July 18, 2014 -- Frankfurt hydrologist Prof. Petra Döll from the Institute of Physical Geography at the Goethe University has been researching what parts of the world and to what degree groundwater reservoirs have been depleting over the past 50 years. Using WaterGAP, the global freshwater model that calculates flows and storages of water on all continents, she has arrived at the most reliable estimate to date by taking into consideration processes that are important in dry regions of the world.
The values calculated were compared with monitoring data from many different wells and data from NASA's Gravity Recovery & Climate Experiment (GRACE) satellites (see: "NASA satellites detect possible disastrous flooding months in advance"). These satellites measure changes in the Earth's gravity field. Döll has come to the conclusion that the rate at which groundwater reservoirs are being depleted is increasing but that the rate is not as high as previously estimated.
Ninety percent of water consumption is due to irrigation for farming purposes. Only the comparatively small remainder is used for potable water and industrial production. As an example, 40 percent of the cereals produced around the world are irrigated. However, in many cases, this results in increased scarcity of water resources and puts a burden on ecosystems. In dry regions, the amount taken from groundwater reservoirs can easily exceed the amount being replenished so that the groundwater reservoir is overused and depleted.
"By comparing the modeled and measured values of groundwater depletion, we were able for the first time to show on a global scale that farmers irrigate more sparingly in regions where groundwater reservoirs are being depleted," said Döll. "They only use about 70 percent of the optimal irrigation amounts."
The rate at which the Earth's groundwater reservoirs are being depleted is constantly increasing. Annual groundwater depletion during the first decade of this century was twice as high as it was between 1960 and 2000. India, the U.S., Iran, Saudi Arabia, and China are the countries with the highest rates of groundwater depletion. About 15 percent of global groundwater consumption is not sustainable, meaning that it comes from non-renewable groundwater resources. On the Arabian Peninsula, in Libya, Egypt, Mali, Mozambique, and Mongolia, over 30 percent of groundwater consumption is from non-renewable groundwater.
The new estimate of global groundwater depletion is 113,000 million cubic meters per year for the period from 2000 to 2009, which is lower than previous, widely-varying estimates. This can be considered to be the most reliable value to date, since it is based on improved groundwater consumption data which takes the likely deficit irrigation into account and since the model results correlate well with independent comparative data.
The increased use of groundwater for irrigation also results in a rise in sea levels: According to Döll's calculations, sea level rise due to groundwater depletion was 0.31 millimeters per year during the period from 2000 to 2009. This corresponds to roughly one tenth of the total sea level rise.
See also:
"Aquifers across U.S. being depleted at accelerating rate, finds study"
"Groundwater Depletion Linked to Rising Sea Levels"
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