In recent years, drought events have become more frequent and severe, affecting human life, the environment, and industry. As a result, monitoring drought characteristics such as patterns, occurrences, intensity, categories, and duration presents a crucial challenge for scientists. These characteristics are usually estimated using hydrological or climate models, which, however, frequently fail to capture actual changes. Consequently, droughts are under-, overestimated or not captured. As remote sensing advances, a near real-time drought assessment would be successfully enabled using data provided by geodetic techniques such as the Gravity Recovery and Climate Experiment (GRACE) and the Global Positioning System (GPS). However, the limitations in GRACE and GPS techniques, data products or their quality, such as the spatial resolution, leakage effect, background models used for GRACE observations processing or systematic errors of GPS technique may face limitations in accurately capturing essential information on drought characteristics. In our study, we assess the impact of errors embedded in GRACE and GPS data on determined droughts. We calculate uncertainties of the Drought Severity Index (DSI) determined from GRACE-derived and GPS-observed vertical displacements. We investigate a number of ways to designate errors, starting with spherical harmonic coefficients errors, TWS errors associated to gridded GRACE mascons, errors in the positions of permanent GPS stations, by GRACE TWS variance-covariance matrices to errors in combining field using the Three-Corner-Hat (TCH) method. We find that maximum error values occur in nearly 30% of drought periods, showing that they are over- or underestimated by geodetic data. For the variance-covariance method, uncertainty of DSI determined from GRACE are identical for the entire European region. On the other hand, we observe that uncertainty of DSI determined from GRACE for both SH errors and mascon TWS errors are coherent in time. Values of GPS-DSI uncertainty are mostly close to zero, although we also identify significant peaks in series over drought and flood periods as sensed by GRACE-DSI. The results obtained for several different methods of error assessment are the next step in examining the reliability of drought characteristics, which can be valuable for decision makers.

How to cite: Lenczuk, A., Ndehedehe, C., Klos, A., and Bogusz, J.: On the quality of drought characteristics by GPS and GRACE signal errors, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2000, https://doi.org/10.5194/egusphere-egu25-2000, 2025.

Glaciers are a dynamic part of the Earth's system. They are vulnerable to the impacts of climate change, making them a dynamic and rapidly transforming element of the Earth system. The consequences of these changes extend far beyond the polar and alpine regions, affecting ecosystems and water resources globally. Glaciers are important for water management, acting as natural reservoirs and providing millions of people with fresh water. However, their retreat can disrupt water supplies, increase flood risks, and lead to hazards such as rock moraine instability. These challenges underscore the importance of understanding this part of the ecosystem. Monitoring and measuring glacial environments are essential not only for mitigating risks but also for advancing scientific knowledge. By studying the dynamics of glaciers, scientists can better understand their interactions with the Earth's climate system and predict future changes. Such insights are critical for developing sustainable resource management strategies and enhancing societal resilience.

The Vernagtferner Glacier has been a research area for geodetic sensors for over 150 years, beginning with Sebastian Finsterwalder's photogrammetric observations in the 19^th century. Since then, the Bavarian Academy of Sciences and Humanities has expanded this database by installing sensors and level bars on and around the glacier. The current challenge lies in leveraging observational data to develop a glacier model capable of assimilating geodetic observations. This research aims to design an optimized geodetic sensor network that enhances the integration of field observations into glacier modeling. Sensitivity studies evaluate the model’s response to various data inputs, identify observation errors, and refine the network design. Starting with the existing sensor infrastructure, the study explores innovative measurement strategies, including low-cost sensors, to increase spatial and temporal data coverage.

At this stage, a preliminary concept for the sensor network is presented, offering insights into its potential to improve network accuracy and to consider future development. This work should lay the foundation for creating a comprehensive geodetic observation system contributing to glacier monitoring and modeling.

How to cite: Lechner, K. and Pail, R.: Preliminary concept for observing the Vernagtferner Glacier with an optimized geodetic sensor network, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4054, https://doi.org/10.5194/egusphere-egu25-4054, 2025.

In the era of global warming, accurate monitoring of terrestrial water storage across scales is essential for effective water resource management and mitigating the impacts of a changing climate. Since 2002, the Gravity Recovery and Climate Experiment (GRACE) satellite mission and its successor, GRACE Follow-On (GRACE-FO), have provided valuable insights into terrestrial water redistribution processes by monitoring Earth’s time-varying gravity. However, their coarse spatial resolution limits their utility for estimating water mass changes at local scales. While various downscaling methods integrating GRACE data with high-resolution land surface models have been proposed, the accuracy of these approaches often depends on the fidelity of the models used. This presents challenges in the regions where water redistribution is driven primarily via streamflow, flood inundation or agricultural irrigation—processes often poorly represented in many land surface models. In this study, we employ soil moisture contents retrieved from the Cyclone Global Navigation Satellite System (CYGNSS) as a priori soil water mass to downscale GRACE-FO data. The downscaling is applied to the Murray-Darling Basin, Australia, with particular focus on interior arid regions where water redistribution is dominated by streamflow and associated inundation of floodplains. The downscaled GRACE-FO data demonstrate superior performance in capturing local hydrological processes, including a major flood inundation event in late 2022, mapped using the Water Observation product (Digital Earth Australia), and regions of intensive agricultural water use identified through the CSIRO MODIS Reflectance-based Scaling EvapoTranspiration (CMRSET) product.

How to cite: Kim, J., Ryu, D., Seo, K.-W., and Fowler, K.: Downscaling GRACE-FO Data with CYGNSS Soil Moisture for Improved Representation of Floodplain Inundation: Application to the Murray-Darling Basin, Australia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7840, https://doi.org/10.5194/egusphere-egu25-7840, 2025.

During the last 30 years, following the breakthrough that was the TOPEX/Poseidon satellite altimeter, improvements of satellite altimetry has been incremental. We have several generations of satellites to build upon, and with the inclusion of SAR processing, we are reaching the limit of conventional satellite altimetry. With the breakthough that is the Surface Water and Ocean Topography (SWOT) mission, we are able to significantly imrpove our knowledge of ocean geodesy, and provide greatly improved references for oceanography and climate research.

The importance of an accurate mean sea surface reference for global climate science, sea level rise and coastal impacts have been shown. With the inclusion of two-dimensional satellite altimetry, we go beyond the limitations obtained from nadir looking altimetry. With the 2D data, the longitudional resolution is substantially improved, and we have seen a substantial improvement in especially coastal zones, eliminating the majority of coastal contamination causing problems in current models.

With almost two years of SWOT data available, we have a near complete global coverage. Longer wavelengths are alreay well resolved with the 30 years of conventional altimetry, and utilizing the short-wavelength improvement obtained from SWOT, we can get a combined solution with the best from both sides.

We present the next generation of mean sea surface reference fields, with major improvements in spatial resolution and noise reduction. The inclusion of SWOT data has shown small scale oceanographic features previously hidden, and will be of critical importance for geodesy, oceanography and climate science.

How to cite: Nilsson, B., Andersen, O. B., and Knudsen, P.:  The next step in marine reference surfaces – the DTU25 mean sea surface, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8074, https://doi.org/10.5194/egusphere-egu25-8074, 2025.

Variations in water mass redistribution are a critical indicator of climate change, revealing processes such as global continental desertification and cryosphere melting. The Gravity Recovery and Climate Experiment (GRACE) and Follow-On (GRACE-FO) satellite missions have provided over 20-yrs of essential records of Earth's mass variations, significantly advancing our understanding of climate-driven processes.

However, GRACE/-FO spherical harmonic solutions suffer from aliasing errors, as well as measurement uncertainty, manifesting as North/South striping due to propagation of correction model errors and temporal averaging of the signal. While filtering is necessary to reveal meaningful geophysical signals, it introduces leakage and bias by causing signals to smear beyond their location, which impacts the accuracy of regional mass estimates. To overcome these limitations, we introduce MILLS (MItigation Leakage through Least Square), a new method for estimating regional mass variations from GRACE/-FO Level-3 solutions. MILLS leverages knowledge on solution-specific spherical harmonic filters to correct signal leakage and bias. It computes the least square affine regression between a filtered artificial uniform unit source signal over the region of interest and the similarly filtered GRACE/-FO solution for each time step. The affine models are then applied to non-filtered unit source signal, effectively mitigating leakage and bias, thus improving time-dependent regional mass estimates.

We validate MILLS over the Caspian Sea, an ideal test case due to its large size, significant mass depletion signal, and minimal contamination by external geophysical signals. Comparison of MILLS-derived mass estimate with independent estimates from altimetry and in situ tide gauges demonstrates the method's effectiveness in isolating sources and resolving the phase of the annual variations, which is usually uncertain in other methods due to the disturbance caused by regional signals. We then apply MILLS to glacial regions to evaluate its capability for monitoring glacier mass change. Although glacier regions present greater challenges than the Caspian Sea due to more complex external geophysical signals to account for, preliminary results of MILLS-derived glacier mass change show good agreement with independent estimates from optical imagery.

How to cite: Gauer, L.-M., Chanard, K., Fleitout, L., Crétaux, J.-F., Grandin, R., Berthier, E., and Blazquez, A.: MILLS: MItigation Leakage through Least Square -- A new method to estimate regional mass variations from GRACE/-FO, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9812, https://doi.org/10.5194/egusphere-egu25-9812, 2025.

The region around Lake Issyk-Kul in the Tian Shan Mountains in Kyrgyzstan is densely observed with in-situ monitoring stations and used to test and validate satellite observations. Although Lake Issyk-Kul lies at 1600 m elevation, it does not freeze over in the winter months. The hydrology of the region is dominated by the storage in several large to medium-sized endorheic lakes (e.g., lakes Issyk-Kul and Balkhash), artificial reservoirs (e.g., Kapshagay Reservoir), and the snow cover during the winter months. In addition, melting glaciers play a significant role in the region’s hydrology. In 2016, the GFZ Helmholtz Centre for Geosciences, Germany, and Central-Asian Institute for Applied Geosciences (CAIAG), Kyrgyzstan, installed two climate monitoring stations and several GNSS-controlled tide gauges for the monitoring of the environment and Issyk-Kul lake level variations. The in-situ observations and the ice-free winters make Lake Issyk-Kul an ideal test site for calibrating satellite altimetry. 

NASA and DLR plan to launch GRACE-C (Gravity Recovery and Climate Experiment – Continuation) in 2028 in the same orbit as GRACE. ESA plans to launch a Next Generation Gravity Mission (NGGM) in 2032, flying in an inclined and lower orbit. GRACE-C and NGGM will form the Mass-Change and Geosciences International Constellation (MAGIC), which aims to increase mass transport products' spatial and temporal resolution significantly. With this study, we investigate if and how we can use the region to evaluate the MAGIC future satellite gravity mission.

In order to assess the suitability of the Issyk-Kul region as a validation site for MAGIC, we investigate the behaviour of the different hydrological storage compartments. The recently published G3P data set (Global Gravity-based Groundwater Product) compiles a harmonised set of satellite observations of root-zone soil moisture (RZSM), snow water equivalent (SWE), glacier mass change, and terrestrial water storage (TWS). Information about surface water storage (SWS) can be derived from satellite altimetry. This data set allows us to understand the hydrological drivers of TWS variability. Variations of SWS explain the strong interannual variations beyond the linear trend well. However, the different lakes of the region show quite distinct interannual variations. With the current spatial resolution of GRACE and GRACE-FO, these variations cannot be separated. However, this separation would be a prerequisite for the region as a test site for future gravity missions.

By simulating realistic MAGIC observations of the region, we can assess their spatial resolution and, thus, if the region around Lake Issyk-Kul may serve as a test site for MAGIC. First results show that with the higher spatial resolution, we can discriminate between the SWS signals of the different lakes.

How to cite: Boergens, E., Wilms, J., Schöne, T., Jensen, L., Haas, J., Zubovich, A., and Dobslaw, H.: Can we use the Issyk-Kul region as a test site for future satellite gravity missions?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10150, https://doi.org/10.5194/egusphere-egu25-10150, 2025.

Established in 2006, the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) is an international observing network of sites worldwide. Currently, 33 reference sites are designed to detect long-term trends of key climate variables above the Earth’s surface, such as temperature and humidity in the atmosphere. They provide high quality long-term atmospheric measurements using balloon-borne sensors, in particular radiosondes. 

Data from co-located GNSS stations are processed at the GFZ with the EPOS 8 software. This provides GNSS Integrated Water Vapor (IWV) time series co-located with the Vaisala RS41 radiosonde.

This study provides the first three-year comparison, between 2021 and 2023 between GNSS and radiosonde IWV time series at nine sites. The study focuses on small dry bias of GNSS to radiosonde comparisons obtained in previous studies. To this end, several GNSS processing options that may contribute to this bias are discussed. Several approaches to GNSS and radiosonde comparisons are presented, leading to different results in the calculated bias. Finally, the time series provided by the radiosondes are examined in detail, and the IWV is recomputed from the cleaned radiosonde measurements to correct some discrepancies found in the radiosonde-provided IWV profiles.

How to cite: Panetier, A., Zus, F., Dick, G., and Wickert, J.: GNSS-radiosonde three-year IWV comparisons in the framework of the GRUAN project, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10161, https://doi.org/10.5194/egusphere-egu25-10161, 2025.

How to cite: Rietbroek, R., Karimi, S., and Shakya, A.: Signatures of drought and flooding events in terrestrial water storage anomalies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18851, https://doi.org/10.5194/egusphere-egu25-18851, 2025.

In this work, the time-variable gravity field data from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) covering the period from April 2002 to September 2023 is used to quantify glacier mass changes in the Alps. We employ a new method that utilizes the vertical surface displacement data to correct the glacial isostatic adjustment (GIA) and tectonic uplift signal. This approach reveals that the mass increases caused by vertical deformation signals with a trend of 0.75 ± 0.11 Gt/yr. We further include two hydrology models Global Land Data Assimilation System (GLDAS) and WaterGAP Global Hydrological Model (WGHM) to correct for hydrological signals in the Alps. Three forward modeling-derived schemes are used to recover the signals from GRACE/GRACE-FO observations. Our results, when compared with the annual glacier mass balance from the World Glacier Monitoring Service (WGMS), indicate that among the three experiment schemes, the global unconstrained forward modeling algorithm demonstrates the best performance in estimating glacier mass change in the Alps. Overall, applying our new vertical deformation correction method, we find that the total glacier mass loss rate in the Alps is -2.54 ± 0.82 Gt/yr using GRACE Level-2 data and -3.42 ± 0.56 Gt/yr using the JPL Mass Concentration (Mascon) solutions. Additionally, our study identifies a three-month lag between land surface temperature and glacier mass variations, which supports the validity of our estimated glacier mass changes.

How to cite: Liu, S. and Pail, R.: The glacier changes in the Alps from the GRACE and GRACE Follow-On Missions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3951, https://doi.org/10.5194/egusphere-egu25-3951, 2025.

The pivotal role of precipitation in driving the terrestrial water cycle is well-known, but quantifying its transformation into terrestrial water storage remains challenging. This study introduces a new metric -- the average daily fraction of precipitation transformed into terrestrial water storage -- leveraging an advanced statistical reconstruction method and data from the Gravity Recovery and Climate Experiment (GRACE) satellites and their follow-on mission. Results show that about 64% of land precipitation contributes to terrestrial water storage across 121 global river basins from 2002 to 2021, with notable variations across climatic and geographical regions. We also analyze changes in this fraction across global mascons. Our findings shed light on the interactions between precipitation, land surface processes, and climate change, providing valuable insights for water resource management and hydrological modeling.

How to cite: Zhong, Y., Tian, B., Cheng, G., Kim, H., Wu, Y., and Wang, L.: Global quantifying the fractions of precipitation transformed into terrestrial water storage and their changes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4854, https://doi.org/10.5194/egusphere-egu25-4854, 2025.

The geocenter motion describes the relative motion between the Earth’s center-of-mass and center-of-figure, representing one of the largest-scale mass redistributions in the Earth system. Accurate determination of geocenter motion is essential for the realization of the terrestrial reference frames (TRF) and for the full-spectrum monitoring of global mass variations. Traditionally, geocenter motion can be estimated directly from Satellite Laser Ranging (SLR) by tracking orbital motion with ground stations since the 1990s or indirectly from gravity fields provided by the Gravity Recovery and Climate Experiment (GRACE) since 2002. However, SLR-derived geocenter motion estimates are generally unsuitable for studying long-term mass changes because the secular trend is absorbed by the linear definition of the TRF. Additionally, only low-degree gravity fields were available before GRACE (e.g., from SLR), resulting in significant signal leakage errors in geocenter estimates. In this study, we derive the geocenter motion from low-degree gravity fields (up to degree and order 5) after properly addressing signal leakage effect. By combining the leakage-corrected land mass patterns with self-consistent ocean mass fingerprints, we generate geocenter motion estimates and compare them with those derived from GRACE, geophysical models, and the SLR direct tracking method. The trends in our estimates are consistent with GRACE and models, while the SLR direct estimates yield opposite trends, leading to significantly underestimated global ocean mass change rates. Our study provides promising results for deriving long-term estimates of geocenter motion, enabling the study of mass changes in the global oceans and polar ice sheets back to the 1990s.

How to cite: Nie, Y., Chen, J., Peng, D., and Li, J.: Deriving long-term geocenter motion estimates for geophysical applications, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5390, https://doi.org/10.5194/egusphere-egu25-5390, 2025.

The Gravity Recovery and Climate Experiment (GRACE) and its Follow-On mission (GRACE-FO) have greatly improved our ability to monitor changes in Earth's mass distribution, providing unprecedented insights into variations in total water storage (TWS). These variations can be expressed as equivalent water thickness, and they can also be derived from other geopotential variations, such as changes in geoid height, gravity anomalies, or vertical displacements. Understanding these variations is essential for comprehending regional hydrology and solid Earth dynamics.

In this study, we use DDK2-filtered solutions from GRACE and GRACE-FO spherical harmonics to compute geoid height variations over the Türkiye region, based on roughly one hundred grid points. The trend in the geoid height changes for this region is approximately at the millimeter level. We also derive TWS time series from these DDK2-filtered spherical harmonics to compare the changes in geoid height with the corresponding equivalent water thickness values, aiming to explore the functional relationship and correlation coefficients between these two geopotential variations. In addition to time-domain analysis, we apply spectral analysis to examine the power spectrum of geoid height and TWS variations in the frequency-domain. This approach helps us understand the spatial and temporal diversity of geoid height changes across Türkiye and provides insights into their underlying patterns and trends. The preliminary results of this study offer an overview of geoid height changes in Türkiye and highlight the potential of GRACE and GRACE-FO data for monitoring mass redistribution on a regional scale.

How to cite: Gunes, O. and Aydin, C.: Geoid Height Changes over Türkiye: Insights from GRACE and GRACE-FO Spherical Harmonics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9662, https://doi.org/10.5194/egusphere-egu25-9662, 2025.