The United States will fall behind the Soviet Union in the overall power of its strategic nuclear forces by 1982 regardless of anything American military planers do in the meantime, Gen. Bernard W. Rogers, the Army chief of staff, said yesterday.

Rogers, who leaves the Pentagon next week to become military commander of the North Atlantic Treaty Organization, said that by 1982 the United States and its allies also will trail the Soviet-led Warsaw Pact in conventional arms, short-range nuclear weapons and manpower.

"There is a very critical window that opens in the early 1980s, Rogers said at a breakfast meeting with reporters.

"The country has not fielded the modern tanks, infantry-fighting vehicles, air defenses and other equipment that we would need [in the event of a war with the Soviet Union]," he said. He added that the Army would fall more than 250,000 soldiers short of the reserve manpower needed to replace combat casualties during the critical period before new recruits could be trained.

Rogers' assessment was far gloomier than the analysis of the civillian Pentagon leaders who believe the essential equivalence of forces can be maintained in spite of growing Russian strength.

In keeping with the Joint Chiefs of Staff's self-imposed policy of refusing to comment on the merits of the Strategic arms limitation treaty (SALT II), Rogers would not say if the treaty should be approved or rejected by the Senate. He said, however, that the treaty would permit the United States to restore nuclear parity with the Soviets if the U.S. government were willing to put up the money. But he said the catch-up process could not be completed before late in the 1980s.

Rogers softened his analysis by predicting that in the event of war, the European members of NATO would be more valuable allies to the United States than such Warsaw Pact members as Poland, Romania and Hungary would be to the Soviet Union.

Rogers, 57, a former Rhodes scholar, battalion commander in Korea and division commander in Vietnam, was named Army chief of staff in 1976 by Gerald R. Ford, then president. He will succeed Gen. Alexander M. Haig in the NATO post.

Rogers said the United States will lose equivalence with the Soviets in strategic forces when Russian rockets become accurate enough to knock out U.S. intercontinental ballistic missiles in their silos. The Pentagon expects the Soviets to have this capability by 1982.

Secretary of Defense Harold Brown agrees that U.S. land-based missiles will become vulnerable to surprise Soviet attack. But Brown argues that the Unites States would retain enough nuclear punch in submarines and bombers to wipe out much of the Soviet Union. Therefore, he contends, the United States will continue to be able to deter a Soviet attack.