Water authority officials in the Washington area will start a two-week dress rehearsal tomorrow of a plan to replenish the Potomac River to ensure that residents will have water if the severe drought continues.

A "drought exercise" is held every year, but this time "there is the real possibility we will be doing this for real in latter summer," said Daniel P. Sheer, director of the Rockville-based Interstate Commission on the Potomac River Basin.

Sheer said that if significant rainfall does not occur, the measures probably would begin for real in mid-August.

There is a 50 percent chance that the measures will be implemented, Sheer added.

So far, the prolonged drought has not affected the area's water supply. However, an Arlington County water main ruptured on Friday, forcing county officials to enact emergency restrictions. And in Fairfax County, officials are asking county water authority customers to restrict watering their lawns as a precautionary measure.

If the drought plan is implemented, it would be the first under a 1982 agreement among local water authorities and the commission. That agreement established a shared system of reservoirs, the largest being the Bloomington Lake Dam in West Virginia, that can be released into the Potomac whenever it falls to a critically low flow level. That level is 700 million gallons of water a day at Little Falls, where local water boards take 500 million gallons of water from the Potomac each day, Sheer said.

Friday, the water flow level at Little Falls was about 2 billion gallons, he said, representing a drop of about 300 million gallons from the previous day. Sheer said that the flow level is dropping rapidly and that, unless this area receives significant rainfall in the next few weeks, the critical level of 700 million gallons would be reached sometime in mid-August. Local water officials met Friday to discuss their 14-day dry run, during which they will inform the commission each day of their previous day's demand for water from their customers. Each water authority then will be assigned the amount of water that it would be permitted to draw from the river the following day. However, the water authorities will not have to abide by their assigned levels during this exercise. At the same time, the commission will estimate how much water should be released from the dams.

An actual release from Bloomington, acting like the rear engine of a freight train, would advance the river's flow.

The effects of the release would be visible about 5 1/2 days later at Little Falls, Sheer said, and the actual water sent into the river would arrive about a month later.

While the current situation is not an emergency, Sheer explained, it is important for local water boards to practice coordinating their actions in a simulated emergency exercise so "we all know what everybody else is doing."

Water officials say they are having no difficulties in meeting unusually high, and in some places record, demands for water in recent days.

Only the Washington Aqueduct Division of the Army Corps of Engineers, which supplies water to the District and Arlington, has not faced an increase in consumption. This is because most of the divison's weekday consumers are at home in the suburbs on weekends, when demand traditionally is highest, said its director, Harry Ways.

Fairfax County Water Authority officials have asked their customers to cut back on watering their lawns one day a week as a precautionary measure. The "voluntary restraints" were requested, officials said, because of an unprecedented demand on the system caused by rapid growth in the county, combined with the heat and drought.

The restrictions were enacted after the system's customers used a record 152 million gallons of water May 30, bringing demand above the system's pumping and distribution capacity of 150 million gallons a day, said James Warfield, deputy director of the Fairfax County Water Authority.

Some residents in the Groveton area of the county experienced problems getting water for a few hours that day, Warfield said, adding that this was attributable to the rupture of a water main by a bulldozer. The main has been repaired, he said.

"We currently are meeting demand that has been very, very high, much higher than ever projected," Warfield said. "If we weren't having a record-breaking drought, we would have no problem whatsoever meeting demand."

To cope with the demand, the authority accelerated its $225 million capital improvements program last month and authorized plans for doubling the size of its newest water treatment plant in Herndon. That plant, which now treats and pumps 50 million gallons of water a day, had not been expected to be enlarged for two to three years, Warfield said.

Warfield acknowledged criticism from some quarters that the county should build plants large enough to cope with even the worst scenarios. But, he said, the authority "has made a policy decision that we're not going to build an exorbitant amount of facilities just to absorb record-breaking droughts."

"We decided that we will have our customers be inconvenienced once in a blue moon and keep rates down," he said, "and we think that's what our customers want."

Most of the increased use of water is for watering lawns, Warfield said, predicting that this soon will decrease as people give up trying to revive their brown, parched lawns.