Groundwater basins and recharge
The Worldwide Hydrogeological Mapping and Assessment Programme (WHYMAP) is a global initiative to better study and manage aquifer resources. Using very diffuse subnational, national and regional data sources of different quality, a consistent picture of the hydrogeological situation on the globe was obtained for the first time. WHYMAP shows "various characteristic groundwater environments in their areal extent:
Blue colour is used for large and rather uniform groundwater basins [...].
Green colour symbolises hydrogeological environments of complex geological structure. [...].
Brown colour outlines areas with local and shallow aquifers in which elatively dense bedrock is exposed to the surface [...]"
Five categories define the potential recharge rates from over 300 mm to less than 2 mm/year (ranging within the three main colours from dark (very high recharge rates) to light (very low recharge potential).
Source: Richts A., WHYMAP, 2011.

Global distribution and availability of groundwater

Groundwater, or water stored below the Earth’s surface in unconsolidated media and porous rock, provides up to 33 % of the total water that supports irrigation and human consumption. It is used to supplement surface water supplies in humid areas, particularly in times of drought, and may provide the primary or sole source of water in arid and semi-arid areas worldwide. Globally, groundwater serves as the primary source of water for around 2 billion people. Consequently, the depletion of groundwater is a well-known global-scale concern. In addition to human demands, there are many groundwater-dependent ecosystems (GDEs), such as rivers, lakes and wetlands, that depend on groundwater to maintain their structure and functioning. These unique, fragile ecosystems comprise a diverse and complex subset of the world’s ecosystems. GDEs are especially vulnerable to pollution, the intrusion of brackish waters and variations in the water table caused by human withdrawals, drought and climate change. Despite its overwhelming importance, global estimates of groundwater volume, rates of recharge (through rainfall, surface flow infiltration, irrigation and transmission losses from canals) and discharge (via outflow to surface waters or withdrawal by pumping) have been either unavailable or unreliable, varying by several orders of magnitude over the past half-century. Currently, total groundwater volume in the upper 2 km of the continental crust is estimated at approximately 22.5 km3 – with an uncertainty of about 30 % - or about three times the quantity of surface water. Given projected rates of human population growth and its attendant demands for water, combined with the predictions of climate change impacts on water resources, this gap in our fundamental understanding of the hydrologic cycle is cause for concern.
Although ubiquitous throughout the terrestrial land mass of the globe, groundwater is not amenable to direct observation and so is largely unknown and unmanaged, particularly in the developing world. Even in developed countries, groundwater management is haphazard and data on groundwater depths and withdrawals are rarely shared among local, regional or national management agencies. As a consequence, there is a general under-appreciation of its value, it is being extracted at unsustainable rates (often promoted by perverse governmental irrigation subsidies) and there is little regulatory overview.
The first global map of aquifer types and status, shown here, was compiled using existing ground observation data. Since 2002, the GRACE (Gravity Recovery and Climate Experiment) satellite system of the United States (NASA) and German (DLR) space agencies has provided a novel means of acquiring coarse resolution data that can be used to measure changes in the mass of stored groundwater. We now have an unprecedented opportunity to monitor, with exceptional precision, groundwater changes across the globe. These data are actively used with models and observations of extraction wells to estimate groundwater recharge and withdrawals.
The advent of this enhanced ability to monitor groundwater will result in more widely available data and will facilitate more coordinated efforts to manage its use and quantify its contribution to the global hydrologic cycle. Because 70 % of fresh water is used to irrigate crops, very small increases in crop water use efficiency achieved with new irrigation technologies, judicious crop selection and other prudent water-management practices, has the potential to yield large water savings. However, to be most effective, all water resources – surface, ground and wastewater – must be viewed and managed as an integrated whole.

Most of the major aquifers in the world’s arid and semi-arid zones are experiencing rapid rates of depletion because of water use, mainly for agriculture. This chart is derived from GRACE data and shows groundwater storage declines in the North China Plain, Australia’s Canning Basin, the Guarani Aquifer in southern South America, the High Plains and California Central Valley aquifers of the United States and the aquifers beneath north-western India and the Middle East.
Source: Data from T. Reager, NASA Jet Propulsion Labouratory, California Institute of Technology, USA. Figure 2. Famiglietti 2014).