If the water beneath the Santa Rosa Plain were a corporate balance sheet, about 250,000 residents, as well as farms and businesses, would be in the red by about 1 billion gallons a year.
That's the average annual difference over the last three decades between the water pumped from the ground in the 261-square mile plain and the amount replenished, primarily by rainfall, water officials say.
"Obviously, we don't want that to continue," said Ann DuBay of the Sonoma County Water Agency.
A group of 30 stakeholders and technical advisers — called the Santa Rosa Plain Basin Advisory Panel — is working on non-regulatory ways to balance the human drawdown and natural recharge of groundwater.
In a series of community forums starting Monday night in Sebastopol, the panel will introduce a brand new tool to assist in that effort: A groundwater model developed by the United States Geological Survey.
"It's pretty cutting-edge," DuBay said, describing the model as a way of connecting surface water in streams, creeks and the Russian River with the unseen water beneath the plain.
The long-term imbalance of 3,300 acre-feet of water a year is "rising to a level of concern" but is not yet a critical matter, DuBay said.
The groundwater model stems from a seven-year study of the Santa Rosa Basin by the federal agency, which is collaborating with the advisory panel.
The panel includes residents, environmentalists, ranchers, business owners and officials from the five cities — Santa Rosa, Windsor, Rohnert Park, Cotati and Sebastopol — that draw some or all of their water from the beneath the plain, which is home to about half the county's more than 480,000 residents.
About 12,000 wells operated by the five cities, the water agency and private homes, ranches and businesses draw water from the ground. Public wells account for 15 percent of the use; private wells 85 percent.