Overdrafting

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Overdrafting is the process of extracting groundwater beyond the equilibrium yield of the aquifer. Groundwater is the fresh water that can be found underground; it is also one of the largest sources. Groundwater depletion can be comparable to "money in a bank",[1] The primary cause of groundwater depletion is pumping or the excessive pulling up of groundwater from underground aquifers.

There are two sets of yields: safe yield and sustainable yield. Safe yield is the amount of water that can be taken out of the ground without there being any undesirable results. Sustainable yield is extraction that takes into account both recharge rate and surface water impacts.

There are two types of aquifers: confined and unconfined. In confined aquifers, there is an overbearing layer called aquitard, which contains impermeable materials through which groundwater cannot be extracted. In unconfined aquifers, there is no aquitard, and groundwater can be freely extracted from the surface. Extracting groundwater from unconfined aquifers is like borrowing the water, it has to be recharged at a proper amount. If recharge is not done in proper amounts there can be many impacts. Recharge may happen through artificial recharge and natural recharge.[2]

Mechanism[edit]

The natural process of recharge takes place through the percolation of surface water. An aquifer may be artificially recharged by pumping reclaimed water from wastewater management projects directly into the aquifer. An example is the Orange County Water District in the State of California.[3] This organization takes waste water, treats it to a proper level, and then systematically pumps it back into the aquifers for artificial recharge.

When groundwater is extracted, the water is primarily pulled from the aquifer which creates a cone depression around the well. When the drafting of water continues, the cone of depression increases in width. This increase in width leads to the negative impacts caused by overdrafting, such as the drop of the water table, land subsidence, and loss of surface water reaching the streams. In extreme cases, the supply of water to naturally recharge the aquifers is pulled directly from streams and rivers, leading to depletion of water levels in streams and rivers. The depletion of water in rivers and streams affects wildlife, as well as humans who might be using the water for other purposes.[2]

Since every groundwater basin recharges at a different rate depending upon precipitation, vegetative cover, and soil conservation practices, the quantity of groundwater that can be safely pumped varies greatly among regions of the world and even within provinces. Some aquifers require a very long time to recharge and thus the process of overdrafting can have consequences of effectively drying up certain sub-surface water supplies. Subsidence occurs when excessive groundwater is extracted from rocks that support more weight when saturated. This can lead to a capacity reduction in the aquifer.[4]

Changes in freshwater availability extend to groundwater and human activities, in conjunction with climate change, interfere with groundwater recharge patterns. One of the leading anthropogenic activities resulting in the depletion of groundwater is irrigation. Roughly 40% of global irrigation is supported by groundwater and irrigation is the primary activity resulting in groundwater storage loss across the U.S.[5]

Around the world[edit]

Ranking of countries that use groundwater for irrigation.[6]
Country Million hectares (1×10^6 ha (2.5×10^6 acres))
irrigated with groundwater
India 26.5
USA 10.8
China 8.8
Pakistan 4.9
Iran 3.6
Bangladesh 2.6
Mexico 1.7
Saudi Arabia 1.5
Italy 0.9
Turkey 0.7
Syria 0.6
Brazil 0.5

The ranking is based on the amount of groundwater each country uses for agriculture. This issue is becoming quite large in the United States (most notably California), but it has been a problem in other parts of the world, as was documented in Punjab, India in 1987[7]

United States[edit]

In the US, an estimated 800 km3 of groundwater was depleted in the past century.[5] The development of cities and other areas of highly concentrated water usage has created a strain on groundwater resources. Surface water and groundwater interactions experience reduced mixing together between the surface and subsurface (interflow) in post-development scenarios leading to depleted water tables.[8] Groundwater recharge rates are also affected by rising temperatures which increase surface evaporation and transpiration resulting in decreased soil water content.[9] These anthropogenic changes to groundwater storage, such as over pumping and the depletion of water tables combined with climate change, effectively reshape the hydrosphere and impact the ecosystems that depend on the groundwater.[10]

Accelerated decline in subterranean reservoirs[edit]

According to a 2013 report by research hydrologist Leonard F. Konikow[11] at the United States Geological Survey (USGS), the depletion of the Ogallala Aquifer between 2001–2008 is about 32% of the cumulative depletion during the entire 20th century.[11] In the United States, the biggest users of water from aquifers include agricultural irrigation and oil and coal extraction.[12] According to Konikow, "Cumulative total groundwater depletion in the United States accelerated in the late 1940s and continued at an almost steady linear rate through the end of the century. In addition to widely recognized environmental consequences, groundwater depletion also adversely impacts the long-term sustainability of groundwater supplies to help meet the Nation’s water needs."[11]

As reported by another USGS study of withdrawals from 66 major US aquifers, the three greatest uses of water extracted from aquifers were irrigation (68%), public water supply (19%), and "self-supplied industrial" (4%). The remaining about 8% of groundwater withdrawals were for “self-supplied domestic, aquaculture, livestock, mining, and thermoelectric power uses.”[13]

Impacts on the environment[edit]

The environmental impact of overdrafting includes:

  • Land subsidence – the collapse of land from lack of support (from the water that is being depleted). The first recorded case of land subsidence was in the 1940s. Land subsidence can be as little as a local land collapsing or as large as an entire region's land being lowered. The subsidence can lead to infrastructural and ecosystem damage.
  • Lowering of the water table, which makes water harder to reach streams and rivers
  • Reduction of water volume in streams and lakes because their supply of water is being diminished by surface water recharging the aquifers
  • Animals that depend on streams and lakes for food and water and habitat will be affected
  • Deteriorating air quality and water quality
  • The cost of water to the consumer rises due to the water table lowering so more energy is needed to pump further down, and to cover that pumping companies need more profit
  • Crop production decrease from lack of water (60% of US irrigation relies on groundwater so this is a large loss)
  • Groundwater depletion also throws off the water cycle

Effects on climate[edit]

Aquifer drawdown or overdrafting and the pumping of fossil water may be contributing factors to sea-level rise.[14] By increasing the amount of moisture available to fall as precipitation, severe weather events are more likely to occur. To some extent, moisture in the atmosphere accelerates the probability of a global warming event. The correlation coefficient is not yet scientifically determined.

Socio-economic effects[edit]

Scores of countries[quantify] are overpumping aquifers as they struggle to satisfy their growing water needs, including each of the big three grain producers – China, India, and the United States. These three, along with several other countries where water tables are falling, are home to more than half the world's people.[citation needed]

Water is intrinsic to biological and economic growth, and overdraft limits its available supply. According to Liebig's law of the minimum, population growth is therefore impeded. Deeper wells must be drilled as the water table drops, which can become expensive. In addition, the energy needed to extract a given volume of water increases with the amount the aquifer has been depleted. The deeper the water is extracted the worse the quality of the water becomes, which increases the cost of filtration. Saltwater intrusion is another consequence of overdrafting, leading to a reduction in water quality.[citation needed]

Possible solutions[edit]

  • Artificial recharge – Since recharge is the natural replenishment of water, artificial recharge is the artificial or man-made replenishment of groundwater. This is the more aesthetically pleasing option though there is a limited amount of water that can be used for replenishing which is a problem.[15]
  • Decreased water use – This can be used in combination with the solution above in areas where recharge alone will not work. Consumptive use refers to the water that is naturally taken from the system (Ex: transpiration). It requires such things as switching to less water-intensive crops.[15]

See also[edit]

References[edit]

  1. ^ "Groundwater depletion, USGS water science". water.usgs.gov. Retrieved 2015-12-31.
  2. ^ a b Lassiter, Allison (July 2015). Sustainable Water Challenges and Solutions from California. University of California. ISBN 9780520285354.
  3. ^ "Orange County Water District".
  4. ^ "Land subsidence". The USGS Water Science School. United States Geological Survey. 2015-08-20. Archived from the original on 2013-11-10. Retrieved 2013-04-06.
  5. ^ a b Condon, Laura E.; Maxwell, Reed M. (June 2019). "Simulating the sensitivity of evapotranspiration and streamflow to large-scale groundwater depletion". Science Advances. 5 (6): eaav4574. Bibcode:2019SciA....5.4574C. doi:10.1126/sciadv.aav4574. ISSN 2375-2548. PMC 6584623. PMID 31223647.
  6. ^ Black, Maggie (2009). The Atlas of Water. Berkeley and Los Angeles, California: University of California Press. p. 62. ISBN 9780520259348.
  7. ^ Dhawan, B. D. (1993). "Ground Water Depletion in Punjab". Economic and Political Weekly. 28 (44): 2397–2401. JSTOR 4400350.
  8. ^ Sophocleous, Marios (February 2002). "Interactions between groundwater and surface water: the state of the science". Hydrogeology Journal. 10 (1): 52–67. Bibcode:2002HydJ...10...52S. doi:10.1007/s10040-001-0170-8. ISSN 1431-2174. S2CID 2891081.
  9. ^ Green, Timothy R.; Taniguchi, Makoto; Kooi, Henk; Gurdak, Jason J.; Allen, Diana M.; Hiscock, Kevin M.; Treidel, Holger; Aureli, Alice (August 2011). "Beneath the surface of global change: Impacts of climate change on groundwater". Journal of Hydrology. 405 (3–4): 532–560. Bibcode:2011JHyd..405..532G. doi:10.1016/j.jhydrol.2011.05.002.
  10. ^ Orellana, Felipe; Verma, Parikshit; Loheide, Steven P.; Daly, Edoardo (September 2012). "Monitoring and modeling water-vegetation interactions in groundwater-dependent ecosystems: GROUNDWATER-DEPENDENT ECOSYSTEMS". Reviews of Geophysics. 50 (3). doi:10.1029/2011RG000383.
  11. ^ a b c Konikow, Leonard F. Groundwater Depletion in the United States (1900–2008) (PDF) (Report). Scientific Investigations Report. Reston, Virginia: U.S. Department of the Interior, U.S. Geological Survey. p. 63.
  12. ^ Zabarenko, Deborah (20 May 2013). "Drop in U.S. underground water levels has accelerated: USGS". Washington, DC: Reuters.
  13. ^ Maupin, Molly A. & Barber, Nancy L. (July 2005). "Estimated Withdrawals from Principal Aquifers in the United States, 2000". United States Geological Survey. Circular 1279.
  14. ^ "Rising sea levels attributed to global groundwater extraction". University of Utrecht. Retrieved February 8, 2011.
  15. ^ a b Lassiter, Allison (2015). Sustainable Water. Oakland California: University of California Press. p. 186.

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