Wednesday, May 31, 2023, at The Page Recharge Pond in Campbell, California. (Nhat V. Meyer/Bay Area News Group)

Anyone driving around the Bay Area can see how the heavy rains that flooded California this winter filled local reservoirs after three years of severe drought. deaf.

However, wet winters have replenished underground aquifers, an equally (if not more importantly) source of water. Communities that depend on groundwater throughout the Bay Area, from Silicon Valley to the suburbs of the East Bay, have experienced significant increases in groundwater supplies in recent months, reaching record levels.

The invisible bounty is dramatic, refuting the misconceptions common to many Californians that groundwater always takes years to recover, or is so hopelessly over-pumped that it can never recover. is. That’s true in some high-yield farm areas in the Central Valley, but after decades of carefully managing groundwater supplies, the Bay Area Water Authority has seen the fruits of its labor this winter, experts say.

Groundwater provides 40% of the water that supplies 2 million people in Santa Clara County. Following a dozen atmospheric river storms this winter, the county’s main water table has risen 35 feet since last June and 51 feet from the drought’s most extreme time in September 2021, with drought back to the previous level. The county’s main groundwater basin is now about 90 percent full.

“All the rain certainly helped,” said Vanessa de la Piedra, groundwater division manager for the Santa Clara Valley Water District. “There’s no question we’ve seen significant increases across the county.”

Measurements two weeks ago showed that groundwater was just 64 feet below the surface at the district’s main watch well near the corner of Hamilton and Lee streets in San Jose. This is the highest level ever recorded since measurements began in 1936.

Similar rebounds have also occurred at Sunnyvale, Milpitas and Morgan Hill wells, with major index wells rising 50 feet since September 2021 and now at their highest level in five years.

A similar trend is occurring in the Alameda County Water District, which supplies water to 345,000 people in Fremont, Newark, and Union City.

Water tables have risen 13 feet since December 31 in the Niles Cone groundwater basin, which supplies 40 percent of the district’s water supply.

“We’re almost full right now,” said district general manager Ed Stevenson.

“We consider groundwater to be the most important source because it is under local control,” he said. “It’s good that the state reservoirs are full now. That’s great. But local groundwater is key for us.”

The district diverts water from Alameda Creek to an old gravel pit in Fremont’s Quarry Lakes Park. A dozen or so pits, where gravel was taken to support the construction of the transcontinental railroad, act as natural seepage ponds, allowing water to gradually seep into the ground.

In Livermore and Pleasanton, the water table has risen 30 to 80 feet, filling the reservoirs, said Sal Seguro, a civil engineer at the Zone 7 Water Authority, which supplies water to 265,000 people in the area. said.

The agency buys water from state water projects and uses it to recharge aquifers that are withdrawn during droughts, he said.

“Each district is trying to remove as many socks as possible while they are still available,” he said. “Especially after a drought.”

Santa Clara County has three times as much water stored underground as the county’s 10 reservoirs can hold when full. But that groundwater is not in a huge open cavern. It fills the space between millions of tons of sand and gravel. Groundwater projects are often cheaper than building new reservoirs and less controversial than building new dams on rivers.

However, due to geology and historical practices, some of the Bay Area’s largest water utilities, such as the San Francisco Public Works Commission’s Hetch Hetch Project and the East Bay Municipal Utilities District, do not hold much groundwater. Some places don’t.

Experts say it will be some time before the full impact of this year’s historically wet winter on groundwater supplies across the state is known. Many well managers report water levels to the state only twice a year.

But some clues are emerging. out of 3,400 wells Oversight by the State Water Resources Authority In the areas where measurements were taken this spring, 35% showed an increase in groundwater of at least 5 feet, while 59% showed no change and 6% showed a decrease compared to levels one year ago. was shown.

Many of the locations where the greatest improvements will be seen are along the coast, the Bay Area, or the Sacramento Valley. The San Joaquin Valley has many wells that continue to decline.

Geology can make a big difference. Experts say that where groundwater is only 25 to 100 feet below the surface, water seeps from below the surface in wet years, and water seeps in from underneath streams and rivers, causing water to seep faster in wet years. It is said to be recharged. Some areas of the Central Valley have groundwater at depths of 500 feet or more. Also, the thick clay layer makes it more difficult to charge in the short term.

“If you divide the state into areas with shallower aquifers, such as the Coast Mountains, you’ll see a more rapid response,” said Tim Parker, a veteran hydrologist and president of Parker Groundwater Company in Sacramento. talk. “But in the San Joaquin Valley, the southern part of the Central Valley, water levels are still fairly low. Much of it is at historic lows.”

Decades of persistent over-pumping by farmers have created a crisis in some areas of the San Joaquin Valley.

a study in december Scientists at NASA and Arizona State University found that the recent drought caused the rate of groundwater loss in the Central Valley to be 31 percent greater than during the previous two droughts.

They also found that groundwater losses in the Central Valley since 2003 have totaled about 36 million acre-feet, about eight times the capacity of California’s largest reservoir, Lake Shasta, near Redding.