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 of 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 said. “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 across the state. Many well operators only report water levels to the state twice a year.