14 mm. Correcting GPS measurements for this predicted elastic loading (1) straightens position‐time series, making the evolution of position more nearly a constant velocity and (2) reduces estimates of subsidence rate in Wisconsin, Michigan, and southern Ontario by 0.5–2 mm/yr, improving constraints on postglacial rebound. GPS records Wisconsin and Michigan to have subsided at 1–4 mm/yr. We find this sinking to be produced primarily by viscous collapse of the former Laurentide ice sheet forebulge and secondarily by elastic Great Lakes loading. We infer water on land in the Great Lakes watershed to be total water change observed by GRACE minus Great Lakes surface water smeared by a Gaussian distribution. Water stored on land each year reaches a maximum in March, 6 months before Great Lakes water levels peak in September. The seasonal oscillation of water on land in the Great Lakes basin, 100 km3(0.20 m water thickness), is twice that in a hydrology model. In the seasons, groundwater in the Great Lakes watershed increases by 60 km3(0.12 m) each autumn and winter and decreases by roughly an equivalent amount each spring and summer. In the long term, groundwater volume remained constant from 2004 to 2012 but increased by 50 km3(0.10 m) from 2013 to 2019. Great Lakes water levels rose 0.7–1.5 m from 2013 to 2019, causing the lake floor and adjacent land to fall 3–23 mmSeasonal water storage on land in the Great Lakes watershed is inferred to be maximum in March and twice as big as in a hydrology modelGroundwater in the Great Lakes watershed has a peak‐to‐peak seasonal oscillation of 60 km3and remained nearly constant from 2004 to 2019

Drought struck California during 7 of the 9 years from 2007 to 2015, reducing the state's available water resources. Pumping of Central Valley groundwater has produced spectacular land subsidence. Uplift of the adjacent Sierra Nevada mountains has been proposed to be either tectonic uplift or solid Earth's elastic response to unloading of Central Valley groundwater. We find that of the 24 mm of uplift of the Sierra Nevada from October 2011 to October 2015, just 5 mm is produced by Central Valley groundwater loss, less than 2 mm is tectonic uplift, and 17 mm is solid Earth's elastic response to water loss in the Sierra Nevada. We invert GPS vertical displacements recording solid Earth's elastic response to infer changes in water storage across the western U.S. from January 2006 to October 2017. We find water changes to be sustained over periods of drought or heavy precipitation: the Sierra Nevada lost 15 ± 19 km3of water during drought from October 2006 to October 2009, gained 18 ± 14 km3of water during heavy precipitation from October 2009 to October 2011, and lost 45 ± 21 km3of water during severe drought from October 2011 to October 2015 (95% confidence limits). Such large changes are not in hydrology models: snow accumulation in October is negligible and long‐term soil moisture change is small. We infer that there must be large loss of either deep soil moisture or groundwater in river alluvium and in crystalline basement in the Sierra Nevada. The results suggest there to be parching of water in the ground during the summer of years of drought and seeping of melting snow into the Sierra Nevada in the spring of years of heavy precipitation. Elastic vertical displacements are inverted to infer change in total water storage in mountain ranges in the western U.S. from 2006 to 2017Sierra Nevada uplift during severe drought from 2012 to 2015 is primarily solid Earth's elastic response to water loss in the Sierra NevadaMore water in the ground in California's mountains is lost during drought and gained during heavy precipitation than in hydrology models