Gov. Mills E. Godwin yesterday declared water emergencies in Fairfax and Loudoun counties and the City of Alexandria, and he empowered the three jurdisdictions to impose mandatory restrictions on consumption.

Although all three jurisdictions had asked Godwin for emergency authority, only Fairfax and Alexandria plan to impose restrictions at noon Saturday. Loudoun wanted to be ready "if the need arises," Board Chairman William C. Crossman Jr. said.

The restrictions set to be imposed in Fairfax will sharply curtail outdoor water use and violations could result in a fine of up to $500.

Alexandria's restrictions are expected to be similar, but they must await enabling legislation which, the City Council is expected to adopt Saturday morning after a public hearing on the issue.

The county and city took action to curtail water use after the Occoquan Reservoir, which supplies drinking water to 600,000 Northern Virginians, fell to about one-third of its capacity. At that level - 3.3 billion gallons - the Fairfax Water Authority has recommended that Northern Virginia governments impose mandatory conservation. The authority owns and operates the reservoir.

Although Prince William County also draws much of its water from the reservoir, its Board of Supervisors on Tuesday refused to ration water usage. Board members argued that voluntary efforts already in effect have reduced consumption in the county by 20 per cent over last year, even counting population growth.

Prince William County's refusal to go along with the regional rationing proposals means that Fairfax and Alexandria citizens may be required to ration water that could be used by Prince William residents. As a practical matter, however, Prince William uses less than 10 per cent of the water processed by the Fairfax Water Authority.

The three jurisdiction meeting requested by the Prince William supervisors as a condition to their imposing mandatory conservation remains unscheduled. Officials have not been able to find a mutually convenient time.

The restrictions adopted by Fairfax prohibit residential and commercial outdoor watering and washing except from a can. They also bar operation of vehicle-washing facilities that do not recycle water, operation of ornamental fountains, refilling of swimming pools and the serving of drinking water in restaurants, unless requested.

Voluntary conservation, which has been in effect since Aug. 6, has resulted in about a 10 per cent reduction in consumption, according to officials. Nevertheless the reservoir has continued to recede, in part because of less-than-normal rainfall.

Officials said they do not know how much more consumption can be cut with mandatory conservation. Some Fairfax supervisors said that although the voluntary efforts have had an impact, some residents have complained that their neighbors are not cooperating.

Loudoun's Board Chairman Crossman said that the county does not plan to impose restrictions now, but its water supplier, Fairfax City, has done so in the city. Herndon, which also is supplied by Fairfax City, has imposed restrictions, too. The Fairfax City problem is not an inadequate supply of water but a lack of capacity in the city's technical plant.