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State-of-art climate and land ecosystem models do not yet include a prognostic groundwater, the largest terrestrial store with the longest memory. This talk presents observation and modeling evidence that suggest multiple pathways whereby the groundwater can influence land surface water, energy and carbon fluxes. Global observations and synthesis of water table depth suggest that the water table is <5m deep under > 30% of the land surface. Model simulations in N. and S. America suggest that groundwater supports terrestrial evapotranspiration in drier climate (e.g., the southwestern US) or climate with a long dry-season (e.g., the Amazon), and that it is a key regulator of wetland distribution and methane flux. As human footprints expand, large-scale groundwater withdrawal may prove to be a major component of the terrestrial water cycle. All the above point to the need to incorporate the groundwater reservoir in the next generation of climate and earth system models.