In what would amount to a major policy shift, the State Water Control Board is expected to propose solutions for Northern Virginia's worsening water-supply problems that emphasize conservation and call for no new reservoirs.

Should the shift occur, it would represent a victory for the State Water Study Commission, which was appointed by the General Assembly last March in effect to ride herd on the Water Control Board as it developed solutions for Northern Virginia's future water needs.

Water Control Board member Kenneth Rollins, who represents Northern Virginia, said earlier this week, "I would assume the board would adopt the commission's recommendations." The board must submit a report to Gov. Mills E. Godwin and the General Assembly by next Thursday.

Earlier this month the Norther Virginia subcommittee of the commission refused to accept a report drawn up by the Water Control Board's regional staff, and instead offered 10 "interim recommendations" of its own.

While the Water Control Board's staff projected continuing increases in individual water demand over the next four decades, the Water Study Commission, in its recommendations, said Northern Virginia and the state should "work toward reducing the per-capita water demand."

The commission suggested revision of plumbing and building codes "to mandate the use of water-saving devices and appliances in new construction" and rate structures for water use that "encourage conservation."

The commission also said that construction of various reservoirs in Loudoun County - key elements of three of the Water Control Board's five alternatives - "are not viable alternatives."

In stressing conservation and de-emphasizing reservoir building, the commission's recommendations bore some resemblance to suburban Maryland's recently completed water supply study. The "bicounty study, which has gained national attention, emphasized "drought management" (periodic restrictions, mostly on outdoor uses of water) as a way of holding down demand so that new reservoirs, which are costly and environmentally controversial, will not have to be built.

For example, the study says that drought management could hold down the need for additional water-storage capacity in the year 2005 to as little as 740 million gallons and no more than 1.3 billion gallons. Either amount could be provided by building a relatively small soil conservation lake that would cost $15 million to $20 million.

In addition, the Washington Suburban Sanitary Commission, which serves Montgomery and Prince George's counties, has adopted a rate structure that benefits those who conserve on usage. The Fairfax County Water Authority, which servces Fairfax and Prince William counties and Alexandria, has a rate structure that critics say encourages waterfulness and penalizes conservation.

Though the Virginia Water Study Commission was supposed to be dissolved when the Water Control Board presented its report to the governor and General Assembly, its life would be extended for a year under the Northern Virginia subcommittee's recommendations.

According to subcommittee member Louis F. Guy, an engineer, the additional time would give the commission a chance to detail the benefits of conservation as well an analyse measures that would increase supply without requiring construction of new reservoirs.

Though most of Northern Virginia was under drought-caused water restrictions from mid-September to early November and planners see demand outstripping present supplies by 1981, the region remains sharply divided on what to do.

The Fairfax County Water Authority, anticipating it will have to serve a growing population, plans to build a $53 million pumping-treatment facility on the Potomac River. But Prince William County wants to break away from the authority and build its own facilities.

Prince William's determination to pull out of the Fairfax County Water Authority complicates long-range regional water planning. If the pullout succeeds - it must be approved by the Water Control Board - the Fairfax Water Authority's need for more water storage capacity would be lessened. If the pullout fails, the authority's needs would be greater. But it could be months, even years, before the answer is known.

Planning is complicated by the already fragmented system of water supply in Northern Virginia. Arlington and Falls Church rely on the District of Columbia for their water and Fairfax City maintains two reservoirs in Loudoun County. In contrast, suburban Maryland is served by a single agency - the WSSC.

When the Water Control Board's regional staff issued a preliminary study assuming that Northern Virginia would be served by one regional authority, Fairfax officials, citing all the present and potential splintering, said the assumption was unrealistic. But that preliminary study was shelved and an expected final report never materialized.