IF YOU WERE in any way discomforted by the recent heat wave, not to mention the absence of what you could rightly call a mass-transit system, you will no doubt be comforted to know that the region is at least being spared one problem: There is no water shortage this year. Nor is one likely, barring some catastrophe. Rainfall has been ample for several months. And there is even better news on the local water front. The area's major suppliers are making real progress toward better management and sharing of supplies - in other words, toward making the region far less dependent on the vagaries of rain.

One big gain came last week when the Army Corps of Engineers finally granted the permit for the Fairfax County Water Authority's Potomac River intake. That project, which will provide up to 50 million gallons per day, will help the county cope with growth and reduce its day-to-day dependence on the Occoquan watershed.

The permit is doubly noteworthy because it is the product of strong regional cooperation, plus considerable prodding by Rep. Herbert E. Harris II (D-Va.) and Army Secretary Clifford L. Alexander Jr. One major barrier was overcome last winter when area jurisdictions and the Corps reached an accord on how to allocate Potomac water when the river's flow is low. As often happens, though, that progress brought another problem to the fore. The Environmental Protection Agency, the Interior Department and some area groups criticized the the agreement because it failed to specify how much water should be left in the river above Little Falls for ecological protection there. Studies are under way. A rock-bottom limit should be set before the Fairfax pipe in finished in 1981.

There are other signs of the growing regional interest in pooling water supplies and thus giving everyone more insurance against future droughts and disruptions of service. With Rep. Harris as a catalyst, Northern Virginia water agencies are overcoming the go-it-alone attitudes that contributed to the shortage in the Occoquan basin last year. Across the river in suburban Maryland, a bi-county task force has won substantial support for major connections between the Washington Suburban Sanitary Commission's Potomac system and its Patuxent River reservoirs.

The next logical - and prudent - step would be to build new links between the suburban networks and the Corps' Washington Aqueduct system that serves the District, Arlington and Falls Church. Most of the officials involved are still skittish about this. Even so, they are gradually forging the political connections that have to come first. This is one field in which the prospects for regionalism have vastly improved.