The United States and the Soviet Union, along with other members of the governing board of the International Atomic Energy Agency, have agreed to draft a legal convention next month that would require signatory nations to report immediately on nuclear accidents that could send radiation over national borders, government officials said.

The convention, which would have the standing of a treaty, is one of several steps being taken through IAEA as a result of the Soviet accident last April at the Chernobyl nuclear power reactor and Moscow's failure to inform other countries of the incident for three days.

The notification convention being proposed is similar to one offered at the IAEA in 1981 by the United States after the reactor accident at Three Mile Island. But at that time, many European nations, including France and the Soviet Union, opposed the idea of formalizing notification in a legal convention.

"There has been a 180-degree change," one U.S. official said yesterday. He also noted that without mentioning their own delay on Chernobyl, "the Soviets have pushed this notification convention as if it were their idea."

He said the U.S. goal is to have disclosure take place "within hours" of a nuclear incident and include what types of radioactive particles were released, approximately how much material was released, the time of the release and which way the wind was blowing.

One stumbling bock, the official noted, was trying to define exactly what level of radiation release would trigger the notification requirement. Another, he said, was the question of whether military as well as civilian nuclear facilities would be covered.

In a related matter, U.S. officials at a Senate Foreign Relations subcommittee hearing yesterday painted a somewhat different picture of the Chernobyl accident than had been offered while it was taking place and fewer facts were known.

For example, the agricultural impact of Chernobyl apparently will be far below estimates made by U.S. farm experts immediately after announcement of the accident.

Cautioning that "what we don't know far exceeds what we do know," Robert L. Thompson, assistant secretary of agriculture, said that "adjustments to the accident indicate that major Soviet farm output, consumption and trade will be minimally affected."

He estimated that based on Soviet announcements only farmland within a radius of 18 miles from Chernobyl had been taken out of production, meaning "less than 0.1 percent of Soviet agricultural production would be lost."

Another indicator, Thompson said, was that Soviet "agricultural imports have not risen above levels anticipated before the accident."

On the other hand, farm produce screening for radiation "extended far from the immediate 18-mile zone," Thompson said.