Northern Virginia's water supply planners, in contrast to those in suburban Maryland, are expecting their residnent's average water consumption to increase steadily even though supplies are harder to find.

While suburban Maryland is moving toward water-saving drought management as a way of holding down the need for new reservoir storage, a new report by the Virginia State Water Control Board's regional office projects a dizzying increase in demand.

The present average per person use in Northern Virginia is 96 1/2 gallons a day, with the range extending from 77 gallons in Loudoun County to 120 gallons daily in Arlington. According to the water control board report, which has just been completed, consumption will increase at the rate of 1.5 gallons daily each year until the year 2020 it is 163 gallons a day.

If the assumptions of the staff report are accepted by the water control board itself, the implications - fiscal and environmental - for Northern Virginians are considerable.

Based on the projections, the region would not be able to meet all its water needs beyond 1990, even if the Fairfax County Water Authority is granted approval to tap the Potomac River as a new source.

Though the water authority considers the Potomac project essential to heading off a shortage as early as two years from now, its prospects remain clouded.

The Loudoun County Planning Commission, concerned over the environmental impact from the pipe that would draw water from the river off the Loudoun shoreline, is holding up approval of necessary permits. The Army Corps of Engineers, which has to issue a permit as well is holding up its approval until Virginia and Maryland both agree to protect the District's share of water from the Potomac.

In addition to these troubles, the Potomac plan tied for last place in an environmental rating of 10 projects included in the new control board report.

In looking at future Northern Virginia needs, the control board report chose the conventional approach - now being abandoned in suburban Maryland - of meeting unlimited future water needs. "There is the feeling that restrictions are less acceptable," said John R. McClain Jr., coordinator of the report.

So the staff report lists a variety of public works projects designed to add capacity. All but one of them would be expensive, environmentally controversial, or both. The exception would be raising the height of the existing Occoquan Reservoir - at present the major water supply for Northern Virginia - but the staff report concedes this "will not greatly increse" overall capacity.

Apparently because restrictions on water use are not considered acceptable, as report coordinator McClain said, the report does not explore drought management as a key element of water-supply planning.

Instead the report accepts the premise of increasing demand, and projects needs accordingly. Carried to the year 2000, the figures show northern Virginia consuming more water than more populous suburban Maryland with moderate drought management.

In an evaluation last fall, the Joint Legislative Audit and Review Commission of the General Assembly accused the water control board of "neglecting its water resource planning responsibilities."

Nothing comparable to the scope of the bicounty water task force study in suburban Maryland is occuring in Northern Virginia. The skimpiness of local efforts was underscored during the current water emergency when the Fairfax water supply committee discovered the existence of a 5-billion-gallon reservoir in Prince William County owned and used by the city of Manassas, which has a population of 15,000.

While Montgomery and Prince George's counties are now working in relatively close cooperation with the semi-autonomous Washington Suburban Sanitary CommissioN, the counties' water and sewer agency, relations between local jurisdictions in Northern Virginia and the Fairfax water authority have become badly strained.

In one case earlier this month, the director-engineer of the authority, James J. Corbalis Jr., said that at the time, there was no need for conservation measures to counteract the gradual decline of the water level at the Occoquan Reservoir. The next morning, the chief executives of the jurisdictions served by the authority invoked water emergency ordinances and asked for voluntary conservation.