In theory it is called "trickle down," in practice "revenue sharing." The federal government, like a giant well-spring, each year shares its wealth with the country's garden of local governments, keeping them green and healthy.

In the past, Montgomery County's portion of federal irrigation has amounted to $8 million in revenue sharing funds. If President Carter's latest proposals for change in the program go through Congress intact, however, that trickle could diminish to something resembling a slow drip.

Under Carter's austerity budget for 1981, Montgomery County stands to lose $2.7 million of its revenue-sharing assistance -- a 33 percent cut that in dollar figures is the largest of any county in the country. County officials, still upset by proposed discontinuation of federal impact aid for education, believe a further cut in federal funds could eventually force taxes up, or local services down.

"We could go either way," says county spokesman Charlie Maier.

Carter proposes not only to trim the $6.8 billion program by $1.7 billion nationally, thereby totally eliminating funds for state governments, but to revise the formula that determines how much each local jurisdiction receives.

The formula, based on per capita income, leaves Montgomery County, one of the richest in the country, particularly vulnerable to a tightening of controls on the federal pipeline. Under the revised formula, local governments with per capita incomes greater than 115 percent of the state average would see their shares diverted to less wealthy areas.

The rate at which they would lose those funds is equal to the amount they exceed the state per capita income average.

In 1977, Montgomery County's per capita income was $11,055. The state average was $7,569.

County Executive Charles M. Gilchrist and Rep. Michael D. Barnes (D-8th District) recently met with members of the president's domestic policy staff to try and persuade them to keep the present formula intact.

Even if the county loses revenue sharing funds under the president's proposal, the loss would represent a miniscule portion of its total $646 million budget. The county may even be able to absorb the loss without breaking its fiscal stride.

What concerns county officials, said Maier, is the timing.

"The trouble is," he said, "the feds operate on a schedule slightly different from ours."

While Congress may not decide what to do with the revenue sharing program until Spetember, "we have to decide in the next month what our budget is going to be. We operate on what we know we're going to have in the fall and we have to keep our balances coming out right just as they do."

Maier said, however, the county will not act on the loss immediately because it will not become effective until the year is almost over. He said the initial blow will be softened by a "fairly good year on our investments."

Future reductions in services or a 4- or 5-cent tax hike are still possible, he said: "But as far as next year is concerned -- that remains to be seen."