THE LOW WATER LEVEL in the Occoquan Reservoir probably won't become a crisis - if everyone in Fairfax County, Prince William County and Alexandria helps by conserving water until the fall rains come. Even so, the shortage is one more warning that Northern Virginia is outgrowing the reservoir. Expedients such as buying water from Lake Manassas won't suffice for very long. That's why Fairfax County officials are so anxious to get the Corps of Engineers' permission to draw from the Potomac. And that's why there is also interest in an imaginative plan, put forth by former State Water Control Board Chairman Norman M. Cole Jr., for pumping water from the Shenandoah River to a tributary of the Occoqua.

Besides tapping new water sources. Northern Virginia should connect any new systems with the old. This would enhance the area's ability to cope with future droughts, toxic spills or breakdowns like the pumping-station fire that caused a brief crisis in the Maryland suburbs recently. It would also be a first step toward the regional approcah, involving major interconnections and coordinated water management, proposed recently by engineer Daniel Sheer.

A key factor in all of this is the Corps of Engineers. Although the Corps is supporting detailed studies of Mr. Sheer's concept, it is also holding up Fairfax's Potomac intake pending a regional agreement on allocating Potomac water when the river's flow is low. We can understand the Corps' desire to ensure that basic supplies for its customers in the District, Arlington and Falls Church will not be preempted, over the years, by the growing suburbs. Yet the Corpc' current posture is also ironic because the Fairfax intake and related interconnections can, in the long run, help make the whole low-flow competition irrelevant.

That is the real beauty of the Sheer approach. It could enable the entire region to use the Potomac when the river's flow is ample, and to stockpile water in the Patuxent and Occoquan reservoirs for everyone's use when the Potomac does run low. Even the District and Arlington could be less dependent on the river than they are now.

The tough political question is how to get from here to there. The timing is tricky; Fairfax County, for instance, will need Potomac water several years before any new regional structure can be planned and built. The county's intake should not be delayed. On the other hand, Northern Virginia and all other water-suppliers need to make a real commitment now to a new, cooperative strategy. Perhaps the logjam could be broken by a short-range allocation accord that includes enforceable deadlines for taking each step toward a regional plan.

In the short run, after all, the problems of low-flow allocation are not likely to be that serious. In the longer run, though, the current fragmented and competitive approach will not ensure enough water for anyone.