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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.

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.

On 26 August 2017, Hurricane Harvey struck the Gulf Coast as a category four cyclone depositing ~95 km3 of water, making it the wettest cyclone in U.S.HistoryWater left in Harvey's wake should cause elastic loading and subsidence of Earth's crust, and uplift as it drains into the ocean and evaporates. To track daily changes of transient water storage, we use Global Positioning System (GPS) measurements, finding a clear migration of subsidence (up to 21 mm) and horizontal motion (up to 4 mm) across the Gulf Coast, followed by gradual uplift over a 5-week period. Inversion of these data shows that a third of Harvey's total stormwater was captured on land (25.7 ± 3.0 km3), indicating that the rest drained rapidly into the ocean at a rate of 8.2 km3/day, with the remaining stored water gradually lost over the following 5 weeks at ~1 km3/day, primarily by evapotranspiration. These results indicate that GPS networks can remotely track the spatial extent and daily evolution of terrestrial water storage following transient, extreme precipitation events, with implications for improving operational flood forecasts and understanding the response of drainage systems to large influxes of water.

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Quantification of uncertainty in surface mass change signals derived from Global Positioning System (GPS) measurements poses challenges, especially when dealing with large datasets with continental or global coverage. We present a new GPS station displacement dataset that reflects surface mass load signals and their uncertainties. We assess the structure and quantify the uncertainty of vertical land displacement derived from 3045 GPS stations distributed across the continental US. Monthly means of daily positions are available for 15 years. We list the required corrections to isolate surface mass signals in GPS estimates and screen the data using GRACE(-FO) as external validation. Evaluation of GPS time series is a critical step, which identifies (a) corrections that were missed, (b) sites that contain non-elastic signals (e.g., close to aquifers), and (c) sites affected by background modeling errors (e.g., errors in the glacial isostatic model). Finally, we quantify uncertainty of GPS vertical displacement estimates through stochastic modeling and quantification of spatially correlated errors. Our aim is to assign weights to GPS estimates of vertical displacements, which will be used in a joint solution with GRACE(-FO). We prescribe white, colored, and spatially correlated noise. To quantify spatially correlated noise, we build on the common mode imaging approach by adding a geophysical constraint (i.e., surface hydrology) to derive an error estimate for the surface mass signal. We study the uncertainty of the GPS displacement time series and find an average noise level between 2 and 3 mm when white noise, flicker noise, and the root mean square (rms) of residuals about a seasonality and trend fit are used to describe uncertainty. Prescribing random walk noise increases the error level such that half of the stations have noise > 4 mm, which is systematic with the noise level derived through modeling of spatially correlated noise. The new dataset is available at 10.5281/zenodo.8184285 (Peidou et al., 2023) and is suitable for use in a future joint solution with GRACE(-FO)-like observations. [ABSTRACT FROM AUTHOR]

The existence of a distinct Capricorn component plate within the composite Indo-Australian plate has previously been questioned. If there is no Capricorn plate, the global positioning system site at Bangalore (on the Indian plate) is predicted to move relative to the Australian plate at a velocity of 10 mm [yr.sup.-1]. If there is a distinct Capricorn plate, Bangalore is predicted to move relative to the Australian plate at a velocity of 17 mm [yr.sup.-1]. Space geodetic data from Australia and India are accurate enough to provide quantitative tests of this prediction and to discriminate between alternative composite plate models. The data best fit a composite plate model that assumes the existence of a distinct Capricorn component plate moving relative to both the Indian and Australian component plates. Keywords: plate tectonics, diffuse plate boundaries, component plates, composite plates, space geodesy, marine geophysics, Capricorn plate.

Geodetic results from very long baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS) are used to estimate angular velocities between the Sierran microplate, Pacific plate, and North American plate. The Sierra-Pacific pole of rotation lies nearer to the San Andreas fault than does the Pacific-North America pole of rotation and leads to different tectonic implications than if the latter is used. The angular velocities show that the San Andreas fault system and central California Coast Ranges accommodate motion of 39 [+ or -] 2 mm/yr, mainly by strike-slip faulting. (All confidence limits following [+ or -] signs in this paper are 95% confidence limits.) Fault-normal motion is small, is mainly convergent (at rates up to 3.3 [+ or -] 1.0 mm/yr), and varies along the coast, but is divergent (at 2.6 [+ or -] 1.2 mm/yr) across San Pablo Bay and associated topographic lows across which the Sierran and Central Valley watershed drains to the Pacific Ocean. The mountain ranges tend to be larger where the fault-normal convergence rates are larger. The low convergence rate (0.5 [+ or -] 1.8 mm/yr) normal to the San Andreas fault in the Carrizo Plain differs sharply from that previously inferred (8.2 [+ or -] 1.2 mm/yr and 4.9 [+ or -] 1.6 mm/yr) by Feigl et al. (1993). The difference is due to differences between their and our elastic strain accumulation models and between how their and our Pacific plate reference frames are defined. The ranges in most places require a minimum of 4 +2/-1 m.y. of fault-normal convergence at the present rate to attain their present cross-sectional area if erosion is neglected, more if it is not. The amount of convergence previously estimated from a balanced cross section across the Diablo Range in central California requires 10 +8/-3 m.y. of convergence at the present rate. The former is consistent with widely held views about the onset of the Coast Range orogeny, but the latter is not. Both are consistent, however, with the recent plate reconstructions by S. Cande, J. Stock, and colleagues, which indicate that Pacific plate motion relative to North America changed to a more convergent direction, 20 [degrees]-25 [degrees] clockwise of its prior direction, at ca. 8 to 6 Ma and not at 3.5 Ma, as had been previously inferred. The inferred change in direction of plate motion is large compared with the present angle of convergence across the straight and narrow segment of the San Andreas fault of 0.70 [degrees]-4.7 [degrees], from which we infer that the Sierran microplate changed motion relative to North America at the same time (ca. 8 to 6 Ma) as did the Pacific plate. We further infer that the motion accommodated across the Great Basin must also have changed at the same time. We also examine the hypothesis that stable sliding occurs along the San Andreas fault and other northwest-striking strike-slip faults in central California where the fault-normal convergence rate is low or negative, and that these faults are unstable where the fault-normal convergence rate is high. Such a relationship appears to hold in general, but fails in detail. In particular, there are substantial sections of fault with small inferred rates of fault-normal convergence across which the San Andreas fault is locked. Moreover, the creeping section of the San Andreas fault (i.e., the section between Parkfield and the Calaveras junction) is the locus of greater fault-normal convergence (3.2 [+ or -] 1.4 mm/yr) than is the locked part of the fault (0.5 [+ or -] 1.8 mm/yr) south of Parkfield. Thus, this hypothesis is at best a partial explanation for the observed distribution of locked and nonlocked sections of the fault. Keywords: Calaveras fault, Coast ranges, Diablo range, San Andreas fault, San Francisco Bay, Sierra Nevada.

Geodetic observations using the Global Positioning System (GPS) and other techniques record a high rate of north-south shortening in an east-southeast-trending, 5-40-km-wide belt in northern metropolitan Los Angeles, California. Downtown Los Angeles is observed to be converging upon the southern San Gabriel Mountains at 6 mm/yr. Aside from the elastic strain that will be released during earthquakes rupturing the San Andreas and San Jacinto faults, east-west lengthening across northern metropolitan Los Angeles is minor,

We use geodetic measurements from very long baseline interferometry to estimate the motion of the Sierra Nevadan microplate, which is composed of the Sierra Nevada and the Great Valley. The motion of the Sierra Nevadan microplate relative to the North American plate is described by a right-handed rotation of 0.61 [degrees]/m.y. about lat 32 [degrees] N, long 128 [degrees] W. This Euler pole lies only 10 [degrees] southwest of the Sierra Nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis. It further predicts a velocity of the eastern edge of the Sierra Nevada (at 38.0 [degrees] N, 119.3 [degrees] W) relative to stable North America of 11 [+ or -]1 mm/yr toward N36 [degrees] [+ or -]3 [degrees] W (quoted uncertainties are plus or minus one standard error), which accounts for about one-fourth of the velocity between the Pacific and North American plates and is 25 [degrees] clockwise of many prior estimates. The velocity nearly parallels the boundary between the Sierra Nevada and the Great Basin, which implies that current motion within the Great Basin results in a rotational, noncoaxial deformation. We use this velocity to estimate how motion is distributed across the broad deforming zone taking up Pacific-North America plate motion. We find that the vector sum of strike slip along the San Andreas fault and motion of the Sierra Nevada relative to stable North America (taken up by deformation within the Great Basin) differs little from the Pacific-North America plate velocity. The difference can be described at 36 [degrees] N along the San Andreas fault by a vector of 6 mm/yr directed toward N20 [degrees] W. This vector resolves into components of 5 mm/yr parallel to the fault and 2 mm/yr perpendicular to the fault with 95% confidence intervals of 0 to 10 mm/yr and -1 to +5 mm/yr, respectively. The component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault-perpendicular shortening. We conclude that motion previously inferred to be taken up by deformation other than strike slip along the San Andres fault or deformation within the Great Basin is much smaller than previously thought.