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Tackling one of the American West's most contentious issues, a new government report assesses the demands on Sonoma County's largest underground water source against a future that includes population growth, agricultural needs and the wild card of climate change.

The report by the U.S. Geological Survey provides a detailed profile of the groundwater beneath the 261-square-mile Santa Rosa Plain watershed, home to about half the county's population, including the cities of Santa Rosa, Windsor, Sebastopol, Cotati and Rohnert Park.

About 12,000 wells operated by the five cities, the county water agency and private homes and ranches on the Santa Rosa Plain pump about 53,000 acre-feet of water per year from the ground, matching the amount of water the agency draws from the Russian River.

"We are planning for an uncertain future," said Marcus Trotta, a Sonoma County Water Agency hydrogeologist.

The 199-page report, packed with hydrologic and geochemical data, is the first study of the underground watershed in 30 years and part of the cornerstone for an ambitious effort to establish a groundwater management plan for the Santa Rosa Plain.

A 30-member Basin Advisory Panel representing cities, water customers, agriculture, industry and environmentalists intends to develop, by late this year or early 2014, a plan to "manage, protect and sustain groundwater resources" in the plain.

It's a chore that will require balancing competing interests, such as urban growth versus agricultural demand, and accounting for a future fraught with more volatile weather — droughts alternating with floods — brought on by climate change.

"Future growth in population and demand for water coupled with a changing climate ... are likely to increase stresses on the region's groundwater resources," said a groundwater primer prepared by the Water Agency.

"We have to maintain it," Sonoma County Supervisor Shirlee Zane said, noting that "we can't build new houses or have new farms without water."

To come to an accord over water, Zane said, will require a third leg to the bromide long attributed to Mark Twain: "Whiskey is for drinking; water is for fighting over."

"Water is for sharing," Zane said. "It's life."

Lex McCorvey, executive director of the Sonoma County Farm Bureau, which has a representative on the advisory panel, said he agrees with that concept in principle.

"We're going to need to learn how to share water and prioritize our use," McCorvey said, acknowledging that a tug-of-war between urban and agricultural interests "certainly is a possibility."

But McCorvey said he will withhold judgment on the groundwater management plan until the details take shape.

Agriculture, an $821 million local industry, is vital to the county's economy, he said, adding that farms "need their fair share of water."

Kathy Hayes, executive officer of the North Bay Association of Realtors, which also has a seat on the advisory panel, said she is looking forward to the USGS report, which will be presented at a public workshop at 6 p.m. today at the Finley Community Center, 2060 West College Ave., Santa Rosa.

"We're not going to presuppose where this process ends up," Hayes said, referring to the management plan.

Jane Nielson, an advisory panel member representing the Sonoma County Water Coalition, said the report documents how a change in water policy can positively affect groundwater supply.

Heavy pumping from municipal wells in the Rohnert Park-Cotati area in the 1980s and '90s had lowered the underground water table by as much as 100 feet, Trotta said.

Water conservation programs and a move to use more Russian River water since 2000 have largely restored what the USGS report referred to as a "pumping depression."

Nielson, a former USGS geologist, said that underscores the need to "guard against overpumping" and to strike a balance between water supply and demand.

Contrary to some popular impressions, groundwater is not an underground lake below the Santa Rosa Plain. Instead, it is water — up to 34,000 years old — that occupies the empty spaces in sand and gravel deposits and fractures in volcanic rocks, Trotta said.

As well pumping and natural processes deplete groundwater, rainfall recharges the subsurface aquifers, he said.

The plain occupies the flatlands between the Mayacmas Mountains east of Santa Rosa and the low hills west of Sebastopol, with both upland areas included in the Santa Rosa Plain watershed.

The Laguna de Santa Rosa runs along the west edge of the plain, fed by streams and groundwater seepage.

Groundwater pumping from 70 wells operated by the five cities on the plain, the water agency and a Larkfield water company ranges from about 2,000 acre-feet to 8,300 acre-feet a year, accounting for about 15 percent of total pumping.

Pumping by private wells on farms and homes is not reported and was estimated in the USGS study based on land-use data and a groundwater flow model.

The USGS study is the first product of a seven-year, $2 million effort funded in equal parts by the federal agency, Sonoma County Water Agency and the public entities that pump from the groundwater basin.

A second USGS report, due for release later this year, will include a computerized, three-dimensional model that couples groundwater and surface water flows.

Trotta said it will provide a "powerful tool" for assessing the future impacts of population growth, climate change and water management strategies.

You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.