water.usgs.gov

Up until this point, there has been no unifying effort to create a global map of groundwater tables.  However, a global, comprehensive map of the location and depth of water tables throughout the world can help with finding global patterns of groundwater movement.  Fan et al. compiled all existing government records of groundwater tables from over a million well sites around the globe.  Where government records were not available, they used data from published literature.  The compiled map was not complete though;  the water tables in many places around the globe remain unrecorded.  In order to look more closely and completely at global, regional, and local trends in groundwater distribution and depth the researchers also used a pre-existing groundwater table model.  They found that groundwater tends to be shallowest in the most humid climates, in wetland regions, in arid valleys and along the edges of continents, especially in areas with long, flat, plains of wetlands leading up to the coast.  Additionally, the model looked for the influence of three forces—climate, terrain, and sea level—on the water table depth (WTD).  While sea level has the strongest influence on WTD globally, regionally climate and topographic gradient are most important, and locally they found that terrain can override climate boundaries and lead to climatic anomalies like oases. —Alison Marks