Residents of suburban Maryland have been so co-operative in conserving water that their utility bills will have to be increased to offset falling revenues, the Washington Suburban Sanitary Commission says.

What is happening in suburban Maryland was foreshadowed during last year's drought in California, when residents of the San Francisco area became so zealous about rationing that some utility officials, in desperation, pleaded for a little less conservation.

The WSSC, promoter of "Camel Week" and other conservation efforts, is not about to tell its customers to waste water. But it wants them to pay more money for less.

The WSSC, which provides water and sewer service for Montgomery and Prince George's counties, has asked for a 22.8 percent increase in water charges for fiscal 1979 - and an average 15 percent increase each year thereafter through 1984.

The current rates, which went into effect Jan. 1, reward conservation. "The rates probably are working too well," WSSC general manager Robert McGarry said.

But according to McGarry, customers apparently began major conservation efforts well before the rates went into effect. "People obviously were reading about the drought out West last year and the problems at the Occoquan Reservoir (in Northern Virginia), and we had our Camel Week."

The WSSC had estimated that the conservation - geared rates would slow the rise in water range. But instead usage has actually declined, despite residential, commercial-and-industrial growth.

In its original budget report for fiscal 1979, the WSSC has estimated that daily water use in its service area during fiscal 1978 would average 139 million gallons. But the agency has revised that figure downward to 134 million gallons daily. It has also lowered its estimate to usage in fiscal 1979, which starts July 1, from 142 million gallons to 134 million gallons a day.

The WSSC forsees conservation making such a big impact that, according to its revised projections, usage in 1980-138 million gallons daily-should only be slightly higher than it was in 1975 - 135.5 million gallons daily - when there was a much smaller population and less commercial and industrial demand.

While conservations means higher rates in short term. McGarry said "there will be a payback in the long run." Less demand, he explained, "allows us to get by longer without building new facilities and bond debt begins to dwindle off."

McGarry said present debt payments on both water and sewer projects built with bond issues amount to $52 million a year. Based on current interest, every dollar borrowed in the bond market must be paid back with two.

Even before the impact of conservation became known, the Bi-County Water Supply Task Force, which includes officials of the WSSC as well as of the county governments, was proposing a "drought management program" that would minimize the need for expansion of the present facitities or building new ones.

Montogomery County Council members John L. Menke, cochairman of the bicounty study, has said that suburban Maryland could avoid expenditure of up to $100 million if residents could avoid watering lawns during severe droughts that occur every 25 to 100 years. Otherwise, he said, the money would have to spent on building new storage capacity.

But even if construction costs are held down, the WSSC, its budget figures show, will continue to need more money from its customers, especially when they practice conservation.The agency says lower consumption does not automatically lower costs.

The budget shows that operating costs will rise by more than 30 percent during the coming fiscal year, despite conservation efforts. Costs will go up the agency says, largely because of inflation and the need for more repairs onthe utility lines.