Because the Occoquan Reservoir is so low and prospects for adequate rain are so dim, Northern Virginia's water emergency could extend through the winter and bring mandatory restrictions on usage.

"We are setting a new record every day," Fairfax County environmental planner John H. Thillmann said yesterday, referring to the badly depleted reservoir.

Fairfax officials like Thillmann are showing increasing concern about the ability of the reservoir to supply the 600,000 people in Northern Virginia who depend on it. What worries them most is that the emergency, viewed as temporary when was declared four weeks ago, may be an almost year-round predicament.

Originally officials saw the emergency ending by Dec. 1, when the reservoir typically has about 8.5 billion gallons. That is a big enough safety margin to offset an extremely cold winter, like last year's, when snow doesn't melt into runoff and the reservoir is not replenished.

But estimates based on figures supplied by the Fairfax County Water Authority indicate that the reservoir will be far short of the safe margin by Dec. 1.

The most optimistic forecast that Thillmann and other officials are willing to make is that the reservoir will have about 2 billion gallons on Dec. 1, of which only 1.5 billion would be usable. With maximum purchases of 2.4 billion gallons from the other sources, the usable storage in the Occoquan could be raised to 3.9 billion gallons.

To keep 600,000 people adequately supplied through the winter, the Occoquan would need more than 3.9 billion gallons. Ordinarily, the reservoir gets additional supplies from runoff created by melting snow. But during the harsh winter that began last December, that didn't happen.

Thillmann said that if the coming winter is as harsh - "and we are going to have cold weather" - then "there may not be enough water."

James J. Corbalis Jr., engineer-director of the water authority, which owns the Occoquan, said rains over the next three months could put the reservoir at its normal range by Dec. 1. He added, though, that if the reservoir is down to 3.9 billion gallons of usable storage (Thillmann's most optimistic scenario), "your would have to go to mandatory restrictions."

While Thillmann said a cold winter is likely, the National Weather Service has not yet made its long-term forecast. Don Gilman, who heads the long-term forecast team at the weather service, said the odds are 40 to 1 against the coming winter being as cold as the past one.

One reason why Fairfax officials are concerned that the water emergency may continue is that the reservoir has been in an almost straightline decline since early May. Ordinarily the reservoir goes down steadily only during July and August, periods of high demand and low rainfall.

Based on rainfall predictions by the weather service and WTOP weatherman Gordon Barnes, who was hired as a consultant by the water authority, the reservoir will probably continue to decline through September.

While there have been no forecasts for October and November, Thillmann, assuming that rainfall would be no heavier than it was during the driest October-November of the last nine years, sees the decline continuing.

If the winter is anything like the last one, mandatory restrictions, as indicated by Corbalis, would be the only alternative. The city of Manassas has said it couldn't supply much more water to the Fairfax authority than it has already promised. Fall Church is also sending the maximum amount it could supply the authority.