UNTIL THAT DAY when the citizens of the greater Washington area suddenly discover, one after the other, that all their faucets are spitting air, it is probably going to be difficult to concentrate regional attention on the complex and interconnected problems of sewage-treatment and water-supply.But nearly any expert around town will tell you that the danger of running dry on a grand scale is a real one and that it cannot be effectively addressed on a provincial, jurisdictional level. Regional cooperation is the only sensible way to deal with the area's water needs. But it is also hellishly difficult to organize. So we are pleased to call attention today to some recent evidence of progress along these lines.

Some good work and an interesting proposal have sprung forth from various parts of the region. The work has to do with an old sewer dispute between the Maryland suburbs and the District; the proposal is an imaginative plan for pumping water from the Shenandoah River to Northern Virginia's perilously dry reservoirs.

The sewer story is about a far-reaching agreement that should speed the end of a seven-year moratorium on new development in parts of Montgomery and Prince George's counties. As reported by staff writer. Thomas Grubisich, the accord also enhances prospects that Montgomery might win a legal battle with the Environmental Protection Agency to build a regional sewage treatment plant at Dickerson. In essence, here's what is proposed:

The city government will join Montgomery in the county's suit seeking to force EPA financing of the Dickerson plant. Last year, EPA vetoed the project on the grounds that it would be too big and too costly. Russell E. Train, who was then EPA administrator, urged Montgomery to seek a scaled-down alternative that would include regional sewage commitments - in other words, participation by the District. But District officials were angered by a Montgomery suit claiming that the city was using more capacity than it was entitled to at the Blue Plains regional treatment plant in Southwest Washington.

Montgomery will drop that suit against the city, thus giving the city an additional capacity at Blue Plains.

The city will now drop its opposition to completion of expanded Anacostia sewage line from Bladensburg to Blue Plains. This would enable Montgomery and Blue Plains. This would enable Montgomery and Prince George's to take new advantage of capacities at Blue Plains.

The two counties will reaffirm their commitment to provide the city capacity at the Dickerson plant, if it is built, and at another treatment plant at Piscataway, when that facility is expanded.

In addition, the agreement is likely to have a effect in Northern Virginia, for it could mean that Fairfax County might get additional capacity from these facilities.

While all this suit-dropping and pledging was going on, a noted Virginia expert on water problems, Noman M. Cole Jr., was generating support for his proposals to build a pipeline that could deliver 100 million gallons of excess water a day from the Shenandoah River to the Occoquan Reservoir. If then tied to Arlington's water system, the entire region could benefit. So far, the proposal by the former chairman of the Virginia State Water Control Board has won bipartisan praise from, among others, Rep. Herbert Harris (D) and Lt. Gov. John N. Dalton (R). An alternate proposal, preferred by the Fairfax Water Supply pointed the committee, calls for a pipeline connecting the Potomac to Occoquan. It might be as effective and not involve expanding the Occoquan treatment facilities as the Cole plan would.

At any rate, maybe at last - through more intensive negotiations between local governments and through the good offices and expertise of the Metropolitan Washington Council of Governments - residents of this area may really notice progress in protecting them against water shortages. There is no question that the old, fragmented, haphazard ways of making decisions have become increasingly risky, expensive and ineffective. It is good news if the local leadership has grown newly sensitive to the political as well aswell as technical needs for serious regional cooperation.