Geohydrology: Groundwater

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 22.6 mkm3, with residence times varying between months to thousands of years. Out of this amount, only 0.1–5 mkm3 of total global groundwater is modern or