As the Fairfax County supervisors began planning yesterday what to do in case of a major water shortage, they learned that the County Water Authority plans to spend up to $300,000 to furnish a new building, complete with decorative fountain costing about $12,000

"A brazen waste of public monies," declared Supervisor Audrey Moore (D-Annandale) as she asked the other supervisors to join her in urging the water authority to reconsider.

"While urging water conservation to the public and at the same time erecting a fountain which they don't need (is) irresponsible on the part of the authority," Moore said.

The supervisors complied with water authority, an autonomous agency had voted to spend up to $10,000 for plants, $5,000 for signs and $5,000 for art work in its new building on the corner of Prosperity Avenue and Rtc. 50 in Fairfax. The $3000,000 figure would not include floor covering, Moore said.

She said that the authority's justification for the new furnishings "is that the present furniture will not be compatable with the decorative concept of the new building." The new building is designed without many interior partitions.

In response to a question about the new furniture, James J. Corbalis, engineering director of the authority replied, "we've been in business for 20 years in rented quarters. The furniture I understand varies from good to poor."

"The old furniture," Corbalis said, "doesn't lend itself functionally to the open space concept."

Asked the reason for the water foundation, Corbalis said "you'll have to ask the directors that question."

The new, three-story building scheduled for completion in mid-November, is financed by a $2 million bond issue approved by the voters in 1970. The authority currently rents space at 8001 Forbes Place in Springfield.

Moore not only attacked the water authority's timing in approving the expenditures only two weeks after authorizing a water rate increase, she also criticized symbolism of a water fountain at a time when water conservation is becoming more urgent.

"It also appears to be that there is supreme irony, and insensitivy in the authority's urging water conservation to the public while indulging itself in a frivolous display that will squandor a resource, we are told, is in ever-increasing short supply," Moore said.

But the authority's public information officer, James A. Warfield, said that the agency has not urged any type of water conservation. "We're meeting demands easily, without any problem," Warfield said.

Warfield also said the proposed fountain would recirculate the water it uses and, according to interior designers, the fountain would need a total of less than 50 gallons of water.

Warfield said the recent water rate increase, including an increase of 45 cents in the peak-time surcharge and a 10 cent regular increase, was necessary by increased electric bills, higher chemical costs and capital improvement costs.

Meanwhile, the Board of Supervisors authorized the county executive to contact the office of Gov. Mills E. Godwin in order to work out a plan of action in the event of a local water emergency. Godwin has expressed his desire to delegate the necessary authority for declaring water emergencies to local jurisdications.

And Supervisors Marie B. Travesky (R-Springfield) and Moore called yesterday for a meeting of all the jurisdictions that share the same water sources as Fairfax County in order to discuss a water emergency conservation ordinance now being drafted by the county.