With strikingly little controversy, Montgomery and Prince George's counties are preparing to meet their future water needs with new and unconventional policies that would require more spartan habits by residential water users in years to come.

Under the policies, approved unanimously by a bicounty task force, the emphasis would not be on the traditional - and costly - one of supplying whatever water is demanded, but getting by with less water instead and even rationing it when necessary.

Besides changing the way people use a basic resource, the Maryland policies would drastically scale down the need for new or expanded reservoirs. Reservoir-building, usually enormously expensive and environmentally disruptive, invariably attracts stiff opposition that can put projects in limbo for years.

Across the river, planners at the Northern Virginia office of the State Water Control Board have just produced a report on future needs that takes an exactly opposite approach. The study assumes individual water consumption in that region will actually increase instead of going down and offers a range of expensive and controversial plans to meet this greater demand.

If water use were unlimited, suburban Maryland by 1995 would need an additional reservoir capacity of 4.6 billion gallons, the equivalent of nearly half the storage of the area's present wo reservoirs on the Patuxent River. Suburband Maryland, the District of Columbia and part of Northern Virginia also use the Potomac River as a water supply source, but there are no dams to store the supply.

Under policies adopted by the task force, the additional reservoir need could be reduced to as little as 900 million gallons, depending on how strict the restrictions are.

"We've redefined 'need,'" said Dr. John J. Boland, a John Hopkins University professor who is chief architect of the policies.

In the past, water utilities, including the Washington Suburban Sanitary COmmission (WSSC), which serves Montgomery and Prince George's counties, have assumed that unlimited needs must be met even in times of drought, when water is in short supply.

The work of the bicounty task force stands that assumption on its head.

The WSSC would be the first major public utility in the country to use drought management as an integral part of meeting water needs. According to Boland, who is an associate professor of geography and environmental engineering at John Hopkins, other utilities in the country are showing interest in the technique.

Boland's plan, still in draft form, is an attempt to deal with the peculiar reality of the Washington area's water supply: Most of the year, there is more than enough that can be drawn from the Potomac River, but there is a critical three-month dry period from August through October when flows could, for a day, week or even longer, fail to meet demands.

The conventional solution has been to build or expand reservoirs, which can be tapped during periods of low flow in the Potomac.

Another solution, one gaining more currency as reservoir building becomes more unpopular, is to connect existing reservoirs with the Potomac so the impoundments can be brimming full at the onset of the dry period and thus able to meet needs during periods of low flow in the river.

Either of these solutions would be costly and raise a host of environmental problems. Drought management, on the other hand, would require neither - or at most, modest expansion of present reservoir capacity, many water experts acknowledge.

This is how drought management would work:

First, MOntgomery and Prince Georges would decide how much they want to spend - in terms of financial and environmental cost - in adding new water storage capacity for future needs. If they want to avoid building any new reservoirs at all, they would have to accept the likelihood of periodic and stringent water restriction, based on past drought patterns.

Under the more stringent of two scenarios prepared by the task force, some restrictions (basically limiting lawn sprinkling to alternate days for one month or less) could occur as frequently as once every five years.

Under the less stringent scenario, the same restrictions could be expected to occur once every 10 years.

Additional restrictions - banning all outdoor water uses for a month or less - would probably occur once every 16 years under the more stringent scenario. They would probably occur once every 40 years under the less stringent one.

"What it really amounts to," Montgomery water planner Edmund Graham told the task force at a recent meeting, "is how much are you willing to accept these restrictions and how much are you will to spend for a project so that you avoid these restrictions?"

Suburban Maryland residents will get an opportunity to give their answer to these questions at public hearings on the task force's proposed policies that will be held next month.

After the hearings, the Montgomery and Prince George's County Councils will take up the issues when they decide how much the counties will spend on construction projects to meet water needs.

While their decisions may not be made until early next year, the direction they are likely to take - limited new facilities combined with at least moderate drought management measures - already has been indicated by the conclusions of the bicounty task force, whose membership includes the chairman of each County Council as well as officials of the WSSC.

Significantly, the WSSC is a willing and even enthusiastic partner in the think-small policies. As part of the task force, the utility supported the decision to scrap controversial plans - proposed by a consultant to the agency - to expand the present Patuxent reservoirs from the current size of nearly 11 billion gallons to 44 billion - or even 65 billion, under one option.

"We looked at the figures, and they said we didn't need what we thought we needed," WSSC Commissioner Johanna S. Norris said. Of the bicounty task force, she said, "We have finally gotten off the bureaucreatic merry-go-round."