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.
|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
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.”
The environmental impact of overdrafting includes:
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.