The Arlington County Board called yesterday for Metro to depend far less on government subsidies for its income and far more on fares, even if that means again raising fares. Fares already are set to rise this weekend, the third time in 17 months.

The county board formally proposed that fares and other revenues cover about 75 percent of the bus and rail system's operating costs, with government subsidies making up the remainder . The current ratio is about 50-50.

Metro Board Chairman Joseph Alexander yesterday dismissed the Arlington proposal as unrealistic, saying that in covering Metro's $300 million-plus annual operating budget, "we feel fairly fortunate to get 50 percent" from revenues.

Arlington's suggestion comes as transit planners in the eight area governments that make up Metro are working out cost and ridership projections to be used at the Airlie conference center near Warrenton, Va., this month in debating a five-year "strategic plan" for the transit system. Arlington's proposal was made in preparation for that conference.

Many Metro officials worry that current trends of rising fares and falling ridership could bring about a destructive spiral toward bankruptcy, as has happened to transit systems in other big American cities. The Metro board has split sharply along jurisdictional lines in devising a solution.

The District of Columbia, with a Democratic-controlled city council and comparatively large numbers of low-income residents, generally has been willing to shoulder high subsidies. The more affluent Arlington, and the other four Virginia jurisdictions, tend to argue that riders should pay. Metro's two Maryland county members have stood somewhere in between.

If current patterns continue, Metro staff projections show, the local governments' subsidy bill will rise from $149 million this year to $344 million in fiscal 1987, as costs rise and federal operating subsidies are phased out by the Reagan administration.

Arlington's proposal, contained in a letter to Metro chairman Alexander, said the 75-25 ratio could be achieved by cutting costs, raising ridership and raising fares. Costs could be cut by eliminating binding arbitration from Metro union contracts, which many local legislators say results in excessively high wages and benefits.

Arlington board member Dorothy Grotos, also a member of the Metro board, said surveys had shown that many Metro riders were financially able to pay higher fares. If local governments wanted to help out low-income riders, she said, they could provide cut-rate tickets to individuals rather than subsidizing the system as a whole.

Arlington also wants subsidy arrangements for the rail system to be revised to give individual governments more control over the setting of fares and subsidy levels for their own riders. Metrobus is administered this way but rail finances are divided up throughout the region, giving local governments little control.

Metro officials acknowledge that labor costs are high but say that federal requirements and contracts inherited from private bus firms they took over in the early 1970s make that difficult to change. Layoffs would bring poorer service, Alexander argued. "In order to cut a million dollars off, you've got to lay off 30 or 40 people," he said.

Alexander and others propose that instead of raising reliance on fare income, Metro should seek creation of a regional tax dedicated to transit to cover the deficits, as other cities have done. The tax might be on sales, or gasoline or payrolls.

The idea has been circulating for years without success. The contrasting financial solvency and political objectives of the eight governments in Metro had made some transit officials skeptical that agreement could be reached to create such a tax.