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Napa County is trying to make the case that it does a good job managing the underground reservoir beneath the Napa Valley that provides groundwater for rural homes, wineries and vineyards.

Groundwater levels beneath the main Napa Valley floor are five to 35 feet deep in the spring and the basin remains “full overall” despite the drought, according to a new, draft Napa Valley groundwater report.

“The conditions in the main Napa Valley subbasin have been stable for many decades,” said consultant Vicki Kretsinger Grabert, who worked on the report.

Napa County wants to show it is already on top of the Napa Valley subbasin groundwater situation. Then it can avoid taking more complicated steps required under the state’s new groundwater regulatory regime, though some residents say this more complicated route would be better for the county.

Much hinges on that newly released, draft report entitled “Napa Valley Groundwater Sustainability – A Basin Analysis Report for the Napa Valley Subbasin.” That’s where the county in 1,151 pages, including the appendices, tries to prove its point.

The county’s Watershed Information & Conservation Council on Nov. 3 held a workshop on the draft report. The Napa County Board of Supervisors on Dec. 13 is to consider submitting the report to the state Department of Water Resources by the Jan. 1 deadline.

The Napa Valley subbasin is under the valley floor from Calistoga south past Napa. It does not include the hillsides or the rural Milliken-Sarco-Tulocay area east of the city of Napa, where groundwater issues are more challenging.

Groundwater use in the subbasin varied from about 10,000 acre-feet in 1988 to more than 20,000 acre-feet in 2008 to 10,000 acre-feet in 2011 to 18,000 acre-feet in 2015, the report calculated. An acre-foot is the amount of water that would cover an acre one foot deep.

In general, groundwater use increased from 1988 to 2015 and surface water use decreased, particularly from the Napa River system, the report said.

“As the basin is currently managed, stable groundwater levels observed during recent drought conditions suggest that recent rates of groundwater pumping have not exceeded the sustainable yield of the subbasin,” the draft report said.

The report recommended that the county keep groundwater use at levels consistent with those between 1988 and 2015. That would continue protecting the portion of Napa River flows during dry weather that comes from water seeping into the channel.

But some residents at this and other meetings have questioned whether the Napa River is getting enough seepage to help native fish. They said they’ve seen less water in the Yountville-to-Calistoga stretch as the years have gone by.

Seasonal and year-to-year variability in rainfall and other factors affects both groundwater and surface water, the county responded in a "frequently asked questions" paper. The Napa River has had periods of no-flow days during dryer years since the 1930s, it said.

Other residents have wondered why the basin analysis plan doesn’t include the parts of the county where well water levels have fallen in recent decades.

California is requiring communities to look at medium and high priority groundwater basins, the county replied in the “frequently asked questions” paper. The state has put only the Napa Valley subbasin under that category within Napa County.

But Napa County claims it isn’t ignoring these other areas. The Milliken-Sarco-Tulocay area is one of the most monitored in the county and has various land use controls in place based on groundwater, the paper said.

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Napa County is among communities throughout the state taking a close look at groundwater. California in 2014 passed the controversial Sustainable Groundwater Management Act as well levels in the Central Valley plunged.

“California will no longer be the only Western state that does not manage its groundwater,” state Sen. Fran Pavley said in a press release. “Thousands of homes and small farms cannot keep pace with the race to drill deeper and deeper wells.”

The law set priorities for groundwater basins throughout the state. Local governments can form Groundwater Sustainability Agencies to create Groundwater Sustainability Plans or risk having the state step in to do the work for them.

An alternative is for local governments to submit a basin analysis report to the state showing they are already doing a good job managing their groundwater. The Napa County Board of Supervisors in 2015 decided to pursue this route.

“This is likely the fastest and least expensive path to compliance with the Act,” a March 2015 county report said.

Patrick Lowe of the Watershed Information & Conservation Council recounted steps the county has taken to address groundwater issues dating back to the 2008 General Plan update. That includes forming a groundwater resources advisory committee that met from 2011 into 2014.

Napa County tracks ground water levels at 113 wells and groundwater quality at 81 sites, the draft report said. The county creates annual groundwater monitoring updates that are presented to the county Board of Supervisors.

If the Board of Supervisors decides to submit the basin analysis report to the state, Lowe said, the state would open a public comment period on the report after Jan. 1 and could finish reviewing it by summer.

“They will ultimately make the decision whether our basin is sustainable based on the information we present to them,” Lowe said.

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