The governments of Fairfax ad Prince William counties and Alexandria yesterday called on residents served by the Fairfax County Water Authority to cut back on water usage "for the next several days."

The action was taken in an effort to build up the dwindling supply of water in the Occoquan Reservoir, which the authority relies on to serve its 600,000 customers in Northern Virginia. The reservoir, which only contains about half of its 9.8-billion-gallon capacity, is at its lowest level for the first week of August in 10 years.

Fairfax County Executive Leonard Whorton and Prince William Executive Clinton B. Mullen, acting under recently enacted local legislation, invoked the first stage of a three-level emergency plan. Alexandria City Manager Douglas Harman joined in the request, but the city has not yet passed legislation that would permit it to invoke mandatory conservation. The first stage, entirely voluntary, suggest:

Reducing lawn watering and car washing.

Eliminating other outside uses of water, such as washing sidewalks and driveways.

Operating automatic washing machines and dish washers on short cycles.

Taking brief showers instead of tub baths.

Areas served by the Fairfax Water Authority include all of Alexandria, south and central Fairfax County and parts of eastern Prince William County (excluding manassas and Manassas Park).

If the drought that has caused the Occoquan reservoir to fall so low continues, officials said, mandatory restrictions will be imposed.

At the present rate of usage and with no significant rainfall, the reservoir would be unusable in about 60 days. The water authority has no major alternate sources of supply.

Rain came to the Occoquan basin yesterday, but, according to the national Weather Service, "there's just not enough to make much of a difference."

The flow of the Potomac River, the source of water for the District of Columbia and large parts of suburban Maryland, is down to about half the-normal August flow of about 2 billion gallons daily, but is still above the critical stage.