Researchers from NASA and the University of California, Irvine used NASA’s Gravity Recovery and Climate Experiment satellite mission to measure mass changes – and hence changes in the amount of water present – in the basin, which has been in drought since 2000. By subtracting the amount removed from surface sources, which is documented by the US Bureau of Reclamation, the researchers were able to calculate the loss of groundwater. The results showed that between 2004 and 2013, the basin lost almost 65 million cubic kilometres of freshwater, of which more than 50 cubic kilometres came from groundwater.

‘We don’t know how much groundwater we have left, so we don’t know when we’re going to run out,’ said Stephanie Castle of UC Irvine, the paper’s lead author. ‘This is a lot of water to lose. We thought that the picture could be pretty bad, but this was shocking.’

The Colorado River, the region’s only major river, supplies water for 40 million people across seven states and 1.6 million hectares of farmland. ‘Combined with declining snowpack and population growth, this will likely threaten the basin’s long-term ability to meet its water allocation commitments to the seven basin states and to Mexico,’ said one of the study’s co-authors, NASA’s Jay Famiglietti.