Eighty thousand tons of food in transit throughout the world have been rerouted hurriedly to Ethiopia in recent days to head off a new short-term famine caused by rapidly filling refugee camps and the slow delivery of larger aid shipments that are not due to arrive until late January, according to officials here.

Among the emergency diversions is a ship that was carrying 10,000 tons of U.S. government grain bound for India. Members of the House Select Committee on Hunger, at the end of a four-day visit to study the famine problem, announced here today that diversion of the ship came after they had sent cables to President Reagan and other U.S. officials explaining the "very severe shortfall of food in December."

Most of the food warehouses at Assab, Ethiopia's major port, are empty. There is only a three-day supply of food left at the Korem feeding camp, home to about 31,000 famine refugees. The British Royal Air Force this week ran out of food to airlift to mountain camps and ended its thrice daily flights to Assab.

With demand for food increasing rapidly at overcrowded feeding centers in north-central Ethiopia, and with almost no scheduled shipments of donated food arriving until late January, the Ethiopian government and private relief agencies became alarmed in recent days that they were approaching a point where they would be unable to keep famine victims from starving in feeding camps.

Unless large shipments of food arrive in Ethiopia by early December, Catholic Relief Services' burgeoning feeding program will cease to grow and will be forced to maintain "starvation-level food allotments" for the 700,000 people to whom it is now supplying food, according to spokesman John Swenson.

Swenson said his organization alone now is distributing about 4,700 tons of donated food a month. It will need 7,000 tons in December and as much as 14,000 tons a month after the first of the year.

The reason for the increases, he said, is requests by volunteer agencies to open 19 to 20 new feeding camps. Swenson also said that Catholic Relief Services has decided to try to increase the daily food allotment from a starvation level of 800 calories a day to a subsistence level of 1,400 calories per day.

Despite the increased food shipments, many questions remain about how much food is enough to rescue the more than 7 million people who the Ethiopian government estimates are in urgent need of food aid.

The government is asking for 1.2 million tons of food next year to prevent what it says could be the death by starvation of tens, perhaps hundreds of thousands of people.

So far about one-fourth of that total -- 325,000 tons -- has been pledged by private and public organizations and by nations around the world, according to Kurt Jansson, assistant secretary for U.N. emergency operations in Africa. The United States is by far the largest donor, having pledged 210,000 tons of food.

Some relief specialists here question whether Ethiopia can efficiently deliver all the food it is asking for in the next year.

Strangely, part of the reason for the December shortfall is that ship deliveries of food were staggered to allow for a backlog at the Assab port. But now there is no backlog.

The Ethiopian government, according to relief experts and members of the congressional delegation, has increased markedly the efficiency of its food delivery system. Swenson of Catholic Relief Services called the Ethiopian food bureaucracy "as good as any in Africa."

The Assab port's capacity has jumped in a few weeks from 400 to 5,000 tons a day. In part because of expected backups at the port, now nearly idle, no major food shipments are expected until the diverted American food arrives around Dec. 11.

Even with that U.S. food, there is a disagreement about whether a December-January food gap will be bridged.

At the press conference this morning, Rep. Margaret S. Roukema (R-N.J.), a member of the committee on hunger, said that with the diversion of the U.S. grain, Ethiopia's minimum requirements will be met.

But shortly after she and other members of the committee boarded a Washington-bound U.S. government jet, Ethiopia's top official for famine relief said the short-term food gap has not been filled.

"It will mean serious malnutrition," said Dawit Wolde Giorgis, minister of the Relief and Rehabilitation Commission.

No matter how much food arrives in Ethiopia, the director of the U.N. Children's Fund (UNICEF) said here today, the famine will continue to take a particularly brutal toll on children.

James P. Grant said that 75 to 80 percent of deaths in Ethiopian feeding camps were among children under four.

"Many others have died on their way to the camps," he said.

Grant, a long time specialist in disaster relief, said the famine crisis in Ethiopia -- particularly for children -- was more severe and larger in scale than any he had seen.

He said conditions are considerably worse in Ethiopia than in the camps that sprang up in 1980 along the Thai-Cambodian border.

Grant had just returned from a tour of two new feeding camps in Tigray Province, an area that has been particularly hard hit by Ethiopia's four-year drought.

He said five to 15 small children were dying daily at Mile Camp, which feeds about 2,800 people. He said that on the day before he visited Bati, a nearby camp, 86 young children had died.

"Whenever they are at a camp, children are getting some food. The real problem is diarrhea, which is epidemic," Grant said.

Diarrhea can be treated in the camps, he said, but for children to recover they need double or triple the subsistence amount of food, 500 grams a day, that is normally allotted them in the camps.

Even in normal years in Ethiopia, UNICEF estimates that 1,000 small children die every day of starvation or diseases relating to the country's chronic poverty.