The United States will send up to $75 million in disaster aid to shore up three Andean democracies stricken by drought and floods, top officials of the Agency for International Development said yesterday.

AID requested the money for Peru, Ecuador and Bolivia several months ago and was turned down by the White House. But the agency then worked with Congress on the aid and this summer received a major new piece of management power: to take the cash out of "failing programs" and put it into the South American emergency, according to AID officials.

In the past, when "bad programs" of foreign aid lost their funding, the cash had to be returned to the Treasury. But under new authority granted by Congress, AID, beginning with the South American emergency money, can keep "de-obligated" funds for re-use rather than return them to the Treasury.

In a White House ceremony last year, AID Administrator M. Peter McPherson presented President Reagan with a several-foot-long copy of a check for $28 million from de-obligated programs he was required to send back to the Treasury.

In the South American emergency, money is being taken from several nations' programs. But in future actions, McPherson said, money can be moved only from one program to another within the same nation.

Still, McPherson called this new power to shift money rather than give it back "the most major management reform undertaken at AID as long as I have been here." He has been AID administrator for 2 1/2 years.

The new power came in a clause of the 1983 State Department supplemental appropriations bill, passed by Congress and signed by the president two weeks ago. The power to reprogram money will have to be reviewed annually by Congress.

The aid for the three Andean countries is needed because the region is suffering its most catastrophic floods and drought in about 40 years.

McPherson said that, without the international aid, the region's situation would resemble the 19th-century potato famine in Ireland.

Two-thirds of the potatoes, Bolivia's staple crop, have been wiped out.

"In an area the size of Texas, the drought has reduced the potatoes to the size of marbles . . . . Inside the potatoes the worms are as big as the whole marble of potato," he said.

Areas of Peru that are near-desert, on the other hand, have been deluged this year with 12 to 14 feet of rain. New lakes have appeared, one of which is several miles long and has severed the Pan American Highway.

AID already has given $52.6 million in food to the three countries, but the disaster has wiped out a significant portion of the nations' entire economic output.

The final total of aid in the new package has not been settled because it depends on shutting down other aid programs, a sensitive matter.

McPherson said additional aid was important for economic and political reasons as well as for humanitarian ones. In encouraging South American democracies, it is important to shore up economies that were already weak and heavily in debt before the weather struck, he said.

One AID official, after returning from a trip to the region recently, said that the three democracies "are under siege, not from Marxist guerrillas, but rather from the forces of nature which continue to punish and undercut support for the democratically elected governments."

Because this is the dry season in South America, the drought has not yet broken in Peru and Bolivia. But in the flood areas, rain is now back to near-normal levels.

At the same time that the new South American aid is being readied, the United States is delivering emergency aid to African countries suffering the effects of a drought that equaled the worst in this century.

Though the period of far-below-normal rainfall has ended, southern Africa's annual dry season is in progress, and water rationing and severe food shortages continue.

The United States has given about $6 million in aid to Ethiopia, but has been criticized for reacting slowly to the crisis because Ethiopia receives substantial Soviet aid.

McPherson said that the disasters have forced the United States during the past few weeks to "crystallize" or "formalize" U.S. policy on the question of aid to unfriendly nations.

"It has always been the policy of the United States to feed kids that are starving, without regard to the politics of their government," he said, and this year the policy was discussed and affirmed explicitly.

The United States gave food aid to Ethiopia, but withheld $700,000 worth of trucks requested to ship the food for fear they would be commandeered by the Ethiopian government for military purposes.

Instead, the United States gave cash to the Catholic Relief Service to rent trucks. AID also persuaded the U.N. Disaster Relief Organization to send workers into Ethiopia to supervise the distribution of food directly. graphics/photo: M. PETER McPHERSON ...U.S. had to "crystallize" aid policy