Groundwater is one of the largest stores of freshwater on Earth. Yet, it remains one of the least directly observed components, with its dynamics often inferred from sparse observations, indirect measurements, models, and satellite and remote-sensing products. This limited observability contributes to persistent uncertainties in how groundwater interacts with other components of the hydrological cycle and regulates hydrological variability across spatial and temporal scales. Groundwater is not an isolated component. Through recharge, storage, subsurface flow, and exchanges with surface water and ecosystems, it can buffer or amplify hydrological variability, sustain river and ecosystem flows, influence drought propagation and recovery, and redistribute water across catchments and regions. Understanding these processes is essential for a complete picture of the water cycle under a changing climate. An emerging challenge is to move beyond viewing the terrestrial water cycle primarily as individual stores, fluxes and water-balance terms toward understanding its integrated connectivity and redistribution structure. Quantifying groundwater contributions and connectivity is important for understanding how the water cycle is organized, where uncertainty is concentrated, and how changes propagate through the wider hydrological system. We welcome contributions addressing, but not limited to:
• Groundwater dynamics and connectivity, including contributions to terrestrial water storage, streamflow, evapotranspiration, coastal discharge, and groundwater–surface-water interactions;
• Observing and quantifying groundwater using in situ measurements, remote sensing, geophysical and tracer-based methods, and integration of heterogeneous observations;
• Modelling groundwater within the hydrological cycle, including large-scale and Earth system models, precipitation/runoff partitioning, groundwater representation and uncertainties;
• AI-powered, data-driven and hybrid approaches, including data assimilation, machine learning, reconstruction, prediction and scaling;
• Groundwater roles in hydrological variability, change and extremes, including drought and flood onset, propagation, persistence and recovery, and responses to climate variability, climate change and human interventions; and
• Scaling and implications from local and catchment to continental and global hydrology, including ecosystems, water availability and resilience.