After months of falling to record low levels, Northern Virginia's Occuquan Reservoir is filled to the top of its dam's wall. Water actually is spilling over the top - something that hasn't happened since May 9.

"There is absolutely no chance we could have another crisis this winter," James A. Warfield Jr., spokesman for the Fairfax County Water Authority, said.

As recently as Oct. 26, the reservoir, which is the main water source for the 616,000 people served by the authority, contained only 1.85 billion gallons, or less than a month's supply. But by last Saturday, 17 days later, the reservoir was filled to capacity - 9.8 billion gallons.

In less than two weeks, five inches of rain fell in the Occoquan watershed. That was three and a half inches more than had been predicted by WTOP-TV weather forecaster Gordon Barnes, who has hired by the authority in September to make some long-range predictions.

Because the rainy days were clustered together, most of the precipitation was converted to runoff that went into the reservoir. During the summer and early fall, little rainfall was converted to runoff. Instead, most of it went into the ground to replen ish the water table, which dropped during the dry winter.

Though water restrictions were lifted last week by Fairfax and Prince William counties and Alexandria - the three jurisdictions served by the authority - consumption rates have not spurted upward - to the surprise of the authority officials.

Consumption since restrictions were lifted has been as low as 52.4 million gallons daily. On Monday, it was 57.1 million gallons. Average daily consumption last November, when there were fewer users in the authority's service area, was 60.7 million gallons.

With the reservoir spilling over the top, the authority has been able to stop purchasing additional water from Arlington and has cut back to normal the amount it has been buying from Falls Church. Both Arlington and Falls Church are supplied through the District, which relies on the Potomac River for water.

The authority also has been able to shelve for the season plans to buy an emergency supply of 1.5 billion gallons from the city of Manassas, which operates a reservoir on Broad Run, upstream from the Occoquan.

While the authority has enough water to get through the winter, the crisis this summer and fall could reoccur next year if there is another abnormally dry winter and if summer and early autumn precipitation is below normal again. Because of growth in Northern Virginia, primarily in Fairfax, there will be more demand on a system that will be the same size.

To meet growing demands, the authority wants to start tapping the Potomac, like the District and suburban Maryland, but its plans have been held up by the Army Corps of Engineers. The corps says it won't grant a permit for construction of an intake in the Potomac until Virginia and Maryland agree to protect the District's allocation during periods of low flow in the river - when supply might not be able to meet all demands from metropolitan jurisdictions.

Although the authority expects that its present supply system, built almost entirely around the Occoquan, will be inadequate as early as 1980 (even if rainfall is normal), the Potomac facility may not be ready until late 1981.

Crucial to the date the facility will be ready are current negotiations between Virginia, Maryland, the District and the army engineers over a low-flow allocation agreement. If the talks drag on, completion of the plant will be delayed that much longer.

In a related matter, the State Water Control Board is holding public hearings next week on the proposals to meet Northern Virginia's long-range water demands. All the proposals hinge on the Potomac as a supplementary supply source.

Two proposals call for recharging the Occoquan with river water during normal dry periods. Three others - perhaps the most controversial - suggest additional reservoir capacity in Loudoun County, an area not severed by the Fairfax Water Authority. There is strong opposition in Loudoun to any reservoir system to serve other parts of Northern Virginia.

The water control board will hold a public hearing at 7 p.m. Monday in the Simpson Middle School in Leesburg in the board room of Fairfax's Massey Building governmental center in Fairfax City.