While more than 600,000 people in Northern Virginia are being forced to limit outdoor water use to keep the Occoquan Reservoir from going dry, the city of Manassas, with a population of 14,000, has 5 billion gallons of water available.

The water, actually five miles to the west of the city, is stored in an impoundment on Broad Run called Lake Manassas.

With the Occoquan Reservoir steadily falling - it recently dropped below the 2-billion-gallon mark - some people have been asking: Can Manassas come to the rescue?

The answer is yes, but only in a limited way and for a limited time. The city has agreed to sell 1.5 billion gallons to the Fairfax County Water Authority, which has been trying, so far unsuccessfully, to stabilize the Occoquan through rationing and purchases elsewhere.

Despite a weekend of rain, probably less than 200 million gallons flowed into the Occoquan and the water level in the reservoir was six inches lower on Tuesday than last Friday. Heavy rains this month could delay the water purchase, but apparently only for a short time.

It the authority uses all 1.5 billion gallons from Lake Manassas, the city of Manassas will get $172,500, or about 11 1/2 cents a thousand gallons.

"We're making no profit," City Manager C. M. Moyer Jr. said. "We're not trying to take advantage of our neighbors. Eleven cents will cover debt service on the water and the other half cent will go for the extra treatment we will have to give our remaining water."

The water will be delivered to the Occoquan at an average rate of about 48 million gallons a day, according to authority engineers. Manassas officials say the rate could be as high as 60 million gallons a day.

To send the water on its 24-hour, 12-mile journey down Broad Run, through Prince William County's Lake Jackson and into Occoquan Creek, Manassas release a cone-shaped plug on a 24-inch pipe at the bottom of the lake's concrete dam.

The water will reach its down-hill destination under its own power. The only cost will be in water lost - perhaps as much as 300 million gallons - through seepage and evaporation.

For the water authority, the purchase, even minus a 20 per cent loss, would buy about 24 days during a drought that will last no one knows how long.

If the authority uses up all 1 1/2 billion gallons and still needs more to save the Occoquan, City Manager Moyer offered this hope: "I think we could make some additional water available . . . We'll have to take another look at the situation. We've got to consider the drought and how long it will take our reservoir to fill up.

Some people, including members of the Fairfax Board of Supervisors, have wondered why a city of 14,000 needs a reservoir capable of holding 5.7 billion gallons, while the Fairfax Water Authority, which serves 616,000 people, has a reservoir that holds only 9.8 billion gallons.

Part of the answer is that the city planned the reservoir during the fastgrowth 1960s when it expected to have a population of 100,000 by the year 2,000. In the slower-growth 1970s, it appears the reservoir will be sufficient well beyond the year 2000.

But, as Moyer points out, the city needed a big storage area because the Broad Run watershed is not very extensive. THat means it takes a lot of rain over a long period of time to fill up Lake Manassas. On the other hand, the Occoquan, with a watershed whose stream network extends through parts of Fairfax, Prince William, and Fauquier counties, fills up faster, though the difference is meaningless in this period of drought and low soil moisture.