Northern Virginia's major water utilities, each of which has generally gone its own way in the past, want ot band together in a federation.

The main purpose of the Federation of Water Suppliers and distributors, as the group would be called, would be to reach agreement on how water could be shared during emergencies that cripple one utility but not necessarily others.

Such a predicament occurred last summer and fall when the Fairfax County Water Authority, whose Occoquan Reservoir fell to record low levels, Had to impose restrictions on its more than 600,000 users in Fairfax and Prince William counties and Alexandria. But at the same time, Arlington Falls Church, Fairfax City and even parts of Fairfax and Prince William counties were unaffected because they are supplied by other utilities.

On May 8 all 16 jurisdictions and water authorities in Northern Virginia will get together in their first formal meeting and try to fashion a water-sharing most of Fairfax and Prince William, Alexandria, Arlington and Falls Church - have already held exploratory meetings.

The movement toward cooperation began after officials of Northern Virginia localities and utilities testified at hearings held by Rep. Herbert E. Harris II (D.-Va.), chairman of the District of Columbia's Subcommittee on Economic Development and Regional Affairs. "They had never sat down together before," Harris spokesman Jack Sweeney said.

"We've taken a step forward," Harris said. "once you al the jurisdications together, you can move toward mutual things like interconnections."

Interconnections on a large scale have been proposed as one way the entire metropolitan region could meet its long-term water demand without building costly and controversial new reservoirs.

At present, there is only a small network of interconnections in Northern Virginia. For example, the Fairfax Water Authority could buy only a maximum of about 16 million gallons daily from Falls Church and Arlington. During the Fairfax Authority's emergency last year, Falls Church and Arlington could have supplied more water - which they buy from the District - but limitations were set by the network of pipes.

Fred P. Griffin Jr., assistant engineer-director of the Fairfax Authority, said formation of a regional federation "could provide the opportunity for constructing more interconnetions."

The most vulnerable of all the Northern Virginia utilities is the Fairfax Authority, which at present must rely on the Occoquan for most of its supplies. According to several studies, the reservoir's capability of supplying adequate water during long droughts may have been overestimated. Furthermore, most of the authority's new demand comes from northwestern Fairfax - a problem of geography that requires water to be pumped uphill.

The Fairfax Authority is ready to construct a water intake on the Potomac River and new treatment facilities nearby, but the project is being delayed pending issuance of permit by the Army Corps of Engineers. The Environmental Protection Agency and Interior Department have filed environmental objections to the project - and until the issue is settled, the corps is not likely to grant a go-ahead.

But if and when the authority does get to build its intake-treatment facilities, it will be a greatly enhanced potential as a major supplier - a fact that probably has not escaped other northern Virginia utilities. In fact, the proposed federation would include jurisdications as distant as Loudoun County and Leesburg, both of which have experienced supply problems recently.

With the memory of last year's drought emergency still fresh and the likelihood of peak-time shortages as early as next year, the Fairfax Water Authority has proposed a "water supply management program" that would go into effect next month. The authority is asking Fairfax, Prince William and Alexandria to approve the program.

The authority commissioners, in action last week, unanimously adopted a program under which the agency would automatically increase its daily purchases from Falls Church from 5 million gallons to 8 million gallons. THe increased purchase, which would cost an additional $300 a day, would stretch the storage capacity of the Occoquan by about 10 percent.

In other steps under the management program, the authority would ask for voluntary restrictions on outdoor water usage when the reservoir fell to a 60-day supply (3.3 billion gallons). The authority would also begin buying additional supplies - from Manassas Park, which maintains a reservoir on Broad Run in Prince William.

If the reservoir fell to a 30-day supply (2.8 billion gallons), the authority would impose mandatory restrictions on outdoor usage and prohibit the adding of water to outdoor pools or using water for air conditioning when interior temperature is less than 76 degrees.

The authority is also considering adding a "flashboard" or addition to the top of the Occoquan dam that would increase the reservoir's capacity by 1.5 billion gallons, at a cost of $50,000.