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The rate at which groundwater reservoirs are being depleted is increasing

17 July 2014 Goethe-Universität Frankfurt am Main

FRANKFURT. In what parts of the world and to what degree have
groundwater reservoirs been depleted over the past 50 years? The Frankfurt
hydrologist Prof. Petra Döll has been researching this using the global water
model WaterGAP. She has arrived at the most reliable estimate to date by taking
into consideration processes which are important in dry regions of the world.
The values calculated were compared with monitoring data from many different
wells and data from the GRACE satellites. 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.

90 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 is 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 modelled 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. They only use about 70 percent of the optimal
irrigation amounts", explains Petra Döll from the Institute of Physical
Geography at the Goethe University.

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 USA, 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 millimetres per year during the
period from 2000 to 2009. This corresponds to roughly one
tenth of the total sea level rise.

The work was funded by the Deutsche
Forschungsgemeinschaft through the priority program "Mass transport and
Mass distribution in the System Earth".

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