Over at the Fairfax County Water Authority, they've studied the charts, analyzed the figures, scanned the heavens and concluded: It may take rainfall of near-Biblical proportions to save many Northern Virginians from a summer of tough water restrictions.

Because of persistent dry weather since August, the Occoquan Reservoir is down to only 25 percent of its capacity. To fill the 11-billion-gallon lake would require three inches of rain in 48 hours, says the authority.

Such a deluge usually accompanies a major tropical storm -- and the odds of such a storm at this time of year are poor. Indeed, the outlook for any kind of significant precipitation during the month is bleak.

February is the driest month of the year. Average precipitation amounts to only 2.45 inches. And the long-range forecast of the National Weather Service says the metropolitan area can expect less-than-normal precipitation this month.

(Monday's rain, counting the slight precipitation during the previous evening, amounted to less than three-quarters of an inch.)

"It's not good news," said water authority spokesman James Warfield.

When the water authority commissioners hold their monthly meeting tonight, they are expected to consider whether to recommend additional mandatory restrictions on water usage. Already, the 700,000 people served by the authority in most of Fairfax County, Alexandria and eastern Prince William County are prohibited from watering their lawns and other greenery and washing their cars.

Since the dry spell began in August, only 9.19 inches of rain have fallen, less than half the normal average of just under 19 inches.

The crisis is a frustrating one for water officials and users because while Fairfax Authority customers are ordered to restrict their usage under the threat of a fine of up to $500, an estimated 250,000 Northern Virginians, who get water from other sources, are free to use as much water as they want. So are residents elsewhere in the metropolitan area.

Why is the crisis confined to the Fairfax Water Authority and its customers?

The short answer is that the authority, unlike suburban Maryland and the District, is not yet able to draw water from the Potomac River. The authority is building a facility to pump and treat river water, but it won't be finished until the end of the year. Apart from some small purchases of water from Falls Church (which also gets water from the Potomac, via the District) and Fairfax City (which has a reservoir in Loudoun County), the authority is almost totally dependent on the Occoquan Reservoir.

If the authority could use Potomac water, it could take pressure off the Occoquan temporarily so the reservoir wouldn't be depleted as rapidly during dry periods.

That is what the Washington Suburban Sanitary COMMISSION (WSSC), which serves Montgomery and Prince George's counties, is doing.

Because it decided to step up withdrawals from the Potomac, the Triadelphia and Duckett reservoirs still have almost half their capacity despite the lack of rain.

But there are limits to even this flexibility. If the drought continues, Maryland water officials said, it will have to impose restrictions on customers by April 1. They would be similar to those imposed in Northern Virginia.

While the Fairfax authority doesn't have as much flexibility as the WSSC, it does have some options for getting additional water.

It can buy more water from Falls Church.Presently, the Fairfax Authority buys 10 million gallons a day, but could increase that to 15 million gallons when Falls Church finishes repairing its Chain Bridge pumping station this month.

It can persuade Manassas to sell more water from the city's big impoundment in Prince William County. (Manassas sold the authority 1.5 billion gallons last fall.)

It can draw water from the Occoquan River estuary just below the dam on the Occoquan Reservoir. This could yield another 5 to 10 million gallons a day, but state Health Department approval would be required.

It could accelerate construction of an intake pipe that will draw water from the Potomac. While the new Potomac treatment plant is still months from completion, the intake pipe could be used to send raw water to the Occoquan plant via water lines intended to carry treated water. The plan, which could result in an additional supply of 15 million gallons, is "far-fetched," says the authority's Warfield, but he says it's an option under consideration.

If all the options were employed, the authority could draw on new supply sources totaling about 35 million gallons a day. Since outdoor water usage was restricted last week, the authority has been able to cut its withdrawals from the Occoquan from about 55 million gallons a day to 45 million gallons. That means -- if the options are used -- there would be only a 10-million-gallon-a-day gap between reservoir withdrawals and other sources, a gap that could be closed by additional conservation.

So the spectre of the reservoir actually running dry is exceedingly remote. But that doesn't mean there won't be some hard times for water users. The level of the reservoir may continue to sink. But the rate at which it drops could be slowed dramatically -- if not entirely offset -- by the range of options available to the authority.

Still, at least for now, the flood that kept Noah in his ark for 40 days and 40 nights isn't needed to help Northern Virginians weather the coming dry spell.