Yesterday's heavy rainfall is expected to add a foot to a foot and a half to the level of the drought-depleted Occoquan Reservoir, its first significant rise since last May.

The downpour was greeted with cautious optimism by Jim Warfield, a spokesman for the Fairfax County Water Authority, as a possible turning point - " the rain that will eventually lead us to recovery."

He said the total rise in the reservoir won't be known until Friday, but that an emergency purchase of water from the Manassas reservoir that had been scheduled for today will be delayed.

Conservation measures, he said, still are needed. "We're still asking people to cut back their (water use by) 12 gallons a day per person."

"If this rain isn't followed up by additional rain, then a lot of its effect will be lost," he said.

The authority's 616,000 customers are already under mandatory restrictions that ban most outdoor uses of water. Additional voluntary cutbacks were urged this week in response to a report that said they are necessary to prevent the reservoir from going dry.

By 3:30 p.m. yesterday, Warfield said, the level of the reservoir had risen 5 inches, from a record low of 94 feet, 10 inches to 95 feet, 3 inches. The reservoir contains about 1.95 billion gallons of water, one-fifth of its capacity.

It takes 24 to 48 hours for rain in the watershed to flow into the reservoir.

A rainfall two weeks ago caused a 2 inch rise in the reservoir level, he said, but that gain was lost after a week.

"It's going to take these types of rains to bring us out" of the water crisis. Warfield said.

He said WTOP forecaster Gordon Barnes, as a consultant to the water authority, has predicted more rain this weekend.

Yesterday's rain was caused by a low pressure system in he vicinity of Cape Hatteras, N.C., according to the National Weather Service.

By early last night, the Weather Service recorded more than an inch of rain in the Washington area including 2.17 inches in Woodbridge at the eastern end of the Occoquan Reservoir.

The rain contributed to a rash of fender bender accidents and created minor floodings.

Police reported dozens of accidents that tied up traffic at many locations. An evening rusth hour accident on the Cabin John Bridge near the Beltway tied up traffic for more than hour, police said.

One southbound lane of the 14the Street Bridge was closed for nearly two hours because two feet of water collected in depression.

Police said there was minor flooding in low-lying arcas in Rockville and Fairfax County.

A Virginia Electric and Power Co. official reported minor power outages afecting 900 customers in the Springfield area.