Water consumption in Northern Virginia has fallen by about 18 percent since three jurisdictions announced mandatory restrictions, the Fairfax County Water Authority said yesterday.

But James Warfield, a spokesman for the authority, conceded that the restrictions themselves -- such as watering lawns and washing cars -- may be having little impact because they curtail types of water usage that are more prevalent in the summer than winter. Restrictions covering what the agency says are primarily outdoor activities became effective Wednesday in Alexandria and portions of Fairfax and Prince William counties served by the depleted Occoquan Reservoir.

Ansering critics on the Alexandria City Council who questioned outdoor restrictions at this time of year, Warfield said the water authority was well aware of the limited impact the water bans would have. "We recognize they are not going to have a big impact directly at this time," he said, "but they will have a psychological effect on a lot of people who may feel it is not necessary to conserve water unless the restrictions are in effect."

The result of the widely publicized restrictions apparently has been that people have chosen to take shorter showers or draw less bath water, even though such usage is not covered by the restrictions, he said.

"After the big blitz of news, consumption dropped about 55 million gallons a day to 45 million gallons," Warfield said. "That says something about the psychological effect."

Warfield added, however, that the outdoor restrictions will begin to have an impact once the lawncare and gardening season begins, in late February.

If present conservation trends continue, the water authority has about a 40-day supply at its Occoquan Reservoir, the main source of water for the agency's 700,000 users. To slow depletion of the reservoir during the current dry period, the agency is seeking more water from Falls Church, which is supplied by the Potomac River.

Falls Church now supplies Fairfax with about 10 million gallons daily and has promised to supply another 4.7 million gallons a day when a disabled pumping station is reactivated in early February.

Separately, officials of the Washington Suburban Sanitary Commission said that water restrictions could be imposed in Montgomery and Prince George's counties by mid-March if the lack of rainfall continues. Robert S. McGarry, the agency's general manager, said the Potomac River currently is capable of meeting customer water needs in the Maryland suburbs.