The region's water-supply planners have a doomsday scenario worse than anything that's happened this century. It goes like this: The coming winter brings scant rain, rivers start drying to a trickle, and Washington gets another arid summer when homeowners keep their sprinklers going full blast.
The planners' conclusion: There would still be enough water for everyone, without limits, until the following winter, when underground springs would replenish the supply. That scenario is becoming the flash point for political debate over the need for water use limits in the Washington region.
No substantial relief is in sight from a drought that has hung on for more than a year, according to the National Weather Service, which predicts only scattered storms later this week. Last month, utilities were forced to release water from the upstream Jennings Randolph Reservoir for the first time since it was built in 1982 to replenish the Potomac River.
Montgomery County Executive Douglas M. Duncan (D) says that should get everybody's attention -- and trigger conservation measures -- because once those reserves are gone, that's it: "There's no backup to the backup," as he puts it.
But District and Virginia officials have declined to follow Maryland in imposing mandatory water restrictions, citing the prediction by the Interstate Commission on the Potomac River Basin that water supplies will remain adequate -- even if the doomsday scenario comes to pass.
"We always have to be prepared for something worse than we've seen before," said Erik Hagen, a water supply engineer for the commission, which coordinates the region's water supply, including when water is released from reservoirs. "You want to make sure you go in with a big buffer, and that's exactly where we are at. . . . There's a wide margin of safety."
District and Northern Virginia officials, who have urged voluntary conservation, say they are not concerned that reservoir water has been released for the first time. Reservoirs are supposed to be used, they say, as long as forecasts indicate that water reserves are ample -- and they do.
"For 20 years we've been operating under this mechanism," said James Warfield, Fairfax County Water Authority executive director. "It's showing through this drought that the plan works. The wild card is politics -- when someone steps out and recommends that we do something different than what all the jurisdictions agreed to do."
Duncan argues that, at the least, voluntary restrictions should automatically kick in once the Potomac dips to a level that triggers any release from the Jennings Randolph or Little Seneca reservoirs.
Said Duncan: "He's [Warfield] on a mission to drain those reservoirs. Then he'll say we have a problem" -- a charge that Warfield vehemently denies.
Duncan and others have demanded to know what conditions would trigger a declared "water supply emergency." After that, local officials would decide what to do next -- mandatory restrictions, voluntary ones or no action. Commission staff members hope to draft that explanation within a week.
They say that any trigger will change by the season -- depending on river flow, reservoir levels and water-use patterns. A low reservoir in October, when demand is dropping and an infusion of winter runoff from groundwater is expected, would be less of a concern than at the start of the summer high-use season, water planners say.
The commission, whose board includes representatives of the District, Maryland and Virginia, based its water-supply prediction on worst-case events that have never happened in the same year this century.
That forecast depends heavily on the existence of the region's biggest reservoir -- Randolph Jennings on the Maryland-West Virginia border, which holds 13.4 billion gallons of water owned by Washington area utilities. Including water stored for other uses, the reservoir is 30 billion gallons in size.
As part of their worst-case scenario, water planners assume that even in a dry winter, the reservoir would be replenished by water from underground streams. Even during the winter of 1929-30, which ushered in the century's worst drought, there was 60 billion gallons of runoff -- enough to have filled up Jennings Randolph twice. Winter runoff has not dropped to such levels since then, according to stream gauges.
Then, the supply planners assume that the Potomac River flow drops to the record low set for each day of the year. That includes the day in 1966 when it hit a low for the century -- 60 percent of its current flow.
The bad-news scenario also assumes that the Occoquan River in Virginia and Patuxent River in Maryland are down to a trickle.
In that situation, regional utilities would need to draw 3.3 billion gallons from Potomac reservoir storage to meet demand, planners conclude, and not until mid-July, when record low Potomac flows collide with high summer demand. Releases would stop in the fall, when demand drops off with cooler weather.
During winter months, when leafless trees have less need to soak up water from the ground, that water then flows to the reservoir, filling it up again.
As of yesterday, Jennings Randolph was 78 percent full. Even without appreciable rain, the reservoir gets some replenishment from underground springs throughout the Potomac's 11,500-square-mile watershed that stretches through Pennsylvania, West Virginia, Virginia, Maryland and the District. The Potomac supplies more than three-fourths of the water used in the region that includes the District, most of Northern Virginia, and Montgomery and Prince George's counties.
Next year, the suppliers that draw water from Jennings Randolph Reservoir -- the Army Corps of Engineers, Washington Suburban Sanitary Commission and Fairfax County Water Authority -- will conduct a study of whether supplies are adequate to meet future demand.
The political jockeying over that study already has begun, with Duncan and some environmental groups arguing that the region needs to increase the minimum flow levels it maintains in the Potomac, to better protect fish and wildlife. Now, officials use reservoir water to make sure the river's flow doesn't drop below the minimum of 100 million gallons a day, not counting the amount drawn off for drinking water.
Increasing that minimum could require releasing more reservoir water during summer months when the Potomac River is low. That, in turn, could force the water planners to rewrite their forecasts -- and their assessment of when the region will risk running out of water.
Staff writers Michael D. Shear and Scott Wilson contributed to this report.
Enough Water for Washington
The prediction that the Washington area will have an adequate water supply despite the longevity of the current drought is based on a scenario that combines worst-case scenarios for summer and winter.
If the Potomac River flow this summer drops to the lowest level of any given day recorded this century.
The regions's utilities would need to draw 3.3 billion gallons of water from the reservoir for summer use beginning in mid-July, when worst-case flows coincided with high summer demand.
Jennings Randolph has more than 13 billion gallons of water available for Washington area utilities.
If the region has a winter so dry that runoff from rain and groundwater is as low as during the area's worst drought in 1930.
Reservoirs must be completely refilled during the winter, in case they are needed druing the high summer demand period.
In the 1930s drought, runoff equaled 60 billion gallons -- double the size of Jennings Randolph.