The Soviet Union is adopting radically new measures to revamp its economy, and the old capitalist tool of financial rewards is winning an unprecedented, albeit modest, victory over the egalitarian principles of communism.

New pay scales are now going into effect permitting wages for outstanding workers to be 50 percent over the average. Those who invent techniques to improve production will be granted patent royalties that could make a number of them millionaires.

A discreet effort is under way to deal with the perennial problem of underemployment. Managers are encouraged to reduce the number of employes and increase productivity.

One such effort was reported in Pravda, the Communist Party newspaper. It said the Kirov foundry in Teraspol, Soviet Moldavia, introduced new technology and "conditionally freed" 530 workers from their jobs.

The weekly Economic Gazette said the drive to "numerically reduce the size of the work force" in various firms is "a new measure" that would require coordination between central and regional authorities.

It is not known what happened to those "conditionally freed" from work -- presumably a euphemism for ousting superfluous workers or moving them to areas experiencing labor shortages.

This is a very sensitive political issue in a country that prides itself on a constitutional guarantee of a job for everyone. So is the issue of wide differences in salaries.

According to the Literary Gazette, an engineer making between 170 to 200 rubles a month would be eligible to get up to 300 rubles monthly -- an instant 50 percent raise -- if his work is regarded as important for production.

This is a revolutionary change apparently designed to encourage initiative and performance. Thus far, salaries have been determined by length of service and steady promotions in a fashion akin to U.S. civil service procedures.

The new reforms particularly encourage financial rewards for those who save energy. Factory managers have been authorized to distribute part of the savings in bonuses.

In a move toward economic decentralization, the government has granted directors of enterprises the authority to grant pay raises and, in consultation with union officials, to decide who should get them.

There is speculation that the crisis in Poland as well as generally bleak economic prospects for the 1980s may have prompted the Soviet leadership to drop its stubborn resistance to financial incentives and embark on a radically new course.

Yet Soviet economic experts, in numerous articles that have been appearing during the past few weeks, say the need for changes was caused by a dramatic slowdown in the productivity growth rate since 1978 and the prospect "in the immediate future" of an even more dramatic decrease in the labor force.

According to Abel Aganbegyan, an economist and member of the Soviet Academy of Sciences, the annual increase in productivity averaged 6.8 percent during the years 1970 to 1975, then dropped to 3.4 percent from 1975 to 1980, and is currently running at 2.5 percent.

"It is clear that we have to increase labor productivity sharply," he wrote in the newspaper Trud. "There is no alternative to this because we have entered a new phase in development."

In this phase, he continued, "the influx of new workers into the labor force will decrease by 75 percent in the most immediate future." At the same time, it has become "more costly and more difficult" to reach natural and mineral resources, which are being rapidly depleted in European Russia and are now available mainly in Siberia.

The demographic problem is particularly acute. The Soviet Union's rate of population growth has been falling since 1960, when it was more than twice the current level of .84 percent annually. The burgeoning Moslem population of Soviet Central Asia, which amounts to 42 million of the country's total of 262 million, is now calculated by Soviet specialists to make up half the growth projected between 1970 and 2000.

This means that the growth rate of the Soviets' European population is almost flat, since the Central Asians shun migration and tend to remain where they grow up.

"The entire economic mechanism must be restructured," one Soviet official said privately. "It simply cannot operate the way it operated 10 or 15 years ago."

The Soviets attempted to introduce economic reforms in the mid-1960s, when the late premier Alexei Kosygin came forward with more modest proposals. His reforms failed to gain wide support and ultimately were abandoned.

This time, the government insists that it is determined to see the changes adopted, however. A ranking official briefing Soviet journalists this week described the scope and importance of the reforms as equal to the period of economic reconstruction that followed the end of World War II.

The decision to make radical changes in the economy was taken at the last Communist Party congress in February of this year. It is being implemented gradually and cautiously. Some officials indicate privately that they expect resistance to the changes that involve the use of financial incentives.

One indication of official caution can be seen in the fact that the changes are not billed as reforms in the press, but rather as a new and inevitable change in the country's economic "development."

While the scope of the reforms is as yet unclear, the Soviet media has embarked on an intensive campaign to prepare the public for the planned changes. They have been citing several ludicrous examples of inefficiency.

Take a story about Moscow's Gorky Park. The 12 swans in an artificial lake near a Ferris wheel there have suddenly become the focus of a public argument. It turns out that the 12 swans are being cared for by seven full-time employes at an annual cost of $18,500.

The Ministry of Geology at the other end of Moscow was reported to have an "experimental art division" with 60 employes. Two Soviet reports on the division said that it had produced a photo album entitled "Creativity of Children in the Nursery of the Soviet Ministry of Geology." This presumably took up most of their time this year.

Yet another instance reported by the newspaper Trud involved the management of a coal mine in Donetsk. The managers decided to reduce the size of its accounting department by buying a computer for $230,000. As a result of this decision, the number of employes in the accounting department has increased by nine.

In the industrial city of Sverdlovsk in the Urals, the manager of a plastics firm reportedly strove to "dream up" work for 120 employes whose machines have been standing idle since January. The Japanese-made equipment, which was bought for $7 million, cannot be used because there are no raw materials.

Similar examples of flagrant waste or bungling provide a glimpse of the inner workings of the Soviet economy.

One report revealed how the Ministry of Oil knowingly allowed $54 million of imported chemicals and machinery to rust away. Another revealed an incident that sounded like a joke: one plant decided to dismantle an obsolete division and created a permanent commission to do such jobs. The commission ended up employing more people than the division that was dismantled.

Yet another press account revealed, for example, that there are a "large number of engineers" working as street cleaners in Moscow. This was partly understandable, since many people do not want to leave the capital and would rather stay here doing odd jobs than go to the provinces in their professional capacity.

But then there is an engineering firm in Vorkuta, in the Far East, where 132 of its 180 white collar workers were doing manual labor for reasons that could not be adequately explained. "Engineers with a broom," commented the newspaper Socialist Industry. "What is to be done when there are not enough street cleaners?"

The proliferation of such dramatic reports is seen by some skeptical observers as being somewhat misleading. Statistically, the Soviet economy is second in size and strength only to the United States'. The Soviets produce more steel than the United States. The Soviets are the world's leader in hydroelectric power, oil production, coal and railroad transport.

Soviet spokesmen, however, now maintain that there are limits to growth and that after decades of emphasizing a high rate of capital investment this country must now concentrate on getting the most out of existing plants and equipment. Moreover, they say, mineral resources and raw materials are being depleted in the European part of Russia. There is still enormous mineral wealth in Siberia, but it is far more costly to reach.

All of this seems valid enough. But this country is also in a period of transition. The economy has grown enormously, but has become more unwieldy. The society is caught up in a generational change. The urbanization process has been precipitous -- in 1960 roughly 60 percent of the population lived in rural areas, compared to roughly 20 percent today.

To top it all, a falling birth rate now threatens a decline of up to 75 percent in the number of young workers joining the labor force, according to official estimates.

"Yes, we are in a transition," said one Soviet official. "We are still a production-oriented society. But for the first time we realize -- not on paper, not merely in words, but in real terms -- what our problems are.

"We don't want to destroy the old system while trying to put into effect a new one. But we are going to carry out these reforms. We are not heroes or idiots, or people who are immune to pain and suffering. We want to join the 20th century."