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.
Rising sea levels
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 millimetre per year. This is surprisingly large when compared to the current annual rise in sea levels, which the IPCC estimates at 3.1 mm.” About half of the current rise in sea levels can be attributed to thermal expansion, a little over one quarter to run off from glaciers and ice caps, and the remaining quarter to groundwater depletion. “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 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 Centre (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.
Depletion worst in certain countries
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.” The hydrologists estimate that global groundwater extraction and depletion have increased by 312 km3 to 734 km3 and 126 km3 to 283 km3 per year, respectively, since 1960.