The crushing drought that has left Dulles International Airport 12 inches below its annual yearly waterfall has been sapping supplies at the Beaver Dam reservoir providing water for eastern Loudoun County.

Loudoun County Sanitation Authority General Manager Dales Hammes asked the county board to urge its customers to water their lawns only once a week to reduce regional water demand.

Although residents responded to earlier calls to reduce outdoor watering in August, the drought persists, and the water supply at the reservoir has plummeted from 1 billion gallons in late July to 436 million gallons on Tuesday.

“We need their help,” Hammes said. “We’re not out of the woods, and we need to reduce our demand even further.”

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The last time the county faced serious problems, in the late summer of 1999, two tropical storms dumped a combined 6 inches of rain on the area.

Unless similar events occur, Hammes warned the reservoir could need another year to recover.

“This 12-inch rainfall deficit is not going to be erased quickly,” he said.

Recreational facilities such as soccer fields and farmers who need water supplies for their livelihood are being asked to water every other day.

“The bottom line is: conserve, conserve, conserve,” Vice Chairman Bruce Tulloch said.

The county already has reduced the amount it pulls from the reservoir each day from about 10 million gallons to about 3 million to 4 million gallons, depending more on water from neighboring Fairfax County.

If the reservoir continues to dry up, the Loudoun County Sanitation Authority might be forced to depend entirely on water from neighboring Fairfax supplies, Hammes said.

dgenz@dcexaminer.com