SANTA BARBARA, CALIF., FEB. 27 -- The nation's largest ocean desalination plant is to begin operation here Friday, ushering in what boosters say is a new era in coastal water development and what skeptics claim is a costly folly.

The plant was conceived during a period of extreme drought but comes on line during an unusual period of water surplus in a city that until recently enforced mandatory 45 percent water conservation and prohibited lawn watering and car washing.

Now, thanks to the "March miracle" rains of 1991 and the "fabulous February" storms in southern California this month, local reservoirs almost dry a year ago are near capacity. Santa Barbara has such abundant water supplies that the new plant is to go on standby after operating only two months.

"Santa Barbara was faced with a real emergency and bought itself some emergency-response equipment," said Robert Almy, the county water manager. "It's like buying a new fire truck. You keep it in perfect working order and hope you never have to use it, but you want it to work if it does."

Although desalinated water is presently superfluous, water officials have said the plant could be the forerunner of similar plants along the California coast as cities rush to find new supplies as their populations boom.

The small community of Morro Bay more than 100 miles north, largely dependent on well water, is trying to build a desalination plant. Other plants are being discussed in the water-short communities of Monterey and Carmel.

The idea that desalinated water would make a significant contribution is dismissed as an empty dream by water economists such as Henry Vaux of the University of California at Riverside, who said the cost is so high that desalination plants will be scarce.

The Santa Barbara plant cost $30 million and is to be paid for through higher water charges for five years. City officials estimated the cost of delivering desalinated water at $1,900 an acre foot. An acre foot is 325,851 gallons.

This compares with the cost of $230 an acre foot for federally subsidized water delivered to the city from nearby Lake Cachuma, a Bureau of Reclamation reservoir. But desalinated water is a reliable supply always available in times of drought.

"It's an insurance policy," said Steve Mack, the city's water supply manager. "The ocean is never going to run out of water."

Last year, in a fifth year of drought, Santa Barbara voters approved the plant and hooking on to a new unit of the State Water Project, a vast system of northern California reservoirs that supplies southern California with water through a huge aqueduct. The cost of this water is estimated at $1,100 to $1,300 an acre foot, but Santa Barbara Mayor Sheila Lodge questioned whether it would ever be needed.

Even if it is needed, it may not be available.

Last year, the State Water Project delivered only 20 percent of its contracted supplies to southern California, and this year's projections are similar. The giant Metropolitan Water District, which serves 15 million southern Californians, continues to require 31 percent water conservation because of the limited state supply.

Santa Barbarans, in contrast, expect to have all the water they need for the first time in six years. But they have formed conservation habits that may be hard to break.

City officials estimated that average monthly water bills of $20 would triple if Santa Barbarans started using all available water.

However, conservation has become such a way of life here that the city estimates that three-fourths of residents will use so little water that they will see no increase in their bills.