Encyclopedia of Geology

Abstract

Groundwater constitutes 30% of the global fresh water resource, and it is a major water supply for over 2 billion people worldwide. While surface water resources are the principal water resource globally, the importance of groundwater has been increasing as a reliable resource during times of drought and for irrigation purposes, particularly in semi-arid and arid regions. Despite the importance of groundwater in the global hydrologic cycle, limited information is available about total groundwater storage as well as groundwater use. Recent estimates indicate a total groundwater storage of about 22.6mkm3, with residence times varying between months to thousands of years. Out of this amount, only 0.1–5mkm3 of total global groundwater is modern or <50years old. This has important implications for water resources management, as modern groundwater is most sensitive to changes in climate variability and land use as well as contamination. Estimated global groundwater withdrawals of 750–1500km3/year have caused groundwater depletion, particularly in the most important agricultural regions of the world. Groundwater depletion has reduced well yields and resulted in land subsidence, water quality problems, and sea-water intrusion into coastal aquifers. Sustainable management of groundwater resources requires accurate estimation of various components of the hydrologic budget, including groundwater recharge, evapotranspiration, and streamflow as well as implementation of management approaches that consider surface water and groundwater as a single linked resource.