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The future of urban growth and agriculture in Napa County will depend on managing existing surface and underground water as well as developing new resources to meet rising demands for endangered-species protection, according to a water-supply expert.

"To continue providing water reliably will mean increases in cost of service and changes in ways we use water to be more efficient," said Felix Riesenberg, principal water resources engineer for the Napa County Flood Control and Water Conservation District. "Conservation and use of recycled water will be more important for reliability and self-reliance on local supplies."

Drought years, such as those the state has been experiencing, and potential climate change could put further pressure on water availability, he noted. Mr. Riesenberg spoke on these competing needs at the Business Journal's Impact Napa conference Aug. 27.

One of the major concerns about reliability in supply involves the North Bay Aqueduct, which supplies Solano and Napa counties via water pumped from Barker Slough in the Sacramento River Delta east of Travis Air Force Base.

The Napa County water district has entitlements from the State Water Project for 29,025 acre-feet of water from the aqueduct a year and sells aqueduct water to American Canyon, Napa and Calistoga. Napa also taps the 31,000-acre-foot Lake Hennessey and 1,400-acre-foot Milliken Reservoir, and Calistoga gets water from 275-acre-foot Kimball Reservoir. A family of four uses two to three acre-feet of water annually.

But allocations from the State Water Resources Control Board vary. The 2009 allocation to Napa County of 25,000 acre-feet requested was just 15 percent, or 3,500 acre-feet, in October amid a declared "critically dry" year. The state water board increased the county's allocation to 40 percent in May because of late spring snowfall in the Sierra Nevada.

American Canyon is completely dependent upon aqueduct water to serve not only residents and businesses in the city limits but also surrounding vineyards and business parks that were part of a water district acquired upon the city's incorporation, according to Michael Throne, the new public works director.

The city has to react to State Water Project cutbacks and make up for daily demand spikes through purchases of aqueduct water from neighboring Vallejo. City concerns about water supply led to a halt in providing written water commitments to builders in the Napa County Airport-area business parks during some of 2007.

"It makes planning sometimes difficult because it is not reliable," he said.

This past spring American Canyon started pumping recycled water to 40 customers, with the goal of reducing about 1,000 acre-feet a year of fresh water used for irrigation. The city is building a storage tank for recycled water as well as more pipelines to supply irrigation needs at city facilities as well as commercial developments such as Napa Junction and industrial areas along Green Island Road.

Supplying those properties will reduce consumption of potable water by an estimated 300 to 400 acre-feet per year. That will help in drought years, but the city continues to look for ways to deal with a 1,000 to 2,000 acre-foot budgeted deficit, according to Mr. Throne.

Meanwhile, Napa Sanitation District is expanding availability of recycled water from its south Napa plant for irrigation in conjunction with the North Bay Water Reuse Authority. Two goals are to deliver recycled water to Napa State Hospital and the groundwater-deficient Milliken Sarco Tulocay area as well as Los Carneros winegrowing region.

Groundwater is the main water source in unincorporated areas, followed by some diversion and storage of stream and river water. The 2050 Napa Valley Water Resources Study of those areas, completed in 2005, found that use and supply are about in balance, though a deficit could come by 2050, according to Mr. Riesenberg. He said that concerns about regulation have posed some challenges in recruiting property owners to install well-monitoring devices to create a more complete water-supply model.

Wine industry programs such as Fish Friendly Farming and Napa Green Winery, as well as the frost-protection watermaster system in place on the Napa River since 1976, are helping rural water supplies become more sustainable, according to county Farm Bureau Executive Director Sally Elles.

Inside Napa, the city has been allowing developers to mitigate water use by purchasing certificates that fund replacement of high-flow plumbing fixtures elsewhere and by funding the construction of recycled-water service to project sites, according to Joy Eldredge, general manager of the city water division.

"As we look at large projects associated with development we look for opportunities to condition mitigation to offset uses to mitigate the cost of extending purple pipe to them," she said, referring to the standard color of lines carrying recycled water. Such an offset has been suggested for the proposed Ritz-Carlton hotel.

As American Canyon adjusts long-range public policy to meet water demand, it and other cities supplied by the aqueduct are concerned about its long-term future. Recent court rulings on when water can be pumped from the Delta's waterways because of the protected smelt could pose problems for the aqueduct's pumping point, according to Mr. Riesenberg.

So the Napa County water district and Solano County Water Agency for more than a decade have been studying two alternative aqueduct source points miles upriver to improve water quality.

Climate-change-protection efforts statewide likely will result in requirements from regional water quality control boards that project proponents employ what is called low-impact development, according to Mr. Riesenberg. That calls for site conditions after development, such as water percolation into aquifers, to mimic conditions before construction.