When Fairfax County board chairman John F. Herrity talks about making do with what the county has, he's looking at $13.5 million less in U.S. funds than the county expected in fiscal 1982.

The county of nearly 600,000 residents will absorb a cut of $687,000 in its budget for social services programs. Herrity has been lobbying for more federal money for the county's refugees and for $4.4 million in impact aid to help with the costs of educating military dependents at Ft. Belvoir.

Some county constituents, like nursing home administrator Joan Bishop, say the federal cuts will affect the quality of care--and "the quality of life"--for groups such as the elderly.

"Without the staff and materials, we won't be able to offer the same activities," said Bishop. He said his Leewood Nursing Home in Annandale has 90 out of 132 residents who will be affected by Medicaid cuts. "We'll regress to an institution-type setting, with people just sitting around watching television."

Herrity is less pessimistic.

"I think we'll be able to work within the context of the budget cuts without an increase in taxes or impairment of services," he said. Instead, the county should be "squeezing the fat" out of a lot of programs it now runs.

"There's no way the county could continue the way it had been," Herrity said, adding that Fairfax residents understand there will be initial problems with Reagan's budget-cuts. "You don't turn a ship around on a dime."

Fairfax Supervisor Audrey Moore (D-Annandale) forcasts tough times ahead for the poor and middle-class and tough decisions for the board.

"Mental health and retardation services have expanded, but that expansion may be checked now," Moore said. The county's garden apartments are increasingly being converted into condominiums, "putting the squeeze on the poor and elderly, the people making less than $15,000 who can't afford to buy anything in this county." She expects the county soon will have to find a way to help them keep their homes.

The county's fast growth has created a demand for more services and strained the programs available now. She said Fairfax needs money for police, is behind in spending for fire fighting services and cannot cut school spending because, unlike other localities, enrollment is not dropping.

The county expects to spend $400,000 to $500,000 to keep day care services going and to supplement other federal funding losses for certain programs. Companion care and chore care programs for the elderly have tightened their eligibility requirements, so relatives will no longer be paid to care for their own.

"The idea," said Moore, is to "save the money for the people who honest-to-God need it, who can't get help any other way."

Moore takes little comfort in the notion that the localities may--at long last--be given more taxing powers by the state: "How marvelous! We can tax more. That's just great."

The county's $1.51 property tax rate already "sticks out" as the highest in Virginia, she said, and could go higher if Fairfax has to make up the cuts to fund essential services.