The drinking water supply for the Washington area is undependable and the risk of summertime water shortages and rationing is real, government agencies, water planners, politicians, dam builders, conservationists and environmentalists agree.

Some are willing to risk shortages and do nothing to store more water; others believe something must to done, but cannot agree what it should be and have so far been unable to build public belief that there is even a problem.

"To say there is a water shortage is to raise an issue without credibility," said S. Buddy Harris, of the Board of Trade's water resources committee. "People look and they see that the rivers are not dry and the reservoirs are not dry."

It is in that context that a slow-motion debate on the Washington water situation has continued for two decades.

During that debate, on a few hot days in every summer since 1966 and on four days last June alone - area water supply systems have taken more water from the Potomac River than the Potomac River had to offer on its driest known day. Fortunately, heavy demand for water has not coincided with low river flow to date. Statistically, it is just a matter of time before that happens, water planners say. "Potomac roulette," they call it.

The driest day was Sept. 10, 1966, when only 388 million gallons were measured by the U.S. Geological Survey as the river's total flow. The heaviest use came on July 18, 1974, when 448 million gallons were taken from the Potomac for area water systems.

Every Washington area jurisdiction is dependent on the Potomac River as the source of either its present or future water needs. If an extended drought were to hit the Potomac Basin, as has happened in the West and Midwest this year, this is what it would mean to area residents:

Rationing would begin with a ban on lawn watering and car washing. Appeals to take shorter and fewer showers and to flush the toilet less often could follow, depending on the severity of the situation. High-penalty water bills for excessive use could be employed. "We have plans to turn water off to individual neighborhoods on a rotating basis if it gets serious enough," said Jean B. Levesque, director of water resources for the District of Columbia. A major concern would be to keep pressure up to fight a big fire.

If the river gets too low, the giant water intakes for the suburban Maryland water supply system will begin sucking air, not water. Air in th system means the individual user has less water at lower pressure. If that reduction is severe enough, it means health problems, because there is a danger of untreated, polluted water from other sources seeping into mains and pipes.

A major water shortage in the nation's capital would get publicity everywhere, a prospect that does not appeal to the Board of Trade. "If word gets out that there is a water problem here," said Buddy Harris, "it will affect this area economically. Major industries will draw a line around us. There is either growth or retrogression - no status quo." No-growth advocates obviously have a different view as to how calamitous a water problem might be.

The water of an almost-dry Potomac would be of much lower quality than water in a full river, because both natural and man-made pollutants would not be as well diluted. Because of that, water taken from a low-flowing river requires much more expensive treatment before it is drinkable. Even then, it might taste odd.

The Potomac's tidal estuary - that portion of the river below Little Falls - could become a smelly field of blue-green algae, just as it did in 1965. Despite much-improved sewage treatment at the various plants that dump into the estuary, a low-flowing Potomac has much less "flushing action" to clean the estuary of the nutrients and pollutants that provide a hospitable environment for algae.

The prospect of water shortage is difficult to believe because the Washington area has had 10 straight years of water prosperity.

Since 1966, annual rainfall has been near or above normal. There have been two major floods in the Potomac Basin - spawned by tropical storms Agnes and Eloise. If water has been a problem, it has been because of too much, not too little.

So far this year, however, the Potomac River is not its average self. In February, for the second straight month, the river was below normal. Just about a third as much water as is usual flowed over Great Falls.

The rainfalls of the past two weeks have brought the March total to within an inch of normal to date, but for the year the area is still four inches short of its average rainfall total.

"It is too early to draw any implications," said Wayne Solley, a hydrologist with the Geological Survey. "March is normally the wettest month and April is also a month of significant precipitation."

Based on what is known about the river's flow in times of drought and the rates of water used on hot days, and assuming population increases, the Corps of Engineers concluded in 1975 that "there exists an increasing potential for" one-day or one-week shortages right now.

Many persons, suspicious of the corps and its dam-building reputation, have quarreled with the corps' population projections and other assumptions for the future. No one could be found who does not think that there are growing risks for occasional shortages.

The corps warns that time is short for decisions because solutions take so long to implement.

The one water supply aid under construction on the Potomac - a dam at Bloomington, Md. - was first authorized in 1962 and will not be completed until 1980.

Enough water flows overGreat Falls every year to supply the Washington area for 10 or 15 years but, in the terminology of the corps, the Potomac is "unregulated." In other words, there is nothing to catch that excess water and save it for the future.

There has been a steady increase in the amount of water taken from the river. In 1965, a total of 94 billion gallons was withdrawn. By 1976, the total was 111 billion gallons.

The reason for the increase is simple more people live here, and they all drink water, taken baths and flush toilets. Total population in the metropolitan area grew by almost a million between 1960 and 1970. Growth has slowed since then, but the upward trend continue.

The 3 million persons who live in the Washington area today are served primarily by three different and virtually independent water systems; each has its unique supply problems.

The largest system, and one that serves about half the people, is operated by the Corps of Engineers and is known as the Washington Aqueduct. It serves all of the District of Columbia, Arlington County and Falls Church. The aqueduct system is almost totally dependent on the free flow of the Potomac for its water. Two small aqueduct reservoirs in the District - Dalecarlia and McMillan - have a total reserve capacity of about one day.

The Washington Suburban Sanitary Commission (WSSC), which serves Montgomery and Prince George's counties, has two major water systems. One of them - supported by two small reservoirs on the Patuxent River - provides about 20 per cent of the WSSC water on an average day. The rest of the WSSC's water comes from the free-flowing Potomac, unaided by reservoirs. There is no major connection between the two WSSC systems, so the Patuxent reservoirs don't help the rest of suburban Maryland in a Potomac water crisis.

The Fairfax County Water Authority operates a reservoir and water treatment plant on the Occoquan River that serves most of Fairfax County, part of Prince William County and all of Alexandria. Although the reservoir has good storage capacity, the treatment and distribution system is nearing capacity, and if growth continues, simple demand is expected to exceed capacity by the end of this decade.

The Occuquan is downhill from the area of Fairfax County's heaviest growth - Reston/Herndon/Dulles Airport - and the logical source for water there is the Potomac.Fairfax is seeking a water intake above the Loudoun County line on the Potomac.

There are small independent water systems for Rockville, Fairfax City and Loudoun County, and many private wells.

None of the area's water supplies is connected with the other in any meaningful way. Good capacity at the Occquan reservoir does not help Washington. The Patuxent dams do not help Arlington.

No plan to solve these problems has been agreed on. It has continued to rain. No one believe, Don Bowman, enviromental director for Fairfax County, put it this way:

"Americans don't begin to fight until they have counted their dead."

NEXT: Solutions and Decisions