The steady decline of the Occoquan Reservoir, Northern Virginia's major source of water, was reversed yesterday by an upsurge in citizen conservation and some long-waited rainfall.

The reservoir, which supplies all of Alexandria, most of Fairfax County and part of Prince William County, gained 1 inch, rising to 102 feet, 2 inches. While the increase was small, it was considered significant because the reservoir has been in a general, drought-caused decline since early May.

Meanwhile, Fairfax Supervisor Audrey Moore (D-Annandale) called for an independent investigation of the Fairfax County Water Authority. Mrs. Moore, a frequent critic of the authority, said at a press conference that officials of the agency "may have withheld information (about alternative water supplies) . . . and promoted scare tactics of shortages in an effort to convince the public that Fairfax should be allowed to withdraw water from the Potomac."

Mrs. Moore's issue centered around the 5 billion gallons of water stored in Lake Manassas, an impoundment upstream from the Occoquan. Last week, shortly before Northern Virginia officials said the reservoir had only a 50-day supply of water, the authority began negotiating with the city of Manassas to buy part of the lake's supply.

Mrs. Moore wanted her proposed investigation backed by the entire Board of Supervisors, but the two members of the vacationing board that she was able to reach declined to join her in calling for an emergency meeting.

Mrs. Moore, acting on her own, called for an investigation by civic, citizen and business groups in Northern Virginia ranging from the Northern Virginia Conservation Council to the Fairfax County Chamber of Commerce.

John F. Herrity, chairman of the Fairfax supervisors and one of the two members contacted by Mrs. Moore, said an investigation, if it is warranted, ought to be done by the State Water Control Board, "There's no way a citizen committee can get together a staff to do the job," he said.

Mrs. Moore and other members of the board have been frequent adversaries on water issues. An opponent of growth, she has been critical of the water authority's efforts to expand its capacity by building a treatment plant on the Potomac River.

The water shortage in the Occoquan has tended to underscore the authority's contention that it needs quick approval from the Army Crops of Engineers to go ahead with the Potomac facility. Mrs. Moore is asking if the shortage was really necessary.

Surveying the latest statistics on water consumption, Fairfax officials said they were encouraged. The first week of the 11-day-old water emergency saw little cooperation with the call for voluntary conservation. The big reductions this weekend came after a second appeal by Northern Virginia officials.