Groundwater
An illustration shows groundwater in aquifers below the water table, with three different wells dug to reach it. Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from the surface; it may discharge from the surface naturally at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.
Groundwater can be thought of in the same terms as surface water: inputs, outputs and storage. The natural input to groundwater is infiltration from surface water, which must then percolate downward to reach the groundwater. The natural outputs from groundwater are springs and seepage to the oceans. Groundwater storage can be much larger (in volume) compared to its inputs than surface water and have a slower turnover rate, though this depends on the features of the aquifer. This difference makes it easy for humans to use groundwater unsustainably for a long time without severe consequences. Nevertheless, over the long term the average rate of infiltration above a groundwater source plus input from streams is the upper bound for average consumption of water from that source. Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table. Groundwater can be a long-term 'reservoir' of the natural water cycle (with residence times from days to millennia), as opposed to short-term water reservoirs like the atmosphere and fresh surface water (which have residence times from minutes to years). Deep groundwater (which is quite distant from the surface recharge) can take a very long time to complete its natural cycle.
Groundwater accounts for about half of the world's drinking water, 40% of its irrigation water, and a third of water for industrial purposes. About 2.5 billion people depend solely on groundwater resources to satisfy their basic daily water needs. Global freshwater withdrawal was probably around 600 km3 per year in 1900 and increased to 3,880 km3 per year in 2017. The rate of increase was especially high (around 3% per year) during the period 1950, 1980, partly due to a higher population growth rate, and partly to rapidly increasing groundwater development, particularly for irrigation. The Asia-Pacific region is the largest groundwater abstractor in the world, containing seven out of the ten countries that extract most groundwater (Bangladesh, China, India, Indonesia, Iran, Pakistan and Turkey). These countries alone account for roughly 60% of the world's total groundwater withdrawal. In India, 65% of the irrigation is from groundwater and about 90% of extracted groundwater is used for irrigation. A majority of extracted groundwater, 70%, is used for agricultural purposes.
Groundwater depletion has been calculated to be between 100 and 300 km3 per year. This depletion is mainly caused by 'expansion of irrigated agriculture in drylands'. In the Punjab region of India, for example, groundwater levels have dropped 10 meters since 1979, and the rate of depletion is accelerating. Subsidence occurs when too much water is pumped out from underground, deflating the space below the above-surface, and thus causing the ground to collapse. The result can look like craters on plots of land. In the first half of the 20th century, the San Joaquin Valley experienced significant subsidence, in some places up to due to groundwater removal. Cities on river deltas, including Venice in Italy, and Bangkok in Thailand, have experienced surface subsidence; Mexico City, built on a former lake bed, has experienced rates of subsidence of up to per year. For coastal cities, subsidence can increase the risk of other environmental issues, such as sea level rise. For example, Bangkok is expected to have 5.138 million people exposed to coastal flooding by 2070 because of these combining factors.
The impacts of climate change on groundwater may be greatest through its indirect effects on irrigation water demand via increased evapotranspiration. There is an observed declined in groundwater storage in many parts of the world. This is due to more groundwater being used for irrigation activities in agriculture, particularly in drylands. Some of this increase in irrigation can be due to water scarcity issues made worse by effects of climate change on the water cycle. Direct redistribution of water by human activities amounting to ~24,000 km3 per year is about double the global groundwater recharge each year. Climate change causes changes to the water cycle which in turn affect groundwater in several ways: There can be a decline in groundwater storage, and reduction in groundwater recharge and water quality deterioration due to extreme weather events. In the tropics intense precipitation and flooding events appear to lead to more groundwater recharge. However, the exact impacts of climate change on groundwater are still under investigation. This is because scientific data derived from groundwater monitoring is still missing, such as changes in space and time, abstraction data and 'numerical representations of groundwater recharge processes'.
Groundwater governance processes enable groundwater management, planning and policy implementation. It takes place at multiple scales and geographic levels, including regional and transboundary scales. Groundwater management is action-oriented, focusing on practical implementation activities and day-to-day operations. Because groundwater is often perceived as a private resource (that is, closely connected to land ownership, and in some jurisdictions treated as privately owned), regulation and top, down governance and management are difficult. Governments need to fully assume their role as resource custodians in view of the common-good aspects of groundwater. Domestic laws and regulations regulate access to groundwater as well as human activities that impact the quality of groundwater. Legal frameworks also need to include protection of discharge and recharge zones and of the area surrounding water supply wells, as well as sustainable yield norms and abstraction controls, and conjunctive use regulations. The Arab region is one of the most water-scarce in the world and groundwater is the most relied-upon water source in at least 11 of the 22 Arab states. Over-extraction of groundwater in many parts of the region has led to groundwater table declines, especially in highly populated and agricultural areas.
Common questions
What is groundwater and where is it located?
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. It exists below the water table within aquifers, which are units of rock or unconsolidated deposits that can yield a usable quantity of water.
How much fresh water is groundwater compared to other sources?
About 30 percent of all readily available fresh water in the world is groundwater. Groundwater accounts for about half of the world's drinking water, 40% of its irrigation water, and a third of water for industrial purposes.
When did global freshwater withdrawal increase significantly between 1950 and 1980?
The rate of increase was especially high during the period from 1950 to 1980 due to higher population growth rates and rapidly increasing groundwater development. Global freshwater withdrawal increased from around 600 km3 per year in 1900 to 3,880 km3 per year in 2017.
Where does groundwater depletion occur most severely today?
Groundwater depletion occurs mainly through the expansion of irrigated agriculture in drylands such as the Punjab region of India where levels have dropped 10 meters since 1979. Cities on river deltas including Venice in Italy, Bangkok in Thailand, and Mexico City have experienced significant surface subsidence.
Why does climate change affect groundwater recharge and storage?
Climate change causes changes to the water cycle which lead to declines in groundwater storage and reduction in groundwater recharge alongside water quality deterioration. Direct redistribution of water by human activities amounting to approximately 24,000 km3 per year is about double the global groundwater recharge each year.