Global Patterns of Groundwater Table Depth

Water Flowing Underground In addition to serving as an out-of-sight, yet much-needed water source, groundwater influences ecosystems on land—especially when the groundwater depth is shallow. Fan et al. (p. 940) used government archives and published studies to construct a global map of groundwater depth based on over 1,000,000 direct well measurements. A groundwater model was then used to construct a continuous global map of groundwater depth. The findings reveal the global influence of sea level and climate on groundwater depths across several regions and ecosystems. Up to 32% of the global land area contains ecosystems that are influenced by shallow groundwater. Shallow groundwater affects terrestrial ecosystems by sustaining river base-flow and root-zone soil water in the absence of rain, but little is known about the global patterns of water table depth and where it provides vital support for land ecosystems. We present global observations of water table depth compiled from government archives and literature, and fill in data gaps and infer patterns and processes using a groundwater model forced by modern climate, terrain, and sea level. Patterns in water table depth explain patterns in wetlands at the global scale and vegetation gradients at regional and local scales. Overall, shallow groundwater influences 22 to 32% of global land area, including ~15% as groundwater-fed surface water features and 7 to 17% with the water table or its capillary fringe within plant rooting depths.