Alexandria and Prince William County, following the lead of Fairfax County, have declared water emergencies in an effort to keep Northern Virginia's principle reservoir from drying up.

Under restrictions on outdoor water use imposed yesterday in Prince William and early this morning in Alexandria, residents served by the badly depleted Occoquan Reservoir will be prohibited from watering lawns, washing cars, operating fountains or filling pools. Commercial car-wash facilities that recycle water will be exempted. Violators will be subject to fines of up to $500.

The restrictions affect almost all of Alexandria and the eastern half of Prince William, or about 70,000 of the county's 145,000 residents. The western half of the county is served by the city of Manassas, whose reservoir is not so low. Similar restrictions that go into effect today have been imposed in sections of Fairfax served by the Occoquan Reservoir, run by the Fairfax County Water Authority.

The authority has told the jurisdictions it serves that because of unseasonably dry weather that began last fall, the reservoir is down to about 30 percent of its capacity, or a 45-day supply.

Ordinarily January is part of the Washington area's "wet" season, which extends from December to May, but precipitation for the month is two inches below normal.

If the water crisis gets worse, the restrictions will become more severe, water officials said yesterday. The water shortage could lead to the curtailment or even closing down of some commercial operations, such as construction sites, some officials said.

The Fairfax water authority, unlike the Washington Suburban Sanitary Commission serving suburban Maryland, is almost totally dependent on the Occoquan. The WSSC's two reservoirs are in better shape because the utility has been able to tap the Potomac River during the dry months.

The Fairfax authority plans to do the same, but its river plant won't be ready until November at the earliest.