THE RAIN MAY BE THE most immediate water problem hereabouts. But leaking roofs and overflowing water mains, you may rest assured, will be replaced in due course on your list of water concerns by the continuing threat of water shortages. Experience shows that apparent progress toward long-range regional remedies should be cheered cautiously, because new diffculties have a way or arising at every step. Even so, there is reason to applaud the gains reflected in two documents released in draft form last week.

The first is the draft environmental-impact statement on Fairfax County's plan to tap the Potomac and the intake improvements long sought by the Washington Suburban Sanitary Commission. The Corps of Engineers had been saying that the study could not be done before next year. In October, though, Army Secretary clifford L. Alexander promised Rep. Herbert E. Harris II (D-Va.) that the draft would be ready be December 15 - and, wonder of wonders, it was. Moreover, the bulky study concludes that the two intakes would not cause much environmental damage unless, during some future drought, they drew so much water that the river above Little Falls ran dry.

That's where the second document comes in. It is the latest draft proposal for allocating Potomac water in times of low flow. This issue has been the real sticking point. It is not settled yet; the latest draft has not been approved by the governors of Maryland and Virginia, Mayor Washington and Mr. Alexander; and some points remain unresolved. Even as a staff proposal, though, the new draft reflects good negotiating and a general desire to recommodate the suburbs' future needs, environmental requirements and the Corps of Engineers' anxieties baout ensuring enough water for its customers in the District, Arlington and Falls Church.

In essence, the latest formual would allocate Potomac water during shortages according to each jurisdiction's overall water use during the preceding five years - minues supplies available to the suburbs in their nearby reservoirs. The formula would be frozen in 1988 unless renegotiated then. This approach would have three important benefits. It would ensure that the District/Arlington minimym supply would decrease gradually, not suddenly, as the suburbs grow. It would encourage the Maryland suburbs to continue their water-conservation programs and Virginia and the District to do more in this regard. Finally, the plan would push the suburbs to keetp their reservoirs full for use when the river runs low.

Other parts of the draft would encourage everyone to develop more water-supply and storage system - and would obligate the suburbs to give the Corps a chance to buy into each new project. This could be a step toward the kinds of interconnections proposed by engineer Daniel Sheer and others. And such regional links could make the District and Arlington much less dependent on the river than they are now. In other words, the draft low-flow agreement contains language that, if implemented, could eventually make the low-flow problem almost disappear.

Again, it's all still tentative, but praiseworthy nonetheless. The Corps is now inviting comment from concerned agencies and groups, and has scheduled a public hearing on January 23 at Mount Vernon College. We only hope that all of the top officials involved will persevere until all details have been settled, so that the suburban projects can get started and everyone's future water worries can be reduced.