Reversing himself and all his predecessors since the 1940s, Interior Secretary Cecil D. Andrus now wants to exempt about one million acres of rich California farm land from federal irrigation law.

The area is the Kings River water district in the Central Valley, where powerful landowners have tried for years -- without success -- to win congressional and court exemptions from the reclamation law of 1902, which limits the number of federally irrigated acres any one farmer can own.

Andrus' change comes at a crucial time. As Congress prepares for its final week, new efforts will be made to adopt a reclamation law revision applying acreage limitations to Kings River and other districts.

As recently as June, Andrus was saying such an exemption would, in effect, destroy the reclamation program that has turned western deserts into agricultural gold mines. Subsidized federal water irrigates 11 million acres in 17 states.

But last week, in a move that caught other Interior Department officials by surprise, Andrus changed his mind and said the law should not cover the Kings River district.

Andrus said yesterday he changed his mind after visiting the area in early summer, following a suggestion by House Interior Chairman Morris K. Udall (D-Ariz.), who had visite there previously.

Seeing the area, he said, he decided that federal denial of water to large so-called excess landholders would also curtail service to about 46,000 smaller farmers who could not operate without the water.

"Mo Udall asked for my opinion and I held off answering until last week because I didn't feel the bill was going to move," Andrus said. "It was a judgment call and I have no apologies to make. There's nothing clandestine about it."

The secretary's flip-flop, if converted to law or federal policy, would free Kings River from the present law's 160-acre limitation or any of the variations now being discussed by Congress.

Using the cheap, federally supplied water, a number of corporations there have put together holdings far in excess of the acreage limitations. Enforcement of the limits would compel them to dispose of excess lands or stop using the subsidized water.

Among the major excess landholders in Kings River are J. G. Boswell Co., the world's largest Cotton grower (88,212 acres); Salyer Land Co. (29,000 acres); Southlake Farms (26,816) and Westlake Farms (19,817); Chevron USA (13,014) and Getty Oil Co. (4,136).

Andrus' position change now gives the California farmers, who blitzed Congress this year and last with a high-pressure lobbying campaign, a new argument and patron in their fight for special treatment -- and the continued federal water subsidy.

Kings River lobbyists have included John L. Harmer, a former lieutenant governor to Ronald Reagan in California whose name has been mentioned for secretary of Interior; James Lake, a former Reagan campaign aide who now works with the president-elect's Agriculture transition team, and Donald Santarelli, a Nixon-Ford Justice Department official now on the Reagan transition team for Justice.

In addition to their intense lobbying, the growers contributed heavily to congressional campaigns and the Democratic National Committee. The Boswell political action committee, for example, had given $116,000 to candidates through Oct. 15, much of it to supporters of the Kings River exemption and to opponents of incumbents who were against the exemption.

To cite a case, Boswell, which grows no cotton in Wisconsin, spent $10,000 against two incumbents who supported tightening of the 1902 reclamation law -- without the exemption. Boswell gave $5,000 to Robert W. Kasten Jr. (R), who defeated Sen. Gaylord Nelson (D), and $5,000 to James A. Wright (R), who lost to Rep. Robert W. Kastenmeier (D).

Andrus alluded to such pressures in a speech in Berkeley, Calif., last May, where he talked about pressure groups' "incremental erosion of the purposes" of the 1902 law. He said they bring "expensive litigation to stalemate an issue" or they resort to "power politics to cow legislators, bureaucrats or policy makers into decisions which allow abuse to continue and sometimes even endorse it."

In one respect, Andrus' change of position may be academic. A Senate-passed bill and a companion measure adopted by the House Interior Committee would exempt Kings River from the acreage limitations.

But in repeatedly opposing the exemptions in the past, Andrus had said he would not hesitate to recommend a presidential veto of a bill that did not meet his standards.

Rep. George Miller (D-Calif.), leader of opposition to exemptions for the big farms in Kings River and elsewhere, was furious over the Andrus reversal, saying it undercuts legislators who have stood steadfastly with the administration.

The crux of the Kings River dispute is whether the 1902 reclamation law should apply, since the dams providing water there were built by the Army Corps of Engineers and not the old Bureau of Reclamation.

Andrus and his predecessors at Interior have consistently taken the position that reclamation law should apply to those districts where irrigation water comes through a corps project. The 9th U.S. Circuit Court of Appeals already has upheld that position.

Both the Senate and the House committee bills exempt those corps projects form coverage. Intense controversy over that and other issues has kept the bill off the House floor, but Chairman Udall will try next week to call up a scaledown compromise for a vote.

A Udall aide said the compromise approach entails dropping the corps project exemption issue from any consideration, leaving it ot be resolved by the next Congress or the incoming Reagan administration.

"I don't think the bill is going anywhere," Miller said."It has serious problems, but Udall's negotiations suggest a way to pass it is to take out Kings River and deal with the rest of it. . . . Here we are at the eleventh hour, and these people get a stay of execution. It's incredible."