The Fairfax County Board of Supervisors, alarmed by the shrinking water supply in the county's principal reservoir, yesterday declared a water emergency, making persons who water lawns, wash cars, or operate fountains subject to a fine of up to $500. The law is effective at noon Wednesday.

Board members acted after learning that the lack of rainfall in the Washington area has reduced the water in the Occoquan Reservoir to about 30 percent of the reservoir's capacity or a 45-day supply.

The supervisors decided to make the restrictions effective on Wednesday to give officials in Alexandria and Prince William County, jurisdictions also served by the reservoir, time to adopt similar conservation measures.

The water-use restrictions, aimed only at those who receive their water from the reservoir, will affect more than half of the county's residents. There are no restrictions on areas, including Falls Church and Fairfax City, that get their water supply from the Potomac River.

With forecasters predicting another 30 days of cold, dry weather, county officials said it could take some time to replenish the reservoir.

Watering of lawns, shrubbery, trees or other vegetation will be prohibited except from a container with a capacity of not more than three gallons. A similar restriction will apply to washing cars and other vehicles. Plant nurseries and commercial agricultural activities are exempted from the restrictions, as are commercial car-wash operations that use county-approved water-recycling systems. Restaurants will serve water on request.