In a lawsuit with far-reaching implications for urban growth in Napa County, a coalition of four small counties and cities is suing the state Department of Water Resources to guarantee deliveries from the State Water Project.

The Napa County Flood Control and Water Conservation District is among those challenging DWR in Sacramento County Superior Court. At stake are state supplies that Napa Valley cities depend on for half their water.

For American Canyon, the stakes are even higher. That city depends on the State Water Project for nearly all of its supplies.

The state cut State Water Project deliveries by 65 percent this year, citing drought conditions that left Sierra Nevada reservoirs below normal.

If the suit is successful, DWR would have to give Napa County and the other plaintiffs 100 percent of their allocation every year, drought or no drought.

The plaintiffs — Napa County, Butte County, Solano County Water Agency and Yuba City — argue that they are entitled to full allocations under the terms of the 1963 contracts that launched the State Water Project.

The project, the largest of its kind in the nation, supplies more than four million acre feet annually to Bay Area, Central Valley and Southern California water districts.

The plaintiffs, who receive 2 percent of this amount, say their 1963 contracts exempt them from dry-year cutbacks because they contribute water to the Sacramento River Delta, from which the State Water Project draws its supplies.

In Napa County’s case, water from the Lake Berryessa watershed finds its way through Putah Creek into the delta, making the county an “area of origin” that qualifies for special treatment, the suit says.

“It’s a contract interpretation issue,” said Felix Riesenberg, principal water resources engineer for the Napa flood district.

The cities of Napa, American Canyon, Yountville and Calistoga are putting up $200,000 as Napa County’s share of the suit, which was filed July 17 in Sacramento, Riesenberg said.

If the suit is successful, local jurisdictions could count on full delivery of each year’s state allocation. Currently, Napa County is entitled to 23,300 acre feet annually, growing to 29,025 acre feet by 2021.

“The reliability of the State Water Project for Napa County is extremely important,” Riesenberg said. “Without it, it creates the possible conflict of forcing cities into using ground water.”

Allocations have fluctuated in recent years, making it hard for cities to plan, Riesenberg said. The 2007 allocation was 60 percent of contract amounts, dropping to 35 percent this year. If the drought continues, next year’s allocation could be even less.

Local cities are making do this year, relying on local reservoirs that recharged due to periods of locally heavy rain last winter.

Napa went into this summer with more water than the year before, but arranged to buy supplemental supplies from outside the county as an insurance policy.

American Canyon, which is nearly wholly dependent on the State Water Project, arranged to buy additional water from Vallejo.

Both cities hired a consultant in June to see if additional long-term supplies can be found. This study will cost $128,000.

Cities have generally considered ground water off limits for municipal uses, leaving it for agriculture and rural dwellers. In a water emergency, underground aquifers would be tempting resource.

If the suit is successful, there should be no net impact on DWR or the environment, according to the plaintiffs’ press release. The suing agencies would get their full allocation, leaving less to be divided among the other 25 state contractors.

State Water Project supplies enter Napa County through Jamieson Canyon using the North Bay Aqueduct.

St. Helena is the only Napa Valley city without a contract for State Water Project water.

No one was available for comment Tuesday at the DWR.

The lead attorney for the plaintiffs, Jeanne Zolezzi of Herum Crabtree Brown, a law firm in Stockton, also was unavailable.