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Global groundwater depletion is growing year by year, reaching 113,000 million cubic metres per year in the past decade – twice as high as the rate between 1960 and 2000.

Using a global water model system, hydrologist Petra Döll from the Goethe University Frankfurt and her colleagues calculated the most reliable estimate to date of the rate of groundwater depletion – especially in dry regions of the world.

The bulk of that water is being used for crop irrigation. Only 10% is used for industry and drinking, with the rest going to farming. In dry areas in particular, the amount of water drained from underground reservoirs for use often exceeds the rate it is being replenished, which could eventually cause them to dry up.

In the Middle East, Saudi Arabia and Iran are the two countries with the highest rates of groundwater depletion.

In the Arabian Peninsula and Libya, Egypt, Israel, Mali, Mozambique and Mongolia, at least 30% of the underground water also came from non-renewable sources. This unsustainable use can further increase water scarcity in the future.

However, Döll points out that her estimates, while showing an increase in underground water depletion, are not as bad as was previously expected. She suggests this is because farmers in dry regions are using less water for irrigation than normal irrigation amounts.

“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% of the optimal irrigation amounts,” explained Döll.

But the researchers point out that model still contains several uncertainties. For example, the groundwater depletion in Saudi Arabia is equal to the independent estimate if optimal irrigation is assumed, while in the Nubian Sandstone Aquifer System the depletion fits best with the reduced irrigation scenario suggested.