NOW THAT THE RAINS have filled up the Occoquan Reservoir again, it may be tempting to dismiss Northern Virginia's recent water shortage as a phony or a fluke. It was neither. It was serious, and the fact that rain arrived when not predicted only underscores how chancy the whole situation is. As long as Fairfax and Prince William Counties and Alexandria rely so heavily on the Occoquan water-shed, they will be captives of the weather - and another long dry spell could bring restrictions more severe than those recently in force.

The long-term remedy most emphasized recently is the proposed Potomac River intake, which has been held up by the Army Corps of Engineers. Now that Army Secretary Clifford Alexander has taken an interest in this tedious dispute, it should be possible to work out a regional low-flow accord soon and get the intake under way. Mr. Alexander's recent "summit conference" with Virginia Gov. Mills Godwin, acting Maryland Gov. Blair Lee 111 and D.C. Mayor Walter Washington at least sounded encouraging.

A second element, we think, should be a regional system of reservoir interconnections and coordinated management, roughly along the lines suggested by engineer Daniel Sheer. Working this out, and especially arranging joint financing, would be a real test of area officials' capacity to cooperate. Yet such a plan would give every water system in the region more protection against future droughts and emergencies. Right now, for instance, is a toxic spill should poison the Potomac near the Corps' intake, the District and Arlington would have to ration water severely if the problem lasted much more than a day.

The final element should be conservation - not the emergency, no-car-washing kind, but permanent changes in water-using systems and habits to keep down the everyday demand. The Washington Suburban Sanitary Commission, which has been very creative in this area, has just adopted a sliding scale of water and sewer rates to encourage conservation. Virginia jurisdictions should pursue similar steps. Fairfax County's rapid growth - which intensifies the pressure on the Occoquan - also provides chances to test new ways of building water-saving into new construction. The county's plumbing code calls for some elementary water-conservation steps, but innovative developers could probably do no more. Mobil Oil, for example, has expressed concern about water for its new Fairfax headquarters. Besides doing their darndest to increase supplies, Fairfax officials might well ask the company to help by reducing its needs as much as possible.