A new age is rumbling in for the nation's public lands, replacing the era when Smokey the Bear symbolized protection of resources and Old Faithful meant parks in which nature was left undisturbed. If Interior Secretary James G. Watt has his way in Congress, the hardhat will soon be as much a part of the scene on public lands as the forest ranger's Stetson.

Watt's proposed budget all but closes down the programs that created and expanded the national parks over the last 16 years. This unprecedented public lands policy is Watt's first major move in office, one that reflects the administration's approach to energy.

Watt's proposal is based on the firm belief that public lands are there to be exploited in the national interest as well as admired. It assumes that the lands are a resource for development and jobs as well as for peace and recreation, and that wildlife, natural beauty and economic growth can coexist.

Parks, Watt argues, have taken enough land out of developmental use and are bedraggled from neglect, choked with tourists who often don't enjoy what they find. Watt wants to end the "park barrel" practice of the past: "We don't need a park in every congressional district," he said. "We need to look at what we have to make sure that it consists only of that which is truly unique and of national significance."

He wants to eliminate appropriations for the Land and Water Conservation Fund which has financed the purchase of millions of acres of parkland since it was set up as part of Lyndon Johnson's Great Society in 1965. He proposes cutting 1982 federal buying power from the $335 million President Carter proposed to a token $45 million to settle outstanding claims against the government.

In addition, Watt wants to cut $185 million from the 1982 program in grants to the states for recreation programs and urban parks, and has asked that $250 million in 1981 money be rescinded. Instead of buying more parkland, Watt has asked Congress to shift $105 million from the Land and Water Conservation Fund to the National Park Service for sprucing up existing parks.

"We took this action because the states, volunteer agencies and the private sector are able to do these jobs without direct federal assistance," Watt told a congressional hearing last Wednesday. "No decision I have made more clearly illustrates this administration's change of course."

What promises to be a long, bitter battle between conservationists and the administration has already been joined. The critics argue that the new approach is oriented, like the business executives who espouse it, toward short-term gains at the expense of the future. The environmentalist are digging in for the long haul, expanding their Washington offices and running major fund-raising and membership drives.

The budget debates under way in Congress are one battleground for this dispute. Bureaucratic decisions pending on future parks and wilderness designations are another.

Rep. Phillip Burton (D-Calif.) called Watt a "Neanderthal" for his parks policy and labeled the Reagan budget "a declaration of unconditional war on our great natural areas." The House Interior Committee, in the first taste of what will be a major fight, refused to go along with the plan. By 20 to 15 it recommended authorizing $282 million for federal purchases and $153 million for the states. The committee report said the members "feel strongly that the federal government will be acting irresponsibly if this backlog situation is allowed to continue."

The House subcommittee on Interior Department appropriations balked too, restoring $250 million Wednesday that Watt had tried to cut from the fund this year. The two actions go to the House Budget and Appropriations committees, and the issue ultimately will be decided by the House and Senate.

Watts' proposed cuts are without precedent.They would leave incomplete some of the nation's most famous parklands: Cape Cod National Seashore in Massachusetts; the Cape Hatteras National Seashore in North Carolina; California's Redwoods National Park, Golden Gate National Recreation Area and Santa Monica Mountains Recreation Area; the Appalachian Trail that runs the length of the eastern seaboard, and many more. It would eliminate the urban parks program that has contributed to Central Park in New York, Rock Creek Park in Washington, D.C., Grant Park in San Francisco and hundreds of vest-pocket parks in cities everywhere, "just when demand on local and state parks is unprecedented," said the National Recreation and Park Association in a recent statement.

Congress long ago pinpointed land it intends to buy for these parks and others, and the owners have what amounts to a cloud on their titles as a result. In fact, $1 billion in authorized purchases are in the backlog and a funding freeze leaves all the landowners in limbo, unable to sell their properties. Meanwhile, land prices go up so that buying the properties later will cost the government more.

Watt also plans no additions to the wilderness system, and instead will make sure that mining companies are allowed to exercise their lease rights to drill or mine in wilderness parks. In what may be a far-reaching decision, he recently declined to appeal part of a federal court verdict that allowed Rocky Mountain Oil and Gas Co. to begin major work at a lease site in potential wilderness where it had done no work before 1976.

In that year the Federal Land Policy Management Act slapped controls on all Bureau of Land Management territory, "grandfathering" existing leaseholders as exempt. But there has been much debate over what kind of work leaseholders can do on land that might eventually be declared wilderness. Interior Department attorneys had argued that the law intended to prevent permanently damaging activity even by grandfathered leaseholders, but Watt's decision drops that point.

The Bureau of Land Management, with Colorado cattle rancher Robert Burford in the saddle, is studying 24 million western acres for possible designation as wilderness, an area the size of Indiana. The formal wilderness designation would exclude any kind of development after 1984, but Watt told a House subcommittee he may ask Congress to extend that deadline.