The 616,000 Northern Virginians who are already under mandatory restrictions to reduce their water use must cut their consumption another 15 per cent to keep the dwindling Occoquan Reservoir from going dry.

That is the conclusion of a special team of experts from the U.S. Geological Survey, National Weather Service and Interstate Commission on the Potomac River Basin. Their report will be given today to the Fairfax County Water Authority, which supplies Alexandria, most of Fairfax County and part of Prince William County.

James J. Corbalis Jr., the authority's engineer-director, said the authority will urge that its customers cut back their water use by another 15 per cent, as the three agencies recommend.

Under the current mandatory conservation, most outdoor uses of water have been sharply restricted.

Proposed now are such additional measures as taking shorter showers, operating washing machines and clothes washers with only full loads, shaving with a basin of water instead of with the tap running, having plumbing leaks fixed and installing various water-saving devices.

Such measures would be voluntary, but without an additional 15 per cent reduction in water use, which they are designed to effect, there is "an almost certain probability" that more mandatory restrictions will have to be imposed this fall, according to one official close to the report.

"This is a very serious situation," the official, who declined to be identified, declared. "The risks are very high."

If more restrictions become necessary, businesses and industry, along with schools and other public facilities, would be hard hit. Depending on the severity of additional restrictions, some businesses and other facilities might be forced to close or curtail their operations.

Although almost three months of voluntary and then mandatory conservation has already reduced consumption in the water authority's service area by up to 20 per cent, the Occoquan Reservoir has continued to fall to new record lows.

The reservoir now contains only 1.95 billion gallons - or about a 36-day supply based on the present withdrawal rate. The water level has fallen almost to the point where the authority would begin making an emergency purchase of up to 1.5 billion gallons from the city of Manassas' reservoir on Broad Run upstream from the Occoquan.

If there is an inch of rain today, as predicted by WTOP forecaster (and authority consultant) Gordon Barnes, the purchase could be delayed.

But an inch of rain would not put the Occoquan out of danger, experts agree. "It could be raining while the report is being given, but you'll still need the 15 per cent reduction," the expert close to the study said.

The study, looking at precipitation records going back more than 80 years, concludes "there is a considerable probability of below-average rain" for October and November taken together, the expert said.

Barnes predicted above-average rainfall for October. He hasn't yet given a long-range forecast for November because his contract with the water authority was only recently extended.

According to Corbalis, the Occoquan crisis could continue until the end of the so-called "water year," usually February or March, when the reservoir traditionally has filled up. He said that the additional conservation to be recommended could be eased as early as mid-December - if there is a "significant" increase in the reservoir's level.

The report going to the water authority will also discuss the Occoquan's "safe yield" - the amount of water than can be taken daily from the reservoir while protecting it from going dry during an extended drought.

The report, according to some sources, will put the safe yield at 52 million gallons daily. The water authority, relying on a consultant's analysis, currently puts the safe yield at 65 million gallons daily.

At present, from 38 million to 42 million gallons are taken from the reservoir daily, with another 18 million gallons daily being purchased from other sources. For most of the year - up until early August when conservation efforts first began - more than 50 million gallons daily was being withdrawn from the reservoir.

While the water authority's customers have been hit by restrictions and now are being asked to save even more, those who live elsewhere in the metropolitan area, even in Falls Church, Arlington ad3qqart of Fairfax County, can use as much water as they wish.

The explanation is that the District, suburban Maryland and those Northern Virginia jurisdictions not served by the Fairfax Water Authority use the Potomac River as a supply source.