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Global groundwater depletion leads to sea level rise

Global groundwater depletion leads to sea level rise

A collaboration between IGRAC and the Utrecht University lead to a joint article which is presently in press in Geophysical Research Letters. The article made the front page in the Dutch Journals! Yoshihida Wada and Rens van Beek used IGRAC's GGIS to research the effect of groundwater depletion on sea level rise, with remarkable results: Large-scale abstraction of groundwater for irrigation of crops leads to a sea level rise of 0.8 mm per year, which is about one fourth of the current rate of sea level rise of 3.3 mm per year.

In the drier areas of the world where surface water is scarce, groundwater is abstracted in large volumes in order to irrigate crops, and to a lesser extent for drinking water supply and industry. If groundwater abstraction exceeds groundwater recharge
for extensive areas and long time, overexploitation or persistent groundwater depletion can occur. The resulting lowering of groundwater levels can have devastating effects on natural streamflow, groundwater fed wetlands and related ecosystems.

To assess where in the world groundwater depletion occurs and at what rate, the scientists combined several pieces of information. From a database of the International Groundwater Resources Assessment Centre (IGRAC), which is part of Deltares, they extracted country-based statistics on groundwater abstraction. By combining these statistics with estimates of water demand based on maps of population density and the location of irrigated areas, a global map of groundwater abstraction could be derived. Using a global water balance model, the researchers subsequently produced a global map of groundwater recharge. By subtracting groundwater abstraction from groundwater recharge they arrived at a global map of groundwater depletion.

The results show that the areas of greatest groundwater depletion are in India, Pakistan, the United States and China. Therefore, these are areas where food production and water use are unsustainable and eventually serious problems are expected. The hydrologists estimate that from 1960 to 2000 global groundwater abstraction has increased from 312 to 734 km3 per year and groundwater depletion from 126 to 283 km3 per year.

Because most of the groundwater released from the aquifers ultimately ends up in the world's oceans, it is possible to calculate the contribution of groundwater depletion to sea level rise. This turned out to be 0.8 mm per year, which is a surprisingly large amount when compared to the current sea level rise of 3.3 mm per years as estimated by the IPCC. It thus turns out that almost half of the current sea level rise can be explained by expansion of warming sea water, just over one quarter by the melting of glaciers and ice caps and slightly less than one quarter by groundwater depletion. Previous studies have identified groundwater depletion as a possible contribution to sea level rise. However, due to the high uncertainty about the size of its contribution, groundwater depletion is not included in the latest IPCC report. This study confirms with higher certainty that groundwater depletion is indeed a significant factor.

Wada, Y., L. P.H. van Beek, C. M. van Kempen, J. W.T.M. Reckman, S. Vasak, and M.F.P. Bierkens (2010), Global depletion of groundwater resources, Geophysical Research Letters doi:10.1029/2010GL044571, in press.