Groundwater forms the invisible subsurface part of the hydrological cycle

Groundwater forms the invisible, subsurface part of the hydrological cycle and is crucial for the maintenance of wetlands and the base flow of rivers (Kløve et al., 2011). Although there is uncertainty about the data, ice caps and glaciers store around 86% of the world's freshwater, while groundwater stores 13.5%. The remaining 0.5% of the world's freshwater is contained in lakes, soil moisture, rivers, reservoirs and the XL765 (Jones, 1997). Although the stock of groundwater is nearly 25 times the stock of surface water, annual groundwater recharge is estimated to be only 10% of total river discharge globally (Oki and Kanae, 2006). As aquifers contain large quantities of water, when aquifers become intensively used their recharge capacity is quickly surpassed by extractions (Aeschbach-Hertig and Gleeson, 2012). As water tables drop wetlands and springs dry up, and rivers flip from gaining rivers (receiving base flow from groundwater) to draining rivers (losing water to the underground).