Donald F. Argus

We estimate changes in lake levels and volumes from October 1992 to April 2026 each month for 14,071 natural lakes and artificial reservoirs larger than 10 km2 contained within the surface water and ocean topography (SWOT) mission’s prior lake database (PLD). We use altimetric lake level estimates from G-REALM, DAHITI, HydroWeb, the SWOT mission, and the Global Lake Water Storage dataset’s hypsometric relationships to establish a complete timeseries of lake water storage over the study interval interpolating and extrapolating a seasonal where data is missing. From this information we construct three data products to disaggregate global lake water from GRACE terrestrial water storage: 1) timeseries of lake level and storage over the study interval, 2) maps of change in lake water for the 11,386 natural lakes and 2,685 artificial reservoirs on a 16th degree global grid each month, 3) lake-generated changes in equivalent water thickness as seen by GRACE for both JPL’s 3-degree mascon solution and for the spherical harmonic solutions of GRACE. Of the 1706 3-degree terrestrial mascons 1109 (65%) are affected by lake water; 808 mascons (73%) experience a decrease in weighted root mean square (WRMS) when lake water is removed, while 301 mascons (27%) experience an increase in WRMS. This dataset shows a 14-fold increase in the number of lakes studied compared to our previous lake model as well as an extension to 2026 (Gaastra et al. 2025). Correspondingly the number of mascons affected roughly doubles and the percentage of mascons experiencing an increase in WRMS has increased as coverage of high latitude lakes is improved. High latitude and high elevation regions experience an increase in WRMS when lake water is removed primarily due to glacial and snow melt fed lakes which experience their maximum seasonal water during the summers when terrestrial water storage is low as snow has melted. We find the regions where lake water disagrees substantially with GRACE/GRACE-FO terrestrial water are mainly concentrated in high latitude regions, alpine regions, and arid near costal mascons.

We use the vertical displacements of 331 Global Navigation Satellite System (GNSS) antennas provided by the Nevada Geodetic Laboratory (NGL) between April 2002 and May 2022 and located in the European Seine River basin. We invert these displacements using elastic loading theory for changes in Terrestrial Water Storage (TWS). We adopt a benchmarking approach to classify GNSS stations as reliable for estimating TWS changes on two pre-defined temporal-scales: seasonal and long-term. We find 302 and 183 of the 331 GNSS stations to be reliable for seasonal and long-term temporal-scales, respectively, and then use them for GNSS inversion. We compare the GNSS-inverted TWS using the benchmarking approach with initial and common approaches that use all available and pre-selected set of GNSS stations for inversion, respectively. We show that the GNSS-inverted TWS using the benchmarking approach is most consistent with ERA5-Land TWS in the western part of the basin, where the densest network of GNSS stations is available. It also detects long-lasting droughts reported earlier with great reliability, providing an alternative to hydrological models. We then recommend for future hydrogeodetic studies to benchmark the GNSS stations for the two temporal-scales, which will improve the comparison with other datasets, enhancing the interpretation of long-term and seasonal hydrological events.

Groundwater flowing within mountainous regions is a critical water source for mountain and adjacent low‐elevation communities and ecosystems. Despite their importance, quantifying storage changes within these groundwater systems remains a challenge. To evaluate the fate of mountain groundwater and the role of extremely wet winters in replenishing groundwater storage, we use high‐precision Global Navigation Satellite Systems surface displacements to constrain changes in groundwater storage within the Sierra Nevada and Cascade mountain ranges, two important mountainous regions spanning much of the westernmost US. We find that groundwater storage has significantly declined over the past two decades, comprising over 90% of the total water storage lost in these regions, associated with recurrent, severe, multi‐year droughts. However, extremely wet winters can replenish groundwater by more than twice the average annual flux over short periods, driving these systems from historical lows to above normal conditions. Furthermore, discharge from these groundwater systems is 2–3 times less variable than recharge, indicating they release a relatively constant amount of water year over year, allowing for surplus storage following extremely wet winter periods. These findings show that extreme precipitation (snow + rain) rapidly recharges mountain groundwater and can offset multi‐year losses. Moreover, mountain groundwater systems can retain storage gains for at least 1 year post‐event, providing a durable source of streamflow, agricultural, and municipal water supply. As extremely wet winters are predicted to become more intense and frequent, we hypothesize they may help offset groundwater loss due to aridification, sustaining ecosystems and buffering against a new, drier climate normal.

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Steady-state secular motions of the Earth’s surface (i.e., motions not influenced by transient processes such as earthquakes or volcanic eruptions) reflect plate boundary interseismic strain accumulation, plate motions, post-glacial rebound, sea-level rise, or dynamic topography. Over the past three decades, the expansion of Global Navigation Satellite System (GNSS) networks has densified the number and spatial coverage of station position and velocity observations with improved measurement accuracy. In this study, we focus on horizontal motions and aim to create the most up-to-date, spatially dense velocity field. We gather secular velocities at ~35000 unique GNSS stations distributed globally, covering both tectonically active and stable regions. Roughly 18000 velocities are determined at the Nevada Geodetic Laboratory (NGL) from time-series of mostly continuous GNSS observations. However, the spatial coverage of the NGL velocity solution suffers from the absence of available RINEX (Receiver Independent Exchange Format) data in places such as most of the India-Eurasia collision zone. We therefore compile about 17000 additional continuous and campaign GNSS velocities from ~400 published studies and transform these auxiliary velocities onto the NGL velocity solution through a least-squares inversion. For several large earthquakes with sufficient GNSS observations (e.g., 2004 M9.1 Sumatra, 2011 M9.1 Tohoku, 2010 M8.8 Maule, and others), we correct GNSS time-series and auxiliary velocities for postseismic viscoelastic deformation using forward modeling based on a gravitational spherical Earth with a 1D rheological structure. For other earthquakes, we correct the GNSS time-series by removing the postseismic time-series fitted by an empirical logarithmic function. Additionally, we develop and apply a velocity outlier detection and removal algorithm to generate our final global velocity database. Our velocity field is an update to the compilation from the 2014 Global Strain Rate Model (GSRM) and greatly extends the scope of existing global velocity solutions. Our new database will be used to produce the next GSRM and to provide a starting velocity field for future integration with InSAR analysis.

Developing a comprehensive understanding of global water resources – and their responses to extreme events and variations in climate – requires integrating diverse modeling and observational approaches across disciplines, with geodesy playing an increasingly integral role. Geodetic measurements and models, including those tracking solid Earth deformation caused by mass redistribution in the hydrosphere, provide key insights into water-cycle processes and systems. This study focuses on Global Navigation Satellite System (GNSS) data from the western United States to examine recurring cycles of severe drought and rapid recovery over the past two decades. Interdisciplinary evidence from hydrology, meteorology, and geodesy suggests that these cycles are strongly associated with variability in the frequency and intensity of seasonal atmospheric rivers (ARs). During Water Year 2023, GNSS data revealed record-breaking water-storage gains in California’s Sierra Nevada mountains and Sacramento-San Joaquin-Tulare (SST) river basins, driven largely by an exceptional series of powerful ARs. In the six-month period between October 2022 and March 2023, water-storage gains in these regions surpassed those of any prior year in the analysis, which began in 2006, with an estimated 80% of the gains delivered by ARs. By early spring 2023, we infer that approximately half of the water-storage gains had infiltrated the subsurface, providing a critical water resource for downstream communities through processes such as mountain block recharge. Our analysis further shows that hydrological drought and recovery, based on GNSS estimates of total water-storage changes, respond more slowly to precipitation patterns than meteorological drought and recovery, highlighting the insulation of subsurface pools from surface fluxes. We find that years with heavy precipitation can help to sustain storage levels into subsequent years with less precipitation. Moreover, as geodetic observational accuracy improves, a deeper understanding of the assumptions, limitations, and opportunities inherent in our models is necessary. To assess the precision of GNSS-informed water-storage estimates, we compare results derived from independent GNSS position estimates and inversion techniques. Additionally, we provide updates on recent progress in developing community-available modeling tools and investigating the effects of 3-D heterogeneities in Earth structure on deformation responses to surface mass loading.

Groundwater flowing through the fractured bedrock composing most mountain ranges has been increasingly recognized as a vital source of freshwater for both low-elevation communities and mountain ecosystems, maintaining streamflow and constituting a large portion of recharge to lowland aquifers used to support human activities. Despite the growing awareness of groundwater’s role in mountain hydrology and the potential impacts of climate change on mountain groundwater, it remains a challenge to study the dynamics of mountain aquifers, largely due to the low density of observational wells and challenges in characterizing the mountain block over large areas and depths. Here, we report on a new approach to characterize the flow and hydraulic properties of mountainous aquifers at a mountain range scale. We utilize high-precision Global Navigation Satellite Systems (GNSS) observations of vertical crustal displacement produced by the redistribution of freshwater on or near the Earth’s surface to estimate changes in groundwater storage within the Sierra Nevada and Cascades Range of the western United States with high spatial (10s of kilometer) and temporal (daily) resolution over the past two decades. We find that on average groundwater annual recharge is less than discharge, driving long-term declines in groundwater storage over the last 19 years. Furthermore, we find groundwater recharge to be up to 3x more variable than groundwater discharge in these mountainous areas, suggesting that mountain aquifers release a relatively constant amount of water to streams and adjacent lowland aquifers despite fluctuating recharge conditions. Utilizing identified periods of groundwater discharge, we characterize the hydraulic conductivity, storativity, and flow path length of these groundwater systems using fluid diffusion models in combination with our GNSS-inferred groundwater estimates. Our initial estimates of these parameters reveal relatively high values of bedrock conductivity (~1x10-3-1x10-4 m/s) relative to expected values based upon each region’s bedrock lithology, suggesting that areas with highly fractured bedrock as well as saprolite may exert a strong control on groundwater discharge at the mountain range scale. Furthermore, our results indicate that groundwater flow paths can span lengths on the order of 100s-1000s of meters, supporting the notion that groundwater can flow over extended areas supporting recharge at both a local and regional scales. Our work seeks to provide a new set of tools for hydrologists to investigate these often poorly understood systems.

Extreme winter precipitation events, associated with frequent and intense atmospheric rivers, deposit significant quantities of water in mountain regions over short periods of time. Precipitation is forecast to become more variable as climate change intensifies; however, it is unclear how that will affect mountain aquifer recharge. Here we use high-precision Global Navigation Satellite Systems (GNSS) surface displacements and elastic deformation models to surface loading to estimate total water storage changes. Using independent estimates of water stored within shallow subsurface and surface reservoirs, we isolate changes in mountain groundwater storage in two important mountain regions of the western US at high spatial (~30km) and temporal (~ 1 week) resolution. We find that groundwater storage is the dominant component of long-term total water loss within the Sierra Nevada and Cascades, composing up to 95% of the total water lost over the past two decades. However, extremely wet winters, such as that of 2023, can recharge groundwater storage by more than twice the average annual amount, driving the state of groundwater storage from historical lows to above or near-normal conditions over relatively short periods. Further, we find gains in groundwater storage associated with these events are relatively durable, persisting over several proceeding years following the extreme recharge event. Mountain aquifers have been increasingly recognized as a dynamic and critical source of water storage and release to adjacent low-elevation communities; however, persistent declines in mountain aquifer storage have been observed across the western US over the past two decades. In a future with increasingly variable precipitation, the strong influence of extreme events may act to maintain mountain groundwater, sustaining ecosystem health and buffering adjacent areas against drought conditions in between events.

We are strengthening the application of GPS's capability to estimate change in total water using measurements of elastic displacements of Earth's surface; breaking down total water into its components such as snow, soil moisture, and groundwater; and integrating GRACE gravity data to infer change in total water in groundwater basins.In California's Sierra Nevada, GPS each day tracks the dumping and dissipation of storm water. In Water Year 2023, total water increased abruptly during each of two sequences of snow-dominated atmospheric rivers. Subsurface water, which we take to be total water inferred from GPS minus snow water equivalent, to rise in early January at the time of the first AR sequence, remain constant from late Jan through March (with no increase during the second AR sequence), and rise from April to June as the snowpack melts. Subsurface water increases in the Sierra Nevada by 0.6 m from Oct 2022 to Jun 2023, 45 per cent of cumulative precipitation of 1.4 m. Such a big rise in subsurface water begins to rejuvenate the Sierra Nevada critical zone (Earth's living outer layer between the top of the trees and the bottom of groundwater) and to replenish subsurface water lost during the prior 3 years of drought from 2020 to 2022.Change in total water in California's Central Valley can be determined neither by GRACE alone nor GPS alone. There GPS records primarily Earth's poroelastic response, from which water change is difficult to infer. GRACE cannot distinguish water change in Central Valley from water change in the Sierra Nevada without assuming a hydrology model. We integrate GPS elastic displacements and GRACE gravity to estimate water change in the Central Valley. In the rigorous inversion, GPS determines water change in the Sierra Nevada and Coast Ranges and the remaining water change from GRACE is placed in the Central Valley. We find Central Valley groundwater increased by 0.75 m in the first nine months of Water Year 2023 (the biggest gain ever recorded), replenishing more groundwater than lost during the prior 3 years of drought.

Increasing climatic and human pressures are changing the world's water resources and hydrological processes at unprecedented rates. Understanding these changes requires comprehensive monitoring of water resources. Hydrogeodesy, the science that measures the Earth's solid and aquatic surfaces, gravity field, and their changes over time, delivers a range of novel monitoring tools that are complementary to traditional hydrological methods. It encompasses geodetic technologies such as Altimetry, Interferometric Synthetic Aperture Radar (InSAR), Gravimetry, and Global Navigation Satellite Systems (GNSS). Beyond quantifying these changes, there is a need to understand how hydrogeodesy can contribute to more ambitious goals dealing with water‐related and sustainability sciences. Addressing this need, we combine a meta‐analysis of over 3,000 articles to chart the range, trends, and applications of satellite‐based hydrogeodesy with an expert elicitation that systematically assesses the potential of hydrogeodesy. We find a growing body of literature relating to the advancements in hydrogeodetic methods, their accuracy and precision, and their inclusion in hydrological modeling, with a considerably smaller portion related to understanding hydrological processes, water management, and sustainability sciences. The meta‐analysis also shows that while lakes, groundwater and glaciers are commonly monitored by these technologies, wetlands or permafrost could benefit from a wider range of applications. In turn, the expert elicitation envisages the potential of hydrogeodesy to help solve the 23 Unsolved Questions of the International Association of Hydrological Sciences and advance knowledge as guidance toward a safe operating space for humanity. It also highlights how this potential can be maximized by combining hydrogeodetic technologies simultaneously, exploiting artificial intelligence, and accurately integrating other Earth science disciplines. Finally, we call for a coordinated way forward to include hydrogeodesy in tertiary education and broaden its application to water‐related and sustainability sciences in order to exploit its full potential.