Groundwater is a major source of freshwater for more than two billion people, supporting global food and water security as well as the sustainability of ecosystems. It is a critical component of the global water cycle, and either directly or indirectly, several of the United Nation's Sustainable Development Goals (SDGs) have significant element of groundwater. Specifically, SDG 6 on clean water and sanitation, SDG 2 on zero hunger, SDG 14 on life below water, and SDG 15 on life on land, all rely upon the sustainable use and protection of groundwater resources. However, this globally important Earth system resource is now increasingly vulnerable to the impacts of a changing climate and unsustainable rates of extraction. Falling groundwater levels in some areas are beginning to jeopardize food security and constraining human adaptation to climate change. On a global scale, there are concerns on the impacts of groundwater decline on water quality, disconnection between surface and groundwater leading to reductions or complete loss of environmental flows including, baseflow that sustain rivers, wetlands, and groundwater-dependent ecosystems, reduced ecosystem resilience to prolonged droughts, and the loss of groundwater-dependent ecosystems such as streams, springs, wetlands, and riparian vegetation that rely on groundwater discharge. Other impacts like land subsidence, saltwater intrusion into coastal aquifers, and depletion of groundwater reserves needed for drinking water, agriculture, and industry have also been identified.

Despite these impacts, understanding of the role of climate change and groundwater extraction on groundwater systems in many regions is still limited, impairing aquifer management. Sustainable and equitable management of groundwater resources requires tools and observational frameworks to quantify and improve understanding of groundwater systems. But the lack of groundwater data in some areas limits our understanding of groundwater systems and their responses to climate change and water extraction. However, several emerging studies have used remote sensing techniques and integrated local-scale groundwater observations with large-scale numerical modelling and satellite data to improve global-to-local scale groundwater monitoring and assessment. By combining detailed local hydrogeological understanding and ground-based monitoring data with regional and global satellite remote sensing and numerical models, there is potential to gain new insights into groundwater resources and their changes across different scales. This Research Topic will enable us to put in one place the current progress on this as well as highlight on challenges and future trajectories.

The aim of this Research Topic is to improve understanding on global groundwater hydrology, documenting the progress made in terms of groundwater measurement and characterization as well as assessing impacts of groundwater extraction and climate change. This Research Topic welcomes all original submissions, including research papers and review articles that use in situ and remote-sensing observations, integrates both in situ and remotely sensed observations, geospatial methods, numerical modelling, hydrological model and climate outputs, reanalysis, data assimilation, etc. to improve our understanding of groundwater systems.

Specific research areas and themes include but are not limited to:

1. Groundwater changes, its availability and usage at all spatial scales;

2. The role of climate change (e.g., droughts, floods) on groundwater recharge and discharge;

3. Impacts associated with groundwater extraction on the environment (e.g., flows, and groundwater-dependent ecosystems such as streams and springs);

4. Groundwater status through accurate assessment of groundwater withdrawals and rates using different techniques;

5. Addressing limitations and challenges in the various techniques of groundwater assessment and how they can be optimized; and

6. Data-assimilation and modelling frameworks for groundwater assessment

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Frontiers in Water

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