Despite two months of voluntary and then mandatory conservation in most of Northern Virginia, the level of water in the Occoquan Reservoir is falling twice as water planners projected a month ago.

"We felt by Oct. 1 we'd have 4 billion gallons in the reservoir," Fairfax County Water planner John H. Thillmann told a joint meeting of the Fairfax Board of Supervisors and the Fairfax County Water Authority Monday night. "But we actually had only 1.9 billion gallons."

"It's a serious situation water authority spokesman James A. Warnerd Jr. said yesterday. "There's no doubt we'll have to buy water from the city of Manassas."

The water authority has contracted to buy about 1.5 billion gallons of water stored in the city's reservoir upstream from the Occoquan. The City of Manassas, however has said it cannot supply any more than the water it already has contracted to sell.

The month of September was abnormally dry in the Occoquan dry in the Occoquan watershed or land area from which water flows into the Occoquan River.

Rainfall recorded at various stations in the watershed averaged 2.09 inches. The normal amount is 3.1 inches. The watershed includes south-western Fairfax, northern Prince William and eastern Fauquier Counties.

WTOP forecaster Gordon Barnes, who had been hired by the authority to help it plan water-management strategy, had predicted from 2.25 to 2.55 inches. The significant rainfall forecast by Barnes and other weather watchers for last weekend, however, did not occur.

"That was a pivotal weekend," the authority's Warfield said. "We expected it to be the first step in a turn-around, but we didn't get what we expected."

For October, Barnes is predicting 3.7 to 4.1 inches of rain. The normal amount for the month is 2.73 inches. Although the National Weather Service does not forecast for the watershed specifically Donald Gilman, chief of the long-range prediction group, said "we're saying at least 2 1/2 inches" for the station at National Airport.

Even if Barnes above-normal forecast is borne out, and even if conservation efforts continue, the Occoquan could be in a precarious condition Nov. 1, water experts say.

That could happen because rain, instead of becoming runoff that flows into the reservoir, ends up in the ground replenishing the water table and soil moisture, both of which remain abnormally low.

For example, Warfield said that the ground is so dry now that 1 inch of rain would put only about 200 million gallons of water in the reservoir in the form of runoff. With wetter soil conditions than presently exist, 1 inch of rain can put from 1 billion to 2 billion gallons in the reservoir.

"You've got to recharge the ground before you can get the water in the reservoir," said Dr. John Schaake, director of the weather service's hydrologic research lab.

While there is no danger that the Occoquan is going to run out of water in 30 or even 60 days - the Manassas purchase would buy close to 40 days of time - reservoir watchers like Thillmann are worried about whether enough water will be available to get through the winter. The water shortage could be particularly grave especially if the season is extremely cold - a condition that produces little precipitation.

The dry winter of 1976-1977, plus the below-normal preceding summer, have been blamed for the drop in the water table and soil moisture now existing in the Occoquan watershed.

Rainfall last month was below normal in the Occoquan watershed, and worse in the Washington area, where it amounted to a scant 32 of an inch according to the station at National Airport.

The amount was less than for any September except 1884 when 14 of an inch fell and 1967 when 20 of an inch fell, according to weather service records.

Because of drought conditions in parts of its extensive watershed, which includes parts of West Virginia and Pennsylvania, the Potomac River flowed at only 42 per cent of its nor-average flow of 1 billion gallons a day records began to be kept.

The months of September and October normally are the driest months in the watershed. So far, the Potomac's flow - even though it is well below normal - has been adequate to meet water demands in its service area - the District of Columbia, suburban Maryland and that part of Northern Virginia supplied by the District.