The Fairfax County Water Authority has decided to add two feet to the dam at its Occoquan Reservoir -- a $100,000 project that officials say would give Northern Virginia an extra 20 days' insurance in case of a drought.

According to water authority estimates, demand could exceed supply as early as next summer -- a year before the new water-pumping facility on the Potomac River is ready.

"We're going to have to go through this summer and the next without the Potomac plant," authority spokesman James A. Warfield Jr. said. "So each summer the reservoir is more vulnerable."

The two-foot addition to the dam -- a removable concrete structure called a "flashboard" -- would add 1.1 billion gallons to the present 9.86-billion-gallon capacity of the impoundment, located on Occoquan River between Fairfax and Prince William counties. Warfield said the extra capacity would add 20 days to the period the reservoir could be used during a drought before running dry.

In 1977, during a summer-fall drought, outdoor water restrictions were imposed for about three months on the more than 600,000 Northern Virginians served by the authority.

Warfield said engineering studies have shown that the dam extension would not endanger residents upstream or downstream. But he said that during a severe storm, such as tropical storm Agnes in 1972, a higher dam would add three feet to flooding upstream.

While the dam extension was approved 6 to 3 by the water authority board, the project will require several permits before construction -- expected to take six months -- can begin.

"There are potential problems upstream and downstream," said Kathleen K. Seefeldt, chairman of the Prince William County Board of Supervisors."I have some concerns and I would like them addressed."

Prince William buys water from the authority, but is considering building its own reservoir system. It is also considering relying on the authority for the next 20 years.