President Reagan planned a horseback ride this morning in the warm sun of his California ranch. But by nightfall, two international crises forced him to decide abruptly to cut short his extended vacation and return to Washington on Friday.

In reaction to the shooting down of a Korean Air Lines passenger airliner by a Soviet fighter plane and the hostilities that have involved U.S. Marines in Lebanon, Reagan scrapped his plans to remain at his ranch, 30 miles from here, until Labor Day.

Soon after he awoke at Rancho del Cielo today, the president was given a written transcript of monitored radio conversations between a Soviet fighter pilot and his military ground control during the interception of the Korean Air Lines plane that had strayed far into Soviet air space.

The transcript, which included precise times each contact took place, indicated that the Soviet pilot had checked with authorities on the ground before arming and firing a missile at the jetliner, according to an official who has seen it.

The transcript, officials said, came from U.S. intelligence sources that monitored the radio contacts Wednesday night. The pilots' comments had been translated from Russian to Japanese and then to English, according to presidential spokesman Larry Speakes.

At first, the White House viewed the incident as not sufficiently critical to bring the president back to Washington. Speakes told reporters in the morning that Reagan would not cut short his vacation. He said the president had "every facility, every capacity, every capability" to deal with the crisis from the five-room house on his ranch.

Soviet failure to acknowledge and explain the shooting down of the airliner and Reagan's desire to meet with congressional leaders on this and the Lebanon crisis prompted him to change his mind, officials said.

Another factor in Reagan's decision, they said, was the need to demonstrate his personal involvement in responding to both crises. At the ranch, Reagan is secluded from public view and talks with aides, cabinet members and others only by telephone.

Reagan plans to fly back to Washington Friday and meet immediately with the National Security Council. On Sunday, he will confer with the bipartisan congressional leadership. Both meetings are to focus on the hostilities in Lebanon as well as the Korean Air Lines incident, Speakes said.

In a statement read to reporters late today, Reagan denounced the Soviet action and said Moscow had "totally failed" to explain it. "Words can scarcely express our revulsion at this horrifying act of violence," said Reagan, who ordered flags flown at half-staff and promised to "make every effort to get to the bottom of this tragedy."

On Wednesday night, the president was called twice by National Security Adviser William P. Clark, who had preliminary information about the incident. But aides did not inform the president that the jetliner had been shot down until today at 7:10 a.m., California time, officials said, because they were struggling to analyze conflicting data through the night.

Speakes said Reagan was first notified at 7:30 p.m. Wednesday, California time, that the plane was in trouble. He said early reports were sketchy. "It was regarded as a missing aircraft for a long period of time," he said.

Shooting the plane down "seemed such an incredible incident that we were very careful in our reporting and were rechecking again and again to make sure we were not misstating anything," Speakes said.

When Reagan was called the second time Wednesday night by Clark, who was at the Biltmore Hotel here, the White House had obtained "harder information" based on intelligence reports.

At this point, Speakes said, the administration had some indication that Soviet radar tracking was extensively involved in the incident. But he said it wasn't until midnight California time that the White House learned the plane was down. "At midnight, we still did not have details," Speakes said, but "we knew it had disappeared" from radar.

Reagan was not contacted again, however, until this morning. Speakes said officials had determined through translation of the radio contacts what had happened.

This morning, Reagan was called at the ranch by White House counselor Edwin Meese III, who is also staying here, and was told that it had been established that the plane had been shot down. Reagan was also given copies of the Soviet fighter pilots' conversations, according to one official.

Reagan, who has made 16 trips to his ranch since taking office, has never before cut short his visit here for such a crisis.

In 1981, White House officials decided not to wake Reagan one night when U.S. fighter planes shot down two Libyan jets. Ever since then, his aides have been sensitive to any suggestion that Reagan is not in touch with late-breaking world events.

Still, Reagan's preference to delegate authority to subordinates remained evident today. Secretary of State George P. Shultz delivered a nationally televised statement on the shooting down of the South Korean airliner before talking with Reagan.

Shultz had conferred instead with Clark. Only after Shultz had finished with reporters at the State Department--and was pointedly asked by them if he had talked to Reagan--did the president call to discuss the matter with him.