In 2023, a parade of atmospheric rivers brought months of heavy precipitation to much of California, filling reservoirs and raising snowpack far above average levels. This flood of water was a major relief after nearly two decades of drought. But a seismic study has now revealed that water on the surface did little to restore the state鈥檚 depleted reserves underground.
鈥淭here was very limited recovery, compared to the groundwater lost over the recent droughts,鈥� says at the University of Texas at Austin.
Groundwater supplies between 40 and 60 per cent of California鈥檚 water. But years of drought and over-pumping, especially for irrigation, has severely depleted the state鈥檚 aquifers. 鈥淧eople don鈥檛 really see what鈥檚 underground, so they tend to think less about groundwater,鈥� says Mao.
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She and her colleagues used the dense network of seismic sensors around earthquake-prone Los Angeles to precisely measure how groundwater changed in the aquifers beneath the city during years of drought and 2023鈥檚 heavy precipitation. They measured the faint ambient seismic waves generated by human activity and natural forces like the tides. When these waves pass through an aquifer, they change velocity depending on the water pressure.
While all that precipitation in 2023 largely replenished the shallow aquifers beneath Los Angeles, the researchers found deeper aquifers only regained 25 per cent of the water they had lost due to pumping during droughts since 2006. This shows one wet year isn鈥檛 sufficient to solve California鈥檚 groundwater crisis, especially given the 鈥渨hiplash鈥� swings between wet and dry years there.
With the state currently facing another extremely wet forecast after a year of drought, California is actively trying to capture more of this stormwater underground. This could involve slowing it down by allowing it to pour out of rivers to flood fields or by directly pumping it underground in wells. The low recovery rate in 2023 suggests there is plenty of scope for such efforts to make a difference, says Mao.
Seismic monitoring could also offer a cheaper and higher-resolution way to monitor the efficacy of replenishment efforts. 鈥淲e鈥檙e pioneering this approach in Los Angeles because of all the existing seismometers that study earthquakes,鈥� says Mao. But she says other areas with networks of seismometers could use a similar approach, from Japan to New Zealand. Eventually, this could provide a near-real-time measure of water storage in aquifers deep underground.
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