Even epic rainfall may not be enough to refill SoCal’s aquifers

Even though immense rains repeatedly pummeled California in 2023, they barely helped recharge aquifers drawn down by decades of drought and human pumping, a new study reveals.

About one-third of the water supply in Los Angeles, which is susceptible to long dry spells, comes from groundwater. But in the first three months of 2023, more than a dozen atmospheric rivers — long, narrow weather systems chock full of water vapor — brought rainfall to the West Coast. Then, in August, hurricane Hilary spilled rain over Southern California. Statewide, precipitation for the year measured well over double its 20th century average. Altogether, the January-through-August precipitation added more than 90 billion gallons of water into surface reservoirs in the Los Angeles area.

That moisture almost completely recharged the region’s near-surface aquifers. But deeper water-bearing layers hardly gained any relief, William Ellsworth, a seismologist at Stanford University, and his team report February 13 in Science.

To make that assessment, Ellsworth and his colleagues looked at how the water that had percolated down into previously parched layers of permeable rock affected the speed of seismic waves traveling through them. Previous teams have used ever-present seismic noise — both from small quakes and from human causes such as traffic and industrial activity — to map faults and other subterranean characteristics.

What many researchers consider seismic noise is “free information, which is there in the earth every day,” Ellsworth says. “To be able to do something with that is really exciting.”

By analyzing vibrations of different frequencies, Ellsworth and the team could identify any changes due to water infiltration as deep as hundreds of meters below the surface.

Overall, the team notes, only about 25 percent of the water lost from the region’s aquifers since 2006 was replenished by the storms of 2023.

“Getting a 3-D picture of water storge in aquifers over time is pretty exciting,” says Roland Bürgmann, a geophysicist at University of California, Berkeley. Although the technique shows promise, many regions don’t have the large dense networks of seismic instruments that California does. But for those areas, researchers might be able to extract useful information from underground fiber-optic networks equipped with the right sensors.