Wile the Occoquan Reservoir, which supplies water for 612,000 Northern Virginians, recedes to record low levels, a nearby reservoir owned by the city of Manassas is nearly brimming with water.

Northern Virginians dependent on the Occoquan may have to continue coping with the system-wide water emergency into September or October. But for the 15,000 residents of Manassas, the outlook is for ample water for years to come.

"I guess we were foresighted," City Manager C.M. Moyer Jr. said.

What Moyer was referring to was the city's decision, when it was still a town in 1968, to build an impoundment that would take care of water needs to the year 2000.

With part of a $5 million bond issue, Manassas built a dam that would impound 5.7 billion gallons of water flowing from Broad Run. The dam was designed so that, with a five-foot extension costing several thousand dollars, another 1 1/2 billion gallons could be impounded.

With its extra capacity.Lake Manassas, as the project was called, could supply enough water for more than triple the city's present population.

Downstream from Lake Manassas, the Occoquan Reservoir has shrunk to a new record low, and now contains only about 4 billion gallons. When full, the reservoir holds 9.8 billion gallons.

As the extent of the crisis became clear last week, the board of the Fairfax County Water Authority discovered that the agency, which owns the reservoir, had no contingency plan to deal with such emergencies.

The contingency plan that is being pieced together for the present crisis involves relying on the foresightedness of water-independent Manassas.

With its princely supply of water, little Manassas can, at a price, offer to help ease the water crisis that affects all of Alexandria, most of Fairfax County and some of Prince William County.

The city could do this by releasing water from its 780-acre lake. The water would pass into Lake Jackson; that lake's owner, Prince William County, would then release it for eventual flow into the Occoquan.

"We want to be a good neighbor," Moyer said. But neighborliness has its conditons. For one, there is the purchase price of 20 cents per thousand gallons, or double the price the Fairfax authority charges when it sells water.

Manassas is also asking that the water authority accept legal responsibility for any suits that might arise from the lowering of Lake Manassas. Boatyards do business on lake, and there are also homes and other properties lining the shores.

Among some Fairfax County officials, there is low-level grumbling about Manassas holding on to its enormous store of water. On Monday night the water, authority went into closed session to discuss whether the city should be releasing more water than it ordinarily does to maintain the flow of Broad Run. The downstream Occoquan Reservoir, as well as Broad Run, would be a beneficiary of any additional release.

While Manassas' right to build Lake Manassas was fought in court in the late 1960s, the Fairfax County Water Authority, after it acquired the Occoquan, had the suit dismissed, thereby ruling out any later legal action.

In other developments, leaders of Alexandria, Fairfax and Prince William announced yesterday at a joint meeting that the jurisdictions would continue the voluntary conservation measures announced Aug. 5.

Consumption for the last three days was down 16 per cent for this summer period (declining to 54.5 million gallons daily), but the reservoir, after inching up Monday, receded again yesterday.

There is difference of opinion, and even confusion, among Northern Virginia officials over how serious the water crisis is.

For example, on Aug. 4, water authority engineer-director James J. Corbalis told his board that "I am not in a position tonight" to recommend that the jurisdictions served by the authority invoke a water emergency. That night Corbalis called for a 10- to 15-day waiting period before action was taken.

But the next morning, all the jurisdictions announced the first, all-voluntary stage of an emergency.

Then, at Monday night's meeting of the water authority, Corbalis noted that he did not recommend such a move. Later in the meeting, he did recommend that the board ask the jurisdictions to impose limited mandatory sanctions.

The board, however, overruled Corbalis - an action that ended the meeting but not the confusion.