California鈥檚 Central Valley has lost roughly 85 cubic kilometres of groundwater storage since 2004 due to intensive pumping during periods of drought.
The Central Valley is one of the most productive agricultural regions in the world, growing 40 per cent of the fruit and nuts produced in the US.听When surface water is inadequate to irrigate all those crops, farmers pump groundwater from the region鈥檚 aquifer. For more than a century, farmers have pumped more groundwater than was replaced by snowmelt and rainfall, causing the aquifer to shrink.
To quantify this change, at Virginia Tech and his colleagues looked at satellite measurements of changes in altitude at 125 points across the Central Valley. As water levels in an aquifer fall, pores that hold water in sediments collapse, causing sinking at the surface. The researchers related these changes in altitude to changes in the amount of water in the aquifer.
Advertisement
Between 2004 and 2019, the Central Valley lost about 60 cubic kilometres of groundwater storage, says Shirzaei. Most of the loss occurred during droughts between 2007 and 2010, and 2012 and 2016, and mainly in the drier southern half of the Central Valley.
This aligns with a study by at Stanford University in California and his colleagues that 67 cubic kilometres were lost between 2002 and 2020.
Between 2019 and now, another dry period, Shirzaei says unpublished results show the valley has lost an additional 25 cubic kilometres of storage. Ahamed says the loss is greater than the combined capacity of all the reservoirs in the state. 鈥淭he numbers are shocking,鈥� says Shirzaei.
at the University of Oregon presented parts of this at the Geological Society of America conference in Denver, Colorado, last week.
While 鈥渟taggering鈥�, the loss in storage represents a fraction of the aquifer鈥檚 overall capacity, says at the Public Policy Institute of California, a nonprofit think tank. There is still more groundwater available 鈥渢han we could conceive of using,鈥� he says.
But the loss of storage has created other problems, says at Colorado State University. Rapidly sinking land has disrupted water infrastructure and complicated planning for a high-speed railway. Water has been in the shrunken aquifer, and farmers have had to drill deeper and use more energy to pump water. Underserved communities who can鈥檛 drill as deep have seen their .
A state law passed in 2014 more sustainable groundwater pumping by 2040, but most of the plans irrigators have submitted so far were judged lacking by the state, says Mount. If new plans aren鈥檛 submitted, the state may step in to enforce changes.
And some of the loss is irreversible no matter the changes in pumping. Natural water pressure isn鈥檛 strong enough to refill some , says Smith. Even where aquifers could be recharged, the loss is effectively permanent, says Mount. 鈥淲e鈥檙e not going to stop and put a hundred years of water back in,鈥� he says.
