Earth has many thousands of aquifers, but only a few dozen aquifers are categorized as major. Thanks to a decade (2003 to 2013) of satellite data collected by a unique NASA Earth-observing mission called the Gravity Recovery and Climate Experiment (GRACE), much is now known about the depletion of many of the major aquifers. Unlike satellite missions that rely on visual images, GRACE measured the tiny space-time variations in Earth’s gravity field, effectively weighing changes in water mass of large aquifers. Figure 70 illustrates groundwater depletion ( – ) or replenishment (+) in millimeters of water per year based on GRACE data for Earth’s 37 largest aquifers.
The GRACE results are consistent with long‑term estimates of groundwater extraction. The estimated evolution of groundwater extraction in India from 1950 to 2010 is shown in Figure 71, together with similar time series for a selected group of countries. India’s groundwater extraction rate in 2010 was approximately 251 cubic km per year, equivalent to a quarter of the global rate. The Ganges‑Brahmaputra basin suffers aquifer depletion and groundwater pollution stresses including swathes of India, China, Nepal, Bangladesh and Bhutan, which collectively is home to about 10 percent of the Planet’s population. Close to 50 percent of the world’s aquifers may be “past their tipping point”, which means that a natural recovery would require centuries.
The Nubian system, the biggest non‑renewable aquifer (fossil water) in the world, occurs under parts of Egypt, Libya, Sudan and Chad. Mostly underlying Egypt and Libya, it is considered under high‑stress due to unsustainable withdrawal rates driven by population growth. Libya depends on the aquifer for about 95 percent of its water.
The Nubian system also features one of the few trans boundary aquifer agreements. Worldwide, six trans boundary aquifers exist with specific agreements, and two aquifers with informal accords. The Northwest Saharan aquifer system that underlies 60 percent of Algeria, almost a third of Libya and part of Tunisia is another trans boundary aquifer with high levels of water withdrawals. It also falls under a cooperation pact and is similar to the Nubian system in that it is non‑renewable water. The water is used mainly for agricultural irrigation and industry.
Projections based on current water usage, anticipated population increase and economic growth scenarios suggest substantial shortfalls in water availability in the coming decade, including nearly 600 aquifers that cross sovereign borders. The depletion of aquifers is the most evident of the Planet’s two groundwater problems, the other problem is groundwater pollution. Pollution diminishes the usefulness of the water that remains after aquifer depletion. The failure to manage Earth’s groundwater resources is a threat to the global stability of human society.