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", 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.
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. 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.
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.
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.
See also: Environmental impact of irrigation
|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 has been a problem in other parts of the world, as was documented in Punjab, India in 1987
In the US, an estimated 800 km3 of groundwater was depleted in the past century. 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. Groundwater recharge rates are also affected by rising temperatures which increase surface evaporation and transpiration resulting in decreased soil water content. 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.
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 is about 32% of the cumulative depletion during the entire 20th century. In the United States, the biggest users of water from aquifers include agricultural irrigation and oil and coal extraction. 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."
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.”
The environmental impact of overdrafting includes:
Aquifer drawdown or overdrafting and the pumping of fossil water may be contributing factors 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[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.
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.