The threat of mandatory water rationing is growing for 700,000 residents of Northern Virginia.

If there is no significant rain in the next week or two, the Fairfax County Water Authority's service areas, which include most of Farifax County, eastern Prince William County and all of Alexandria.

Unusually dry weather also has reduced the Potomac River -- the District of Columbia's only source for suburban Maryland -- to a fifth of its normal flow, but there is still enough to supply those areas without rationing.

Most of the restrictions facing Northern Virginians would affect outdoor use, such as watering lawns and other greenery and washing cars. But under the "Stage Two" emergency that may be imminent, localities could halt any industrial or commerical activity, including construction, which uses water beyond drinking and sanitary needs.

"We have a very serious situation," said Fairfax County Board of Supervisors Chairman John F. Herrity. "This is the worst problem we've had since the drought of 1977." Restrictions on outdoor water use were imposed during that crisis, but lifted after fall rains replenished the Fairfax Water Authority's Occoquan Reservoir.

The reservoir, located on the Fairfax-Prince William border, is fed by Bull Run and several other streams. It would ordinarily be close to or at its 11-billion-gallon capacity at this time -- the height of the so-called "wet" season. But because of below-normal rainfall since last June and abnormally cold weather that has hampered recharging of surface and groundwater sources, the reservoir is down to 3 billion gallons.

If the reservoir falls to 2.8 billion gallons, the authority will declare its "Stage Two" emergency. During the continuing dry period, the reservoir has been losing about 35 million gallons a day as the authority has supplied its customers.

"It doesn't look good," said water authority official James Warfield, noting that long-range forecasts for January through March called for cold and dry weather.

Relief, though, may be on the way. According to meteorologist Robert R. Diockson at the National Weather Service's climate analysis center in Camp Springs, Md., the current long-range forecast for the 30-day period beginning last Wednesday indicates the Washington area "will be getting a normal amount of precipitation or a bit above normal. . . Things are looking a little brighter than they did."