The Interior Department yesterday unveiled a sweeping plan that would require giant corporations, absentee investors and big growers in the West to sell more than half a million acres of rich, federally watered farmland back to small family farmers.

The plan, announced by Interior Secretary Cecil D. Andrus, could, if implemented, alter significantly the patterns of land ownership in Western farming.

The regulations will go into effect in 90 days unless public comments in that period persuade the government to alter them or they are blocked by court suits. Officials said they expect that the rules will be challenged strongly by landowning interests.

In the last 20 years, outside investors and agribusinesses have accumulated large holdings of profitable vegetable and fruit acreage irrigated by water projects fo the federal Bureau of Reclamation. These projects, which cost billions of federal dollars, coverted vast tracts of arid, marginal land in California and elsewhere into some of the nation's most valuable agricultural land.

But critics have charged in various lawsuits that the benefits have gone mainly to powerful business and financial interests.

Andrus said yesterday that Congress designed the water projects "primarily to place small farmers on the land . . . (and) these proposed regulations are an attempt to carry out that congressonal directive."

In 1902, Congress limited ownership of land receiving water from dam projects in the Rocky Mountains and Western rivers to 100 acres. Ranchers and farmers were required to sell any acreage in excess of that. However, the law was enforced erratically over the years, and today over 1 million of these acres have not been sold.

More than half of these acres receive water from federal projects, and the rest could, if landowners requested it.

The new rules are aimed at shifting this, land to family farmers. They would require that the land be divested to farmers who reside within 50 miles of it -- guaranteeing that absentee owners would be excluded.

Although the value of the irrigated land is more than $1,000 an acre, financing presumably would be available from commercial banks and federal loan facilities, since the productivity of the land has been demonstrated.

A number of large operators and growers in such choice farming areas as the central valleys of California have signed agreements with the federal government to divest themselves of the land after receiving the benefits of the water for 10 years. But the new rules would seek to prevent "insider deals" inwhich the land couldbe divested to corporations or friends instead of actual farmers.

All sales of the irrigated acreage would be approved by the Bureau of Reclamation in the Interior Department. When an owner with "excess" land sold the excess, buyers would be drawn from a list of eligible family farmers drawn up by the bureau. If there was more than one eligible buyer, the government would hold a lottery to select the purchaser.

In announcing the proposed rules yesterday, Andrus also indicated that the government is considering ways of establishing a residency requirement for owners of irrigated land not facing divestiture.

He said that residency is "believed to be a legally required oondition of receiving federally subsidizing water, and regulations spelling out how the residency requirement will be implemented across the board will be prepared as soon as practical."

Such a rule could have broad implications, since a substantial amount of land receiving federal water in such locations as California's Imperial Valley is owned by absentee investors or corporations.

Under the rules, farms utilizing federal water could be larger than 160 acres -- but not substantially larger. A farmer and his spouse could each own 160 acres, and they could each lease another 160 acres, for a total of 640 acres, or one square mile. Ownership of tracts of that size by sons and daughters could further increase the ownership.

The plan announced yesterday was spurred by a law suit filed in 1975 by National Land for People, Inc., representing small farmers in the Westlands Water District in California. Westlands is the most extensive and expensive irrigation effort the federal government has undertaken.

In 1976, a federal district court in Washington ordered the Bureau of Reclamation to clarify its rules for distributing land receiving subsidized federal water.

The rules could affect substantial amounts of acreage from which the nation gets cotton, rice, barley alfalfa seed and vegetables. Westlands produced an estimated $360 million in agricultural produce last year.

Water has been the key to agrucultural development all over the West since the beginning of this century. The agricultural boom in California has, from the start, been predicated upon water channeled to the rich central valleys from distant rivers and dams.

Before the water was available, much of California was used mainly for growing grain and pasturing cattle. Butwhen wate began water began irrigating the fertile valleys, the structure of agriculture, and land ownership was radically altered.

As land values soared so did the investments of capital from coroprations and outside financiers.

Hailing the decision yesterday,David Weiman, an outspoken Washington advocate of land reform and the family farm, called the step "extremely significant."

Weiman said that some land in the Westlands district has attracted enormous outside investment - including some from corporations based in the Netherlands Antiles a well-known tax haven for multinational corporations.