suburbia got a good soaking early Monday, but unless there are many more heavy downpours soon, residents of Montgomery and Prince George's counties will have to keep their garden hoses rolled up this spring and possibly summer.

The Washington Suburban Sanitary Commission (WSSC), the bicounty water and sewer utility, says that unless the area gets 8 or 9 inches of rain during the next 7 1/2 weeks, it will impose mandatory restrictions on outdoor water use by April 1, a time when thousands of suburbanites are tending to their lawns and gardens and washing winter's dirt off their cars.

Despite Monday's moderately heavy rain -- almost three-quarters of an inch -- the outlook for badley needed precipitation is not good. In fact, it is bleak.

According to the National Weather Service's climate analysis center, suburban Maryland and the rest of the metropolitan area are likely to get below-normal precipitation during February. Even in normal times, February is the driest month of the year, recording an average of only about 2 1/2 inches of precipitation. March normally is a little wetter, averaging about 3 1/2 inches.

Since the dry spell began in August, only 9.19 inches of rain have fallen, less than half the average of just under 19 inches.

Meanwhile, the WSSC has proposed a 19 percent increase in water rates and a 9 percent boost in sewer rates for Montgomery and Prince George's customers. It would raise the average annual residential bill about $29, from $183 to $212.

Suburban Maryland has been able to escape restrictions that were imposed LAST WEEK ON 700,000 (northern Virginians because the WSSC has been able take pressure off its two steadily falling reservoirs by taking more water from the Potomac River.

The Fairfax County Water Authority, which serves most of Northern Virginia, does not have that flexibility because it cannot yet draw water from the Potomac (its river facility is still under construction).

The two Patuxent River reservoirs -- Duckett and Triadelphia -- still have about half their 12-billion-gallon capacity, while the Fairfax County Water Authority's 11-billion-gallon Occoquan Reservoir is three-quarters empty.

However, the WSSC's flexibility goes only so far. If the current drought is not broken by spring, the WSSC may be forced to draw even more water from the Potomac. If the river's resources are used too heavily, the WSSC and other utilities may find themselves rationed by late summer or early fall, when water demand is heaviest.

Under an agreement covering the Potomac's present and future users, when demand reaches 80 percent of the river's flow, withdrawals are to be strictly allocated. Already the river's flow is down to its lowest seasonal rate in 10 years. The normal January flow is 12 billion gallons daily. This January it was only 1.3 billion gallons daily.

Paradoxically, suburban Maryland's -- and the metropolitan area's -- long-range outlook is better than the short-range one.

In fact, one expert who has been deeply involved in regional water planning -- engineer Daniel P. Sheer -- says confidently, "The long-range problem is solved."

Sheer's optimism is based on acomputer analysis of reservoir storage, river flows and precipitation records and the construction of new water storage facilities in Maryland.

There are three main elements of the solution: Bloomington Dam on the North Branch of the Potomac, the proposed Little Seneca Lake in upper Montgomery and the Potomac intake-treatment plant that theFairfax County Water Authority is constructing.

The three elements would work this way: When the Potomac's flow falls dangerously low, it would be bolstered with about 110 million gallons daily from Little Seneca Lake. If the low flow persisted for more than a few days and there were no prospect of rain, up to 300 million gallons could be released daily from Bloomington Dam.

Seneca, which could be finished by 1986, would provide a quick fix -- water released from the lake would reach the nearby Potomac in less than a day. Water from the Bloomington impoundment, scheduled to be completed this summer, would take seven days to travel down the Potomac and reach the Washington area.

With Little Seneca and possibly Bloomington replenishing the Potomac during a period of drought, the WSSC, the Washington Aqueduct in the District and the Fairfax Water Authority all could continue to tap the river.

The only potential hang-up is a failure by suburban Maryland, the District and Northern Virginia to agree on a regional plan to pay for and then operate Little Seneca. There was a temporary falling out when Montgomery wanted to make its sewage problems part of the negotiations, but has dropped that demand. mTalks are back on track.