Groundwater is back to pre-drought levels
Public can't see the aquifers, but winter storms elevated Bay Area water tables significantly and boosted supplies for future dry years
Anyone driving around the Bay Area can see how the drenching storms that soaked California this winter filled local reservoirs after three brutal years of drought.
But the wet winter also refilled underground aquifers that communities across the Bay Area rely on. From Silicon Valley to the East Bay suburbs, the majority of cities have measured big increases in recent months in their subterranean supplies and some of the highest levels on record.
The unseen bounty is dramatic, and rebuts a common misperception among many Californians that groundwater always takes years to recover, or is all so hopelessly overdrawn it can never be restored. While that is true in some heavily pumped farm areas in the Central Valley, experts say, water agencies in the Bay Area that have carefully managed groundwater supplies for decades saw the payoff this winter.
Groundwater provides 40% of the water supply for 2 million people in Santa
Clara County. Following more than a dozen major atmospheric river storms this winter, the main water table in the county has risen 35 feet since last June — and is up 51 feet since the most extreme part of the drought in September 2021 — returning to pre-drought levels. The county's main groundwater basin is now about 90% full.
“All the rain certainly helped,” said Vanessa de la Piedra, groundwater unit manager at the Santa Clara Valley Water District. “We definitely saw big increases throughout the county.”
Readings taken two weeks ago show that groundwater is just 64 feet below the surface at the district's main monitoring well in San Jose near the corner of Hamilton and Leigh avenues. That's the highest level ever recorded since readings began there in 1936.
Similar rebounds have occurred in wells in Sunnyvale, Milpitas and Morgan Hill, where the main index well came up 50 feet since September 2021 and is now at its highest level in five years.
A similar trend has unfolded at the Alameda
County Water District, which provides water to 345,000 people in Fremont, Newark and Union City.
There, the water table has risen 13 feet since Dec. 31 at the Niles Cone Groundwater Basin, which provides 40% of the district's supplies.
“It's pretty dang close to full now,” said Ed Stevenson, general manager for the district.
“We consider groundwater to be our most important supply because it is under local control,” he added. “It's good the state's reservoirs are brimming full right now. That's fantastic. But the local groundwater is key to us.”
The district diverts water from Alameda Creek into old gravel pits at Quarry Lakes park in Fremont. The dozen or so pits, where gravel was taken to help build the transcontinental railroad, act as natural percolation ponds, allowing water to gradually seep back into the ground.
In Livermore and Pleasanton, the water table has risen between 30 and 80 feet, and groundwater basins are full, said Sal Seguro, a civil engineer with the Zone 7 Water Agency, which supplies water to 265,000 people in the area.
The agency is taking water it purchases from the State Water Project and using it to recharge aquifers that were drawn down during the drought, he said.
“Districts are trying to sock away as much as they can while they have it,” he said. “Especially after the drought.”
In Santa Clara County, there is three times as much water storage underground as the county's 10 reservoirs can hold when full. That underground water isn't sitting in giant open caverns, however. It is filling the spaces between millions of tons of sand and gravel. Groundwater projects are often cheaper than constructing new reservoirs and have less controversy than building new dams on rivers.
But because of geology or historical practice, some large Bay Area water providers don't have much groundwater, including the San Francisco Public Utilities Commission's Hetch Hetchy project and the East Bay Municipal Utility District.
It will be a while, experts say, before the full impact of this year's historically wet winter is known on groundwater supplies. Many well operators only report water levels to the state twice a year.
But some clues are emerging. Of 3,400 wells monitored by the State Department of Water Resources where measurements were taken this spring, 35% showed groundwater increases of at least 5 feet — however, 59% showed no change and 6% showed a decrease when compared with levels a year ago.
Many of the places showing the most improvement are along the coast, in the Bay Area or in the Sacramento Valley. The San Joaquin Valley has many of the wells showing continued decreases.
Geology can make a big difference. Places with groundwater only 25 feet to 100 feet below the surface recharge more quickly in wet years from rain, and water seeping from below creeks and rivers, experts say. Some areas in the Central Valley have groundwater 500 feet or deeper. They also have thick clay layers that make it more difficult to recharge over short time periods.
“If you break the state up into areas where you have shallower aquifers, like in the Coast Range, you are going to see quicker response,” said Tim Parker, a veteran hydrologist and president of Parker Groundwater in Sacramento. “But in the southern part of the Central Valley, the San Joaquin Valley, the water levels are still quite low. A lot of them are at historic lows.”
Decades of relentless overpumping by farmers have created a crisis in some parts of the San Joaquin Valley.
A study in December from scientists at NASA and Arizona State University found that during the most recent drought, the rate of groundwater depletion in the Central Valley was 31% greater than during the two previous droughts.
They also found that groundwater losses in the Central Valley since 2003 totaled about 36 million acre-feet, or about eight times the capacity of Shasta Lake, California's largest reservoir, near Redding.
Santa Clara County has a long history of groundwater struggles. The water table fell nearly 200 feet from 1915 to 1960 as farmers and residents of growing suburbs increased their water use. That overpumping caused the ground to fall as much as 13 feet around San Jose. But the water table began to slowly recover when the Santa Clara Valley Water District began to construct local reservoirs, import water from the Delta and impose what's commonly known as “the pump tax” to fund groundwater recharge programs. The district now has 102 percolation ponds where it moves water to recharge underground supplies.
The main underground supply, called the Santa Clara Sub Basin, is currently back up to the same level it was 100 years ago, despite the fact that the county's population has grown from 83,000 in 1910 to 2 million today.
In 2014, concerned about massive overdraft in some parts of the state, Gov. Jerry Brown signed a sweeping law aimed at better-regulating groundwater in California. It requires local government agencies in places where groundwater is most at risk to draw up plans to recover it.
Critics, including some environmental groups, say the law will take too long to make an impact. As part of a compromise with farmers, it gives areas until 2040 to bring their groundwater pumping back to sustainable levels. Nevertheless, the law could take 500,000 acres out of farm production in the San Joaquin Valley, according to some estimates by the Public Policy Institute of California.
Although Bay Area agencies have had percolation ponds and other recharge programs in place since the 1960s and before, some Central Valley communities are just getting started on major recharge efforts.
“Very few people know about these groundwater basins,” Stevenson said. “Groundwater isn't as exciting as a reservoir you drive past, or the snowcapped mountains you can see. But it's just as important a water supply.”