The Fairfax County Water Authority may have overestimated by as much as 26 per cent the capacity of the badly depleted Occoquan Reservoir to withstand a long dry period, according to preliminary analyses by three state and federal agencies.

There is no major alternate supply if the Occoquan goes dry - a possibility if there is no significant rainfall in the next few months, according to Fairfax water planners. If that happens the water authority, which supplies most of Fairfax County, all of Alexandria and half of Prince William County, has contingency plans to pump water from Burke Lake and other sources.

The new analyses, done by the Sate Water Control Board, the Northern Virginia Planning District Commission and the U.S. Geological Survey, come at a time when the reservoir contains only about a quarter of its usable supply.

Already the jurisdictions served by the Occoquan have imposed mandatory restrictions on outdoor water use, and if the reservoir falls another seven feet, the water authority plans to start using 1 1/2 billion gallons of water if bought from the city of Manassas.

The steady decline of the Occuquan, which has occurred while rainfall was below normal but no less than last year's amount, has puzzled water experts. The answer most frequently given is that the rain, instead of running off into the reservoir, was aborbed by the ground to build up the water table, depleted by a dry winter.

The new analyses suggest that the reservoir's "safe yield" may have been estimated too generously. Safe yied is the maximum amount of water that can be taken out of the reservoir daily to withstand an extended drought.

The water authority, basing its estimate on a 1976 analysis by its consultants, puts the safe yield of the Occoquan at 65 million gallons daily for a drought as bad as any in the past 40 years.

The State Water Control Board says that if the authority planned for the drought of 1930, the wrost in the last 100 years, the reservoir's safe yield would only be 48 million gallons daily.

The question is, how much safety do you want to pay for?" a water control board spokesman said.

The Northern Virgina Planning District Commission and the Geological Survey, in their preliminary estimates, have estimated the reservoir's safe yield at between 50 MGD and 54 MGD.

Water authority chairman Fred C. Morin said, "I believe they are in error, but at this point I don't really know. Until somebody gives us something conclusively different, we have to stick with what we know."

If the safe yield has been overestimated, the jurisdictions erved by the reservoir may have to maintain at least some restrictions for an extended period, perhaps year-around, until the Potomac River can be tapped as supplementary supply.

There is another, longer-range implication. A lower safe yield could dash plans to rely on a Potomac-Occoquain interconnection for the next 30 years, the proposal of a citizens' water supply committee appointed by the Fairfax Board of Supervisors. The Board has endorsed the proposal.