Tom Watson was 13 under par starting today's third round of the $200,000 Tucson Open. That would be enough to win many tournaments on the professional golf tour. But here in the Arizona desert, when par cracks as readily as the mud in the sunbaked stream beds, Watson knew his position was far from safe.

I'll have to pick up four or five more strokes, in these next two days," the 1977 PGA player of the year said as he left the putting green. "I was four under at the turn here one winter, playing with Johnny (Miller), and was one stroke back."

Eleven players were within four to eight shots of Watson when play began in today's 70-degree weather.

Bobby Wadkins, the younger brother of 1977 PGA and World Series winner Lanny Wadkins, began his challenge with birdies on the third, fourth and fifth holes. He birdied five holes in a row Friday, starting with No. 2.

Eight to 10-foot birdie putts got Wadkins going today. He and Watson both bogeyed the ninth - the course's first dogleg, with an elevated green - as Watson made the turn with his lead halved but still two strokes in front.

Wadkins closed to one stroke with a birdie on the par five 11th, only to take a bogey four on the 12th, which Watson parred.

The day's only other change was made by Jerry McGee, who birdied 11, 12, 13 and 14 to go nine under before giving back two shots with bogeys. McGee had started the round back in the pack, at three under, 10 shots off the pace.

J.C. Snead and Bill Rogers played steady if unspectacular golf to remain in slight contention. Snead actually was superior to Watson from tee to green most of this round, except for double-bogey six on No. 7 when his drive caught the water on the right.

Watson's tee shot on nine landed in one of the lakes that border the right side of the fairway. The leader recovered beautifully, with a midiron to midgreen, then was weak with the 10-foot putt neede to preserve his par.

Watson, after opening rounds of 63 and 68, was unable to pick up any of those four or five shots he believed he would need in order to win this $40,000 first prize. Last year's Masters and British Open champion had to be content with one birdie, two bogeys and 15 pars for a 73 and a 54-hole total of 204, 12 under par.

Fortunately for Katson, no one really capitalized on the "one bad round" that seems to mark many of his tournaments. Birdies were surprisingly difficult to come by for everyone. Wadkins shot a one-under 71 to cut Watson's lead in half but otherwise, the leader board underwent little important change.

The most noteworthy performance today might have been Johnny Miller's 42 on the front nine, where he posted three double-bogey 6s for a 76-221. Miller won this tournament in 1974, 1975 and 1976, setting the record of 236 in 1975.

Lee Trevino is the crowd favorite here, playing 60 miles from the Mexican border. He was six shots behind Watson beginning today's action after putting together rounds of 70 and 67.

Slightly more than a year ago Trevino suffered a back injury that forced his hospitalization. A disc was removed from his spine. But he won the 1977 Canadian Open, earned $85,108 last season, and believes he is ready to consistently play top golf this year, with the aid of a gravity machine.

"It attaches on a door," Trevino said. "What it makes you do is to hand upside down, in traction, and do sit-ups and back-bends. I can't say I exactly enjoy it, or getting up nearly three hours early in order to get ready for a round, but this is something I'm going to have to live with the rest of my life.

"I don't intend to quit for many years," Trevino added. "I hope I wind up tripping over my beard, playing out here."

Trevino was 38 Dec. 1.