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.
Natural process of recharge is done through percolation of surface water. Artificial process of recharging the aquifer is through means of pumping reclaimed water from wastewater management projects directly into the aquifer. An example is the Orange County Water District in the State of California. This organization take 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 drafting of water continues the cone of depression increases in width. The increase in width leads to the negative impacts caused by overdrafting, such as 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 has an effect on wildlife, as well as humans who might be using the water for other purposes.
Since every groundwater basin recharges at a different rate depending upon precipitation, vegetative cover and soil conservation practises, 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.
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¨, The primary cause of groundwater depletion is pumping or the excessive pulling up of groundwater from underground aquifers.
Around the world
|Country||Million hectares (1×106 ha (2.5×106 acres)) irrigated with groundwater|
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 is also worth noting that it has been a problem in other parts of the world, as was documented in Punjab, India in 1987
Accelerated decline in subterranean reservoirs
According to a 2013 report by research hydrologist, Leonard F. Konikow, at the United States Geological Survey (USGS), the depletion of the Ogallala Aquifer between 2001–2008, inclusive, is about 32 percent of the cumulative depletion during the entire 20th century (Konikow 2013:22)." In the United States, the biggest users of water from aquifers include agricultural irrigation and oil and coal extraction. "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."
According to another USGS study of withdrawals from 66 major US aquifers, the three greatest uses of water extracted from aquifers were agriculture (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.”
Impacts on the environment
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 water quality
- The cost of water to the consumer rises. This is 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
Aquifer drawdown or overdrafting and the pumping of fossil water may be a contributing factor to sea-level rise. 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.
Scores of countries 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 a number of other countries where water tables are falling, are home to more than half the world's people.
Water is intrinsic to biological and economic growth, and overdraft limits its available supply. According to Liebig's law of the minimum, 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 from 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.
- 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.
- Decreased water use - This can be used in combination with the solution above. Consumptive use refers to the water that is naturally taken from the system (Ex: transpiration ). So decreasing that use is what needs to happen in areas where recharge alone will not work. It requires such things as switching to less water-intensive crops.
- Lassiter, Allison (July 2015). Sustainable Water Challenges and Solutions from California. University of California. ISBN 9780520285354.
- "Orange County Water District".
- "Land subsidence". The USGS Water Science School. United States Geological Survey. 2015-08-20.
- "Groundwater depletion, USGS water science". water.usgs.gov. Retrieved 2015-12-31.
- Black, Maggie (2009). The Atlas of Water. Berkeley and Los Angeles, California: University of California Press. p. 62. ISBN 9780520259348.
- Dhawan, B. D. (1993). "Ground Water Depletion in Punjab". Economic and Political Weekly. 28 (44): 2397–2401. JSTOR 4400350.
- 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.
- Zabarenko, Deborah (20 May 2013). "Drop in U.S. underground water levels has accelerated: USGS". Washington, DC: Reuters.
- Maupin, Molly A. & Barber, Nancy L. (July 2005). "Estimated Withdrawals from Principal Aquifers in the United States, 2000". United States Geological Survey. Circular 1279.
- "Rising sea levels attributed to global groundwater extraction". University of Utrecht. Retrieved February 8, 2011.
- Lassiter, Allison (2015). Sustainable Water. Oakland California: University of California Press. p. 186.
- The Perils of Groundwater Pumping, Issues in Science and Technology