Ground Water and Sea Level
We use ground water for many purposes. Large-scale groundwater extraction for irrigation, drinking water or industry may result in an annual rise in sea levels of approximately 0.8 mm, accounting for about one-quarter of total annual sea-level rise (3.1 mm). According to hydrologists from Utrecht University and the research institute Deltares, the rise in sea levels can be attributed to the fact that most of the groundwater extracted ultimately winds up in the sea. The hydrologists explain their findings in an article to be published in the near future in the journal Geophysical Research Letters. Alternately severe ground water extraction near the sea will also tend to have the sea water replace the ground water table.
At present, sea levels around the world are rising. Current sea level rise potentially impacts human populations (e.g., those living in coastal regions and on islands) and the natural environment (e.g., marine ecosystems). Global average sea level rose at an average rate of around 1.7 ± 0.3 mm per year over 1950 to 2009 and at a satellite-measured average rate of about 3.3 ± 0.4 mm per year from 1993 to 2009.
Two main factors are known to contribute to observed sea level rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the contribution of land-based ice due to increased melting. The major store of water on land is found in glaciers and ice sheets. Now there may be a third reason: ground water extraction.
Groundwater extraction is more common in more arid regions of the world, where there is less available surface water. It is used for crop irrigation, drinking water or industrial purposes. Aquifer levels will decline if over a prolonged period more groundwater is extracted at more locations than can be replenished by means of rainwater recharge. As a consequence, rivers and wetlands will run dry and aquifer levels will fall to such a depth that pumping becomes impossible. Aquifer depletion can eventually spell ecological disaster or even lead to famine.
With the knowledge that most of the extracted groundwater eventually winds up in the sea, the researchers at Utrecht University and Deltares calculated the contribution groundwater extraction makes to rising sea levels. Researcher Marc Bierkens says, "We calculated it at eight-tenths of a millimeter per year. This is surprisingly large when compared to the current annual rise in sea levels, which the IPCC estimates at 3.1 mm."
"Although the role of groundwater depletion in rising sea levels had already been acknowledged, it was not addressed in the most recent IPCC report due to a lack of reliable data to illustrate the severity of the situation. Our study confirms that groundwater depletion is, in fact, a significant factor."
The researchers also looked at a combination of information to identify the areas in the world where groundwater extraction leads to groundwater depletion. An estimate of the amount of groundwater extracted annually in most of the world’s countries could be obtained from a database of the International Groundwater Resources Assessment Center (IGRAC), which is affiliated with Deltares. Combining this information with the estimated demand for water, based on population density and data on irrigated areas, the researchers were able to produce a map of global groundwater extraction. A water balance model was then used to map out global groundwater aquifer recharge, i.e. precipitation that seeps through the soil to recharge groundwater aquifers. By subtracting the figures of the groundwater extraction map from the figures of the groundwater aquifer recharge map, the researchers were able to compile a map of global groundwater depletion.
According to Bierkens, "The study reveals that depletion is the most acute in areas of India, Pakistan, the US and China, which are also the regions without sustainable levels of food production and water consumption and which are expected to experience major problems in the long run."