The Prince William County Board of Supervisors voted unanimously yesterday to impose water use restrictions on about 70,000 county residents supplied from the Occoquan Reservoir, which has dropped to an all-time low.

The Prince William measure, effective at noon Saturday and affecting about half the county's population, is virtually identical to one enacted in Fairfax County and ends a political split. Fairfax County and Alexandria, which share Occoquan water, imposed mandatory restraints two weeks earlier.

The reservoir, owned by the Fairfax County Water Authority and which supplies water to about 600,000 people in the three jurisdictions held steady at the all time low 100-foot level between the Monday morning reading and the 7 a.m. reading yesterday. The level was 100.2 feet on Sunday and 100.6 feet on Saturday, according to water authority spokesman James A. Warfield Jr. A light rain on Sunday night provided some relief, he said.

Voluntary conservation in effect in the three jurisdictions since Aug. 5 had reduced consumption by about 12 per cent, according to Warfield. He said mandatory restrictions, which include barring outdoor watering, car washing, street or parking lot washing and filling of swimming pools, have brought the saving up to about 15 per cent.

Warfield said the reservoir now holds 2.8 billion gallons, compared to a capacity of 9.8 billion, and could supply water in a "worst case" no rain situation for 51 days. Recent consumption has ranged from 45.8 million gallons on Monday to 42.2 million gallons last Thursday, he said.

Should the level reach 93 feet, the water authority will buy 1.7 billion gallons of water, about a 30-day supply, from the well-stocked City of Manassas. If that is used up and the level drops to 92 feet, more stringent restrictions would have to be imposed, Warfield said.

While the Occoquan's level remained critical for a huge suburban population, the outlying towns of Leesburg and Warrenton have been forced to impose more severe restrictions.

Warrenton engineer Harold Wilkinson said the town reservoir now holds an 18-day supply of water, compared to a capacity of 140 days.

He said the sown of 5,000 is now using about 700,000 gallons per day compared to normal rate of 825,000. Wilkinson said the town is negotiating with the owners of a private lake that drains into the town reservoir to buy an additional 30-day supply of water.