With spring weather and the cherry blossoms about to alter the Washington landscape, tourist industry observers are looking cautiously for slight increases in the city's vital $1 billion tourist trade.

"It's tough to make real solid judgments, but we're seeing a considerable increase in phone calls to our tourist information center," said Austin Kenny, of the Washington Convention and Visitors Center. Similarly, a representative of the National Part Service says queries to their information office, on the eve of this week's Cherry Blossom Festival, are way up.

Occupancy is up 5 to 8 percent at area hotels and motels so far this year and museum attendance, at least at the Smithsonian Institution, is also rising slightly.

But the hotel figures are less than conclusive because they are the result of the usual heavy local convention trade and the winter's inauguration festivities. And, museum attendance may be the result of Washingtonians staying at home during the winter months rather than taking expensive vacations.

Nationally, hotel occupancy, certainly a key measure of tourist activity, was off 2 to 4 percent in 1980, according to Fred Mosser, a hotel industry analyst with Laventhol & Horwath, an investment firm. "We see things going back up to normal," he said. "We think this area will conform to things in general.

Nevertheless, there will not be any dramatic increase in the area's tourist flow this spring, industry observers agree. "We sensed things were very flat last year," Kenny said. "And as long as the economy is soft, it's got to affect pleasure travel."

Leonard Hickman, spokesman for the Hotel Association of Washington, the trade group that represents the downtown hotel owners, concurs with Kenny's cautious assessment. Although Hickman said hotel occupancy appears to be "on a par with last year," he also said that 1981 is "the first time in my 14 years here that I'm sort of confused.

"It's a very mixed bag," Hickman said of the figures he sees from downtown member hotels. "Some are doing a little better, and some are doing much worse."

Hickman said that he has analyzed February hotel occupancy data for member hotels and is confounded by the lack of a measurable trend. "I thought that perhaps occupancy would be consistently up at hotels near Capitol Hill because of all the congressional activity. But, that's not true," he said.

"Normally in the first quarter of a new administration, the figures go way up, but that's not applying across the board. Outside of those four days in January," he said, refering to the inaugural period, occupancy at some hotels has been "low."

Jerry Bynum, director of travel marketing for Quality Inns, the Silver Spring-based chain that has 12 franchised and two owned facilities in the metropolitan area, said that bus-tours trade and, in particular, a continuing stream of visitors from abroad, appears to mean a good spring for his company.

"The activity this spring is better than expected," Bynum said. "We have already met the budgeted goals through March. We are in an excellent position to enjoy the benefits of recessionary times, since we are a medium-priced chain."

And, the Smithsonian apparently continues to thrive. At the Air and Space Museum, for example, total visitors for the first two months of the year rose sharply. For January, attendance rose from almost 270,000 to nearly 330,000 while February figures rose from 301,000 to 340,000.

All 13 Smithsonian facilities showed similar increases, as total attendance rose from 816,000 in January 1980 to 1.04 million for the first month of 1981. February totals rose from 936,000 in 1980 to 1.08 million this year.

A spokeswoman for the National Park Service said that crowds seem to have hit many of Washington's key tourist spots sooner this year than in the past. At the monuments managed by the park service, crowds seem to be up anywhere from 10 to 30 percent.

For example, the line to ride to the top of the Washington Monument, which takes the form of a circle surrounding the base of the 550-foot-tall structure, averaged about a five-minute wait at the beginning of March. Now that wait is averaging 25 to 30 minutes, she said.

Those free tourist attractions and discount weekend hotel packages are the heart of what the local tourist industry is telling the nation about Washington. With consumer dollars tight, the availability of hotel packages and the plethora of free tourist attractions have economic appeal to the East Coast population centers that are within five hours of Washington.

"Our basic pitch that we're trying to get across is the weekend in Washington," said Kenny. "Basically everything is free all day, and at night we have a very cosmopolitan city. This is one of the best restaurant towns in the world right now. We can sell Washington as a complete vacation."

In out-of-town newspaper advertisements showing postcards, the hotel association says that Washington is "a wonderful city . . . wish you were here."