Hydrogeodesy is an interdisciplinary field of study that combines geodesy—the science of measuring Earth's size, shape, gravitational field, and their variations over time—with hydrology, the study of water's distribution, movement, and properties^[1]. This field focuses on using advanced geodetic technologies to monitor and analyze changes in water resources, including surface water, groundwater, and terrestrial water storage (TWS)^[1].

Hydrogeodesy integrates several spaceborne and terrestrial geodetic technologies that allow for precise measurements of Earth's hydrological systems. The main tools used in hydrogeodesy include:

• Altimetry: Satellite altimetry measures the distance between a satellite and the Earth's surface, typically using radar or laser signals. This technique is widely used to track the elevation of bodies of water, such as lakes, rivers, and reservoirs, as well as ice sheets and glaciers^[2]. Altimetry has become increasingly valuable for monitoring water levels in remote or hard-to-reach regions^[3].
• InSAR (Interferometric Synthetic Aperture Radar): InSAR uses radar signals to measure surface deformations, such as subsidence or uplift, with millimetric accuracy. This technology is particularly useful for monitoring changes in groundwater storage, wetland dynamics, and other hydrological processes by detecting the vertical displacement of the Earth's surface^[4][5].
• GNSS (Global Navigation Satellite Systems): GNSS includes systems like GPS, GLONASS, Galileo, and BeiDou, and is used to measure precise changes in Earth's surface position^[6]. By monitoring ground deformation and vertical displacement, GNSS can provide insights into changes in groundwater storage, snow depth, and soil moisture^[7][8].

Hydrogeodesy plays a crucial role in monitoring and managing global water resources, providing valuable data for a variety of applications^[1]:

• Water Resource Management: Hydrogeodesy enables the monitoring of water availability across large regions, offering insights into trends in freshwater storage and distribution. It is used to track changes in groundwater, lakes, rivers, and reservoirs, facilitating the management of water resources in regions facing scarcity or over-exploitation.
• Climate Change and Sustainability: Hydrogeodetic technologies are vital for understanding how climate change is affecting the global water cycle. By measuring shifts in water storage, snowpack, and glaciers, hydrogeodesy helps assess the impacts of warming temperatures, changing precipitation patterns, and the resulting threats to water supply systems.
• Groundwater Monitoring: Through gravimetric and InSAR measurements, hydrogeodesy allows for the detection of groundwater depletion, a critical issue in many arid regions. It provides an alternative to traditional groundwater measurement methods, which can be logistically challenging and limited in scope^[9].
• Flood and Drought Prediction: The ability to measure changes in water storage at large scales enables hydrogeodesy to support early warning systems for floods and droughts. By tracking shifts in water availability across rivers, lakes, and groundwater systems, scientists can better predict extreme events.
• Wetlands and Permafrost: Although less commonly studied, hydrogeodesy holds significant promise for monitoring ecosystems such as wetlands and permafrost, which are increasingly affected by climate change. InSAR, for example, can be used to track the seasonal changes in wetlands, while gravimetry can help measure the thawing of permafrost and its effects on water systems.

The field of hydrogeodesy has seen significant advancements in recent years due to technological progress and the increasing availability of satellite data. Some notable developments include:

• Integration of Multiple Technologies: The combination of altimetry, InSAR, gravimetry, and GNSS has led to more comprehensive hydrological models, improving the precision and accuracy of water resource measurements. For instance, using InSAR and altimetry together allows for detailed estimates of floodplain storage and the tracking of water levels in hard-to-reach regions^[4].
• Artificial Intelligence and Big Data: The growing use of artificial intelligence (AI) and machine learning in processing hydrogeodetic data has expanded the potential applications of these technologies. AI can help automate the analysis of large datasets, providing real-time monitoring capabilities and more accurate predictions.
• Open Data and Collaboration: The increasing availability of open-source satellite data, such as those from the GRACE and GRACE-FO and Sentinel1 missions, has helped expand hydrogeodesy’s reach. Open-access platforms allow researchers, policymakers, and stakeholders to utilize hydrogeodetic data, enhancing collaboration and data-driven decision-making.

Despite its potential, hydrogeodesy faces several challenges and limitations^[1]:

• Data Integration: Combining data from different hydrogeodetic technologies, such as altimetry and InSAR, can be technically complex. Standardization of data formats and methodologies is essential to ensure that results from different missions and sensors are compatible and useful^[2][4].
• Spatial and Temporal Resolution: Some hydrogeodetic technologies, such as gravimetry, have limited spatial resolution, making it difficult to monitor small-scale water resources, such as individual aquifers. The temporal resolution of some technologies is also a limitation, as many missions provide data on a monthly or yearly basis, which may not be sufficient for real-time water management needs.
• Cost and Expertise: While satellite-based hydrogeodesy offers significant advantages, the high cost of missions and the specialized expertise required to process and analyze geodetic data can be barriers to widespread adoption, especially in developing regions.

The future of hydrogeodesy is bright, with several promising directions for development^[1]:

• Improved Data Availability: As more satellites with hydrogeodetic capabilities are launched, and as existing datasets become more widely accessible, hydrogeodesy will become a more powerful tool for global water monitoring.
• Integration with Other Earth Science Disciplines: By combining hydrogeodesy with other fields such as atmospheric science, land use change, and ecosystem modeling, it will be possible to gain a deeper understanding of how water systems interact with broader environmental factors.
• Education and Training: To fully realize the potential of hydrogeodesy, it is essential to incorporate it into university curricula and research programs. Interdisciplinary education will help build the necessary expertise to apply these technologies to real-world challenges.

Hydrogeodesy stands at the intersection of geodesy, hydrology, and environmental science, offering critical insights that can guide sustainable water resource management in an increasingly water-scarce world. By combining cutting-edge technology with a deep understanding of water systems, hydrogeodesy is helping to ensure that humanity can continue to meet its water needs while preserving the planet’s precious resources.

• Geodesy
• Hydrology
• Earth Observation Satellite