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Napa Valley’s annual groundwater checkup concluded that water levels in a majority of monitoring wells were stable in spring 2018, despite a drop in overall groundwater storage following a subpar rainy season.

The valley’s vast underground reservoir that farmers tap to irrigate the region’s world-famous vineyards held an estimated 210,000 acre-feet of water. That’s about seven times as much water contained in a full Lake Hennessey.

It's also 9,300 acre-feet less than in spring 2017, according to a 268-page groundwater report done for Napa County by consultants Luhdorff & Scalmanini. But the higher 2017 subbasin storage came after one of the wettest rainy seasons on record.

Luhdorff & Scalmanini concluded that “the Napa Valley Subbasin has continued to be managed sustainably through 2018.”

The report also took a longer view of Napa Valley Subbasin groundwater storage from 1988 through 2018. The cumulative annual storage change over 30 years is positive 4,388 acre-feet, reflecting “a basin in balance,” the report said.

“The fact is, we are pumping,” Supervisor Alfredo Pedroza said after hearing the Luhdorff & Scalmanini presentation at Tuesday’s Board of Supervisors meeting. “But the fact is it is within the sustainable yield.”

Local environmentalist Chris Malan voiced concerns to supervisors during public comments. Among other things, she wants the county to take further steps to make certain well pumping doesn't dewater streams and cause fish kills.

Malan stressed to supervisors during public comments that “this is a finite resource.”

The Napa Valley Subbasin extends under the valley floor from Calistoga to south of the city of Napa. One goal of the annual monitoring reports is to make certain farmers and rural residents aren’t pumping too much water with their wells.

Over-pumping can lead to more than the need to drill ever-deeper wells. Plunging groundwater levels in the Central Valley have caused the ground there to subside as much as 28 feet in some locations since the 1920s. Depleted groundwater levels locally could mean less water seeping into the Napa River during hot summers for fish.

“Everyone living and working in Napa County has a stake in protecting the county’s groundwater resources, including groundwater supplies, groundwater quality, and associated watersheds,” the monitoring report stated.

One change for the Napa Valley Subbasin comes from the state. The Department of Water Resources in 2018 increased the priority designation for the subbasin from “medium” to “high.”

At first glance, that might indicate that the state views groundwater conditions as growing worse. But the Luhdorff & Scalmanini report said the reason is primarily due to revised future population estimates, an increased assessment of well totals and a revised approach to evaluating water quality.

“Just because it’s high priority doesn’t mean there’s something concerning,” said Vicki Kretsinger Grabert of the consulting firm.

Malan had a different interpretation. She said the county’s trajectory is heading in the direction of unsustainability.

Grabert later responded that the state laws implemented in January 2015 do not require groundwater basins to be returned to pristine conditions, as though there had never been development anywhere in California.

The monitoring report said farmers and residents pumped a total of 17,889 acre-feet of groundwater in water year 2018. That rose from 15,831 acre-feet the previous year, but was still within the 17,000-to-20,000 acre-foot range that the report considers the annual sustainable yield.

Weather played a role in slightly less robust subbasin groundwater storage in 2018, the report said.

Spring 2018 followed a rain season that saw 19.3 inches of rain fall at Napa State Hospital. That compares to an annual average of 24.86 inches from 1920-2015.

A fuller Napa Valley Subbasin in spring 2017 followed a rainy season with 45 inches, the third wettest year on record at Napa State Hospital since 1892.

Napa County could soon have more work to do for its annual groundwater monitoring program. The state is looking at reclassifying the Napa-Sonoma Lowlands Subbasin in the Carneros and American Canyon area from low priority to medium priority.

If that happens, the county would have to come up with a groundwater management strategy under the state Sustainable Groundwater Management Act for the Napa-Sonoma Lowlands, in addition to the Napa Valley Subbasin.

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Napa County Reporter

Barry Eberling covers Napa County government, transportation, the environment and general assignments. He has worked for the Napa Valley Register since fall 2014 and previously worked 27 years for the Daily Republic of Fairfield.