Taiwan is situated in the collision zone between the Philippine Sea Plate and the Eurasian Plate, where these two plates are converging at an average rate of 8.2 centimeters per year, leading to significant crustal deformation on the island. Utilizing data from GPS (Global Positioning System) measurements processed and analyzed using Bernese software, the average velocity field of crustal movements can be estimated, providing a more comprehensive understanding of crustal deformation. The combination of GPS and seafloor geodesy observations can aid in unraveling the seismic processes along plate boundaries. Due to the inability of GPS signals to penetrate seawater, acoustic methods are employed to make ocean bottom pressure (OBP) measurements, serving as a valuable and unique tool for monitoring integrated ocean currents and observing sea level changes. OBP measurements have been applied for various geophysical purposes, including ocean physics and marine geodesy. Seafloor Absolute Pressure Gauges (SAPG) based on quartz oscillation principles have been employed to record phenomena such as tsunamis, ocean tides and non-tidal sea level variations, as well as seafloor vertical deformations. These instruments play a crucial role in marine physics research. In recent years, the Academia Sinica has also conducted research in the surrounding waters of Taiwan using acoustic positioning methods for seafloor geodetic observations. In conjunction with seafloor geodetic observations, ocean bottom pressure (OBP) measurement is another method employed. The seafloor absolute pressure gauge (SAPG) developed by the Academia Sinica is composed of a Paroscientific Inc. quartz vibrating pressure sensor, integrated with an OEM data logger from RBR-Global Co. (http://www.rbr-global.com/products/bpr) and components such as the BART Boards with Regular Tuning ROUND and Acoustic Transducer that made by EdgeTech Co. The assembly of SAPG has been completed, and it has been deployed in the waters off the eastern coast of Taiwan for long-term observations. This paper will introduce the instrument assembly of SAPG, pre-deployment testing, and preliminary analysis results of the marine data.

Using continuous seismological data of Central Weather Administration (CWA) Seismographic Network and Broadband Array in Taiwan for Seismology (BATS), we applied the envelope correlation method of Mizuno and Ide (2019) to identify tectonic tremors in Taiwan 2012 to 2022. With a large number of seismic stations used in this study and removal of short-lasted events (< 10 s), we successfully detected ~7000 events. Except for the tremor zone previously observed at southern Central Range, we reported the new tremor “hotspots” across the mountain range of the island, over a distance of 200 km. Different from the fluid-rich environment previously established for tremors in subduction zones, the newly discovered tremor zones in Taiwan coincide with the spots with high geothermal heat flux, indicating that the temperature effect may be the common mechanism for tremor generation in a mountain belt of Taiwan. Other than tectonic tremors, several seismic phenomena are believed to be driven by aseismic slip process such as repeating earthquakes and earthquake swarms. The three catalogs may provide new insight into the controls of quasi-periodic aseismic slip and the role of deep fluid in their generation mechanism. We found that only < 5% of repeating earthquakes and swarms are located in 5 km of the tremor clusters, indicating that the deep-seated tremors might be engineered differently, comparing with the shallower repeaters and swarms. The spatial association is only observed underneath the southern Central Range, where the shallow swarms (< 15 km) and deep tremors (20-50 km depth) are likely interactive. We found 69-80% tremors and 86-96% swarm events occurred at the lower ground water level, respectively. This is contradictory with opposite clamping effect of hydrological/tidal stresses on thrust faulting (tremor) and normal faulting (swarm) slip. We hypothesized that in the lower crust where the thrust-faulting tremors are generated, the vertical fluid mobility could be easily elevated during the decreasing ground water level under the condition of near-lithostatic pore-fluid pressure. The upward migration of fluids may play an important role in the occurrence of swarm activities above the tremors. The continuous magnetotelluric monitoring at the location of active swarms will help us to confirm and further establish the temporal variation of fluid flow.

Nearby faults interact with each other through stress fluctuation incurred by seismic rupture, aseismic slip, and viscoelastic flow in the lithosphere. Understanding fault interactions and their temporal variation under different geometry are critical to regional seismic hazard and risk assessments. However, the complex interplay between adjacent faults is often unclear due to insufficient observations of large earthquakes with prolonged recurrence intervals. The 2022 Chihshang earthquake sequence in eastern Taiwan provides unprecedented insights into the interaction between two head-to-head thrust faults during and after a major earthquake. The Chihshang sequence was initiated by an Mw 6.5 foreshock on 17 September, followed by an Mw 7.0 mainshock 7 km to the north and 17 hours later. Based on the coseismic displacements constrained by field survey, optical satellite images, interferometric synthetic aperture radar (InSAR) data, and a dense network of Global Navigation Satellite System (GNSS) measurements, we map the major coseismic rupture on the east-verging Central Range fault (CRF), and the secondary induced slip on the west-verging Longitudinal Valley fault (LVF). The induced slip on the LVF accounts for 9-15% of the total moment release (Mw 7.1). Before the Chihshang earthquake sequence, the seismic hazard along the CRF was much overlooked due to the high seismic activity of the LVF. The 2022 Chihshang earthquake sequence demonstrates for the first time that the CRF is capable of generating earthquakes of Mw 7. The early afterslip primarily took place on the downdip extension of the CRF at great depth, indicating a contribution of ductile deformation there. Incorporating historical earthquake records over the past 120 years, we demonstrate that a rupture on the CRF or LVF reduces the stress level on the other, causing periods of seismic quiescence and an out-of-phase moment release pattern over time between the two faults. These results not only illuminate the fault geometry at the plate suture zone of eastern Taiwan, but also revise the conventional view of the nearby fault interaction. Integrating geometric complexity and fault slip history among adjacent faults in future modeling is essential for assessing realistic seismic hazards in similar structural settings.

The 2013 Ruisui earthquake is the first unequivocal evidence of the seismicity activity of the Central Range Fault (CRF) in the central Longitudinal Valley in Taiwan, and hence reveals the existence of aseismic slip on the CRF. The finite-fault coseismic model obtained from the Bayesian joint inversion of GNSS and strainmeter data suggests that the rupture area is mainly distributed on a 26 km × 22 km fault plane located at the depth of 3 to 19 km with a maximum slip of about 0.5 m. A variational Bayesian independent component analysis (vbICA) technique is applied to the detrended GNSS time series to extract postseismic deformations in the near-source region. Although the afterslip distribution was not able to be well inverted due to the lack of observation on the western side of the fault plane, using rate-and-state friction rheology to simulate the surface displacements generated by the stress-driven afterslip model, we infer for the first time the existence of a shallow velocity-strengthening region on the CRF, which is capable of hosting and sustaining aseismic transient deformations over months.

The hydrological loading displacements measured by continuous Global Navigation Satellite System (GNSS) networks can provide critical constraints on total terrestrial water storage (TWS) anomalies. We invert sparsely distributed GNSS vertical positions for daily large-scale water heights based on Slepian basis functions and devise a novel GNSS-based drought severity index (GNSS-DSI) dataset for drought characterization in Brazil. The spatiotemporal patterns of GNSS-inferred water estimates agree with the TWS observations derived from Gravity Recovery and Climate Experiment (GRACE) spherical harmonic solutions. Both GNSS and GRACE capture notable annual water oscillations in the Amazon River Basin, with an annual amplitude close to 500 mm, larger than that of 100-200 mm in the other geographical divisions. The newly-developed monthly GNSS-DSI time series correlate well with the well-accepted GRACE-DSI dataset, and 95% of stations feature moderate-to-strong correlations (greater than 0.50) between these two DSI data sets. The new drought monitoring tool solely based on GNSS-inferred water storage deviations succeeds in identifying the well-documented historical droughts and provides a quantitative characterization of these drought extremes in the four large river basins in Brazil. Our results demonstrate that a sparsely instrumented continuous GNSS network could be taken as an independent tool to remotely monitor large-scale TWS variations and to quantitatively characterize regional-scale hydrological extremes.(C) 2021 Elsevier B.V. All rights reserved.

Continuous Global Navigation Satellite System (GNSS) measurements allow us to track subtle elastic crustal deformation in the response to hydrological mass variations and provide an additional tool to independently characterize hydrological extremes (e.g., droughts and floods). In this study, we develop a time-varying GNSS imaging strategy that depends on the principal component analysis of GNSS-sensed vertical crustal displacement (VCD) in 2006-2020 and the monthly images of hydrology-induced deformation are generated for drought characterization across the contiguous United States. The first 12 principal components are selected in our time-varying imaging system, which account for 85% of the data variance. Considering that surface water loads are inversely correlated with the induced elastic vertical motions, we reverse the signs of the GNSS-imaged time series in all grids in subsequent studies (referred to as negative VCD (NVCD)). The GNSS-NVCD data generally correlate well with the water estimates from the Gravity Recovery and Climate Experiment (GRACE) and North American Land Data Assimilation System (NLDAS). Using the GNSS-imaged gridded NVCD products, we produce a GNSS-based drought severity index (GNSS-DSI) based on the climatological methodology, which is implemented by standardizing the GNSS NVCD anomalies that deviate from climatological normal. In most regions, strong linear correlations are accessible for GNSS-DSI relative to GRACE-DSI and the self-calibrating Palmer Drought Severity Index (scPDSI). The new drought monitoring tool, which is based solely on GNSS-measured vertical positions, is used for hydrological drought characterization (onset, end, duration, magnitude, intensity, and recovery); it succeeds in identifying well-documented historical droughts from the US drought monitor (USDM). Our study presents a new drought characterization framework using solely GNSS-measured hydrological loading displacements from a dense GNSS network, which has great potential to strengthen operational drought monitoring and assessment.

The correlation between physical and chemical weathering in mountainous rivers is debatable. A reliable geochemical tracer for physical erosion and chemical weathering is necessary to obtain crucial information on the relationship between weathering processes and environmental changes on the Earth's surface. Small mountainous rivers (SMRs) of Taiwan are characterized by extremely high physical and chemical weathering, and therefore, they provide the foremost natural laboratory for studying weathering processes in tectonically active high-standing islands. In this study, water samples collected from the Kaoping River, the largest river catchment in southern Taiwan, were analyzed for elemental concentrations, as well as for U and Li isotopes, to understand the weathering processes and to evaluate the use of the U-234/U-238 activity ratio, hereafter (U-234/U-238), as a probe for weathering. The average (U-234/U-238) of the Kaoping River is significantly higher than that of other rivers worldwide, and variations in (234U/238U) reflect the degree of physical and chemical weathering. The weathering environment corresponds to the weathering-limited regime, and the mountain area is a natural example of the kinetic-limited control on chemical weathering by intense physical erosion. High physical erosion predominates in the mountainous region, which increases (U-234/U-238), relatively reduces U concentrations, and inhibits chemical weathering. The decreased (U-234/U-238) and increased Na/Li and delta Li-7 downstream clearly indicate enhanced chemical weathering in the alluvial plain. Our results demonstrate that dissolved (U-234/U-238) in the SMRs of Taiwan can reflect the relative contributions of physical and chemical weathering within the river catchment. The coupling between chemical weathering and physical erosion can be observed downstream and the relationship becomes less pertinent in the mountain area due to changes in weathering kinetics. A positive correlation between (U-234/U-238) and uplift rate in the Kaoping River suggests that the riverine (U-234/U-238) is mainly controlled by uplift-induced physical erosion, which may be influenced by landslide activities in the rapidly eroding mountainous river of Taiwan.

Stress on seismogenic faults provides critical information about how much elastic energy is stored in the crust and released by earthquakes, which is crucial in understanding earthquake energetics and recurrence. However, determining post-earthquake stress states on faults remains challenging because current borehole methods are rarely applicable to damaged fault zone rocks. We applied neutron texture analysis to gouge samples of the 1999 Chi-Chi earthquake in Taiwan to infer the stress state after the earthquake. Results indicate that the clay fabric within the principal slip zone is orthogonal to the fault plane, whereas outside the principal slip zone the fabric is predominantly parallel to the bedding-parallel fault plane. We suggest that the clay fabric in the slip zone was first neutralized by the coseismic fluidization caused by thermal pressurization and later re-oriented to the new direction of post-earthquake principal stress. Such stress orientation is consistent with the orientations inferred from core-scale fault slip data and dislocation models constrained from global navigation satellite system displacements. If thermal pressurization is a ubiquitous process during earthquakes, gouge fabrics can be used to help probe the post-earthquake stress state of faults.

Technological advances in global navigation satellite system (GNSS) offer a novel environmental sensor to measure terrestrial water cycles and provide independent constraints on total terrestrial water storage (TWS) changes over various spatiotemporal scales. This study aims to develop an open-source MATLAB-based tool for inferring daily TWS changes based on the relation between GNSS annual vertical displacement and hydrological cycles. The widely used spatial-domain Green's function approach is used to estimate regional equivalent water height changes. To recover daily water storage fluctuations, we integrate the principal component analysis into our time-varying inversion strategy. To demonstrate the implementation of the inversion tool, we invert the daily TWS changes in the Pacific Northwest River Basin, the United States of America, using GNSS-measured vertical surface motions. The primary goal is to share this inversion software for hydrogeodetic scientific communities to fully use the GNSS technique for hydrological applications.

Rainfall is one of the most important triggers of both shallow debris flows and deep-seated landslides. The triggering mechanism involves the process of water infiltration into the failure zone. For deep-seated landslides, the deeper and more extensive failure surfaces delay the effect of the process and thus delay landslide initiations. The delay is difficult to assess, especially if the sites only have scarce or insufficient monitoring data. Under these circumstances, we illustrate that the occurrences of landslides can be estimated by their correlations with the phenomenological water storage index (WSI) of a given catchment. In the present study, a total of five deep-seated landslides in TienChih (4) and SiangYang (1) are investigated. The displacements of the landslides were recorded by global positioning system (GPS) and the WSI was modelled using the tank model. The result demonstrates that the WSI correlates closer in time to the landslide motion than the rainfall, and the WSI thresholds for the landslides are inferred. Thus, this technique can be applied as an associated method to evaluate landslide initiation.

Ambient noise interferometry is a powerful technique to continuously measuring crustal seismic velocity changes (dv/v) and studying crustal behaviors over time. However, the interpretation of such dv/v variations is not straightforward since multiple causes including internal (tectonic/magmatic) processes of the crust and external (environmental) factors could both affect dv/v simultaneously. To differentiate the interplay between the internal and external processes in dv/v variations is an essential step toward accurate crustal monitoring. In this study, we apply the single‐station cross‐component (SC) method to 15 selected stations from the Broadband Array in Taiwan for Seismology (BATS) to investigate the temporal evolution of crustal seismic velocities across Taiwan. We process the continuous BATS seismic recording from 1998 to 2019, construct the daily SC correlation functions, and compute dv/v values by the stretching technique in a frequency band of 0.1–0.9 Hz. We observe both strong annual dv/v variations and co‐seismic velocity drops associated with regional moderate‐to‐large earthquakes. Systematic spectral and time‐series analyses with the weather data suggest that the rainfall‐induced pore‐pressure change plays a predominant role in driving the dv/v seasonality, reflecting a diffusion process from meteoric water into shallow crust. The effects of other factors are relatively local and secondary. We also demonstrate how understanding and correcting rainfall effects could critically improve the resolution and accuracy of internal crustal damage related to earthquakes.

Long-term seafloor geodetic measurements are important for constraining submarine crustal deformation near plate boundaries. Here we present an integrated analysis of a decade of GNSS/acoustic data collected at a site 60 km to the east of northeast Taiwan near the axis of the Okinawa Trough back-arc basin. We obtained a time-series of horizontal and vertical positions based on 18 measurements from 2009 to 2019. These data reveal a southeastward movement at a rate of 43 ± 5 mm/yr since 2012 with respect to the Yangtze Plate. The horizontal motion can be explained by the clockwise rotation of the Yonaguni Block and northern Central Range. In addition, the vertical displacement of the transponder array shows rapid subsidence of 22 ± 9 mm/yr from 2012 to 2019. The fast subsidence rate and negative free-air gravity anomaly in this region indicate that crustal thinning is compensated mainly by surface deformation rather than upward migration of the Moho. Taking into account the offset in 2012 owing to the replacement of the transponder array, the horizontal position time series of our site are best explained by two linear lines with a slope change in July 2013. The timing of the velocity change coincides broadly with a change in the nearby seismicity rate and dike intrusion 150 km away from the site. Our results highlight the potential of seafloor geodesy in assessing temporal changes in deformation near the spreading center of the Okinawa Trough, which cannot be one using data from onland GNSS stations.

Recent studies suggested that transient and long‐term stress changes caused by Earth's surface processes (e.g., extreme weather events, annual variations on groundwater storages) can affect earthquake activities in the subsurface. However, these studies may be limited by the completeness of standard earthquake catalogs, especially during or right after extreme weather events. Here we apply the template matching method to build a more complete earthquake catalog in Taiwan spanning seven months before and 12 months after 2009 typhoon Morakot, which brought the highest rainfall in southern Taiwan in the past 60 years and triggered numerous landslides. We then use the enhanced catalog to investigate possible influences of typhoon‐driven Earth's surface processes (atmospheric pressure, precipitation, and erosion) on local seismicity. We find that the seismicity rate of a 40‐day earthquake sequence in northeastern Taiwan was reduced significantly right after the passage of typhoon Morakot's eye center. In the typhoon‐triggered landslide zone in southern Taiwan, we find a slight increase in background seismicity rate in the next year after Morakot, matching the results of a recent study. However, we do not observe a clear change in the Gutenberg‐Richter b ‐value in this zone, which is different from the recent study. Station outages during and right after Morakot prevents us from better understanding short‐term precipitation effect on local seismicity. Overall, except for a reduction in seismicity rate near the typhoon's low‐pressure eye center in northeastern Taiwan, we do not observe other clear seismicity changes that can be attributed to surface changes induced by typhoon Morakot.

Surface displacements measured by the Global Navigation Satellite System (GNSS) integrate the elastic response of the solid Earth to regional and local hydrologic loading signals and provide an opportunity for near-real-time monitoring of terrestrial water storage variations. Here, we estimate the spectral information of hydrology-induced vertical surface deformation based on spherical Slepian basis functions and recover the daily changes in continental water storage over mainland China from January 2010 to December 2019. Our inversion results, depending on a sparsely-distributed GNSS network, indicate the largest seasonal fluctuation of hydrological mass loads in southwestern China. The GNSS-inferred equivalent water height has an annual amplitude of up to 314 mm, which is greater than 243 mm from the Gravity Recovery and Climate Experiment (GRACE) and 150 mm from the Global Land Data Assimilation System (GLDAS). We also find notable seasonal water oscillations of 153-166 mm in the middle and lower Yangtze River Basin. This feature appears in the GNSS inversion model and GRACE Mascon solutions but is invisible in the GLDAS model, mostly indicating the existence of significant changes in surface water storage that is unmodeled in the GLDAS model. There is board agreement in the water estimates derived from multiple data sets in South China, in contrast to weak temporal coherence of various water estimates in North China. We also demonstrate the ability of GNSS for tracking the drought and wet periods, which can serve as an independent means to characterize hydrological extremes. Our study emphasizes that a sparse GNSS network can still be considered as a complementary tool to map large-scale spatiotemporal variations of water storage and benefit the hydrological community for assessing water storage changes and hydrological dynamics.

Delineation of physical factors that contribute to earthquake triggering is a challenging issue in seismology. We analyze hydrological modulation of seismicity in Taiwan using groundwater level data and GNSS time series. In western Taiwan, the seismicity rate reaches peak levels in February to April and drops to its lowest values in July to September, exhibiting a direct correlation with annual water unloading. The elastic hydrological load cycle may be the primary driving mechanism for the observed synchronized modulation of earthquakes, as also evidenced by deep earthquakes in eastern Taiwan. However, shallow earthquakes in eastern Taiwan (<18 km) are anticorrelated with water unloading, which is not well explained by either hydrological loading, fluid transport, or pore pressure changes and suggests other time-dependent processes. The moderate correlation between stacked monthly trends of large historic earthquakes and present-day seismicity implies a modestly higher seismic hazard during the time of low annual hydrological loading.

We invert daily GNSS vertical position time series for terrestrial water storage (TWS) changes and characterize their spatiotemporal patterns using various geodetic and hydrological data in Sichuan which features complex climatic and geographical conditions. Our inversion strategy depends on a variational Bayesian principal component analysis (VBPCA), which recovers the equivalent water height (EWH) changes using three principal components that explain 82% of the data variance. Various data sets from GNSS, GRACE, and GLDAS indicate an increasing seasonal water variability from northeast to southwest Sichuan. The GNSS modeling results reveal significant annual EWH amplitudes (up to 300 mm), which are largely underestimated in both GRACE‐EWH (∼120 mm) and GLDAS‐EWH (∼100 mm) products. The GNSS‐EWH also shows considerable seasonal changes in the high‐altitude plateau but subtle seasonal oscillations in the lowland basin, whereas this sharp contrast is invisible in GRACE‐ and GLDAS‐EWH primarily due to coarse spatial resolution. The GNSS inferred water products are used to determine the scaling factor for GRACE, with a value of 2.1 in Sichuan. The regional average precipitation peaks in July, about 1–2 months ahead of the maximum of monthly water heights. A bimodal feature in daily precipitation data is most notable in the low‐elevation Sichuan Basin and results in two peaks in daily GNSS‐EWH time series. Our results demonstrate that GNSS can serve as an independent tool for measuring water storage with high spatiotemporal resolution and provides additional constraints for understanding hydrological dynamics.

Ambient noise interferometry is a promising technique for studying crustal behaviors, providing continuous measurements of seismic velocity changes (dv/v) in relation to physical processes in the crust over time. In addition to the tectonic-driven dv/v changes, dv/v is also known to be affected by environmental factors through rainfall-induced pore-pressure changes, air pressure loading changes, thermoelastic effects, and so forth. In this study, benefiting from the long-term continuous data of Broadband Array in Taiwan for Seismology (BATS) that has been operated since 1994, we analyze continuous seismic data from 1998 to 2019 by applying single-station cross-component (SC) technique to investigate the temporal variations of crust on seismic velocity. We process the continuous waveforms of BATS stations, construct the empirical Green’s functions, and compute daily seismic velocity changes by the stretching technique in a frequency band of 0.1 to 0.9 Hz. We observe co-seismic velocity drops associated with the inland moderate earthquakes. Furthermore, clear seasonal cycles, with a period of near one-year, are also revealed at most stations, but with different characteristics. Systematic spectral and time-series analyses with the weather data are conducted and show that the rainfall-induced pore-pressure change is likely the main cause to the seasonal variations with high correlations. The strong site-dependency of these seasonal variations also precludes air pressure and temperature which varies smoothly in space from being dominant sources and suggests spatially-varying complex hydro-mechanical interaction across the orogenic belt in Taiwan.

To investigate the inflation characteristics of subsurface volcano-hydrothermal activity, we examine the spatial variability of the stress field in the Tatun Volcano Group (TVG) of Northern Taiwan using 1016 earthquake focal mechanisms at depths shallower than 5 km from 2012 to 2017. We determine these focal mechanisms using the data from a dense seismic network composed of 40 seismic stations in an area of approximately 10 x 10 km(2) in the NG. We find that this area is subject to the northwest-southeast trended extension and subvertical compression. The stress magnitude ratios (empty set) near the Dayoukeng area are significantly lower than that in the other areas. This feature is likely related to an inflating source buried underneath the Dayoukeng area. Our model shows good agreement between the observed and predicted empty set values when the inflating pressure source located at a depth of approximately 2 km and with the average potency ranging from 1.4 x 10 (5) to 7 x 10(5) m(3) near the Dayoukeng area. We suggest that this inflation is associated with the local volcano-hydrothermal activity. This inflation is the principal pressure source in the shallow crust (<= 5 km at depth) in the TVG and may persist for at least a decade. (C) 2019 Elsevier B.V. All rights reserved.

Global Navigation Satellite System (GNSS) instruments provide a powerful tool to investigate spatiotemporal variations in regional-scale terrestrial water storage based on the solid Earth's elastic response to hydrologic loading signals. Here, we implemented an independent component analysis-based inversion method to investigate water storage changes and hydrometeorological extremes (e.g., heavy precipitation and droughts) in Yunnan. Our time-varying inversion allows us to reproduce the spatiotemporal evolution of terrestrial water storage changes. Three independent components (ICs) were chosen in our time-varying inversion model. The first two ICs contribute to 96.9% and 2.1% of data variance, respectively, and reveal annual water storage changes at different temporal scales. The third IC slightly explains the network time series and possibly correlates with the nonlinear water changes. The averaged GNSS, Gravity Recovery and Climate Experiment (GRACE), and Global Land Data Assimilation System (GLDAS) based multi-annual seasonal water changes have good consistency in their spatiotemporal signatures. All datasets suggest a gradually increasing trend in seasonal water storage variations from northeast to southwest Yunnan. The peak annual amplitudes of similar to 355 mm in the GNSS-inferred water estimates are larger than those of similar to 167-226 mm derived from the GLDAS and GRACE models. Hydrometeorological extremes are tracked in the various water time series, and GNSS-derived water deficits contribute to hydrological drought characterization. We also find good agreement between daily precipitation anomalies, GNSS-, and GLDAS-based water changes during the 2015 winter rainstorm. Our results demonstrate that a continuously operating GNSS network is a complementary tool to remotely measure terrestrial water storage changes and to provide valuable insights into operational hydrological monitoring.