Bangkok — By 1986 Asia's ''rice bowl'' might be better known as a tourist tureen.
Rice exports have traditionally been Thailand's top foreign currency earner. But tourism's rocket rise has taken it from fifth to second place in foreign-exchange earnings since 1975. If it continues on this path, tourism may move into first place within four years.
The implications are puzzling to Thai officials, who historically have been most adept at assimilating foreign invasions of one sort or another.
''We are still not clear on tourism's role in our economy,'' says Dr. Snoh Unakul, secretary-general of the National Economic and Social Development Board.
Last year, rice exports earned $1.3 billion while tourism brought in $1 billion. Ten percent growth last year, with another 10 percent expected in 1982, assures the tourist industry a dominant role in the Thai economy very quickly.
High world air fares have halted growth of Thailand's most popular fans, West Europeans. Short flights from the newly prosperous nations of Asia, especially Japan, make those countries targets of Thailand's new promotion campaigns.
Dusty studies on development of tourist spots are now getting attention. The first is Phuket, a beach paradise island in the south, which is scheduled for $ 41 million in investments. Some officials suggest that Bangkok become a tax-free ''shopping haven'' to rival Hong Kong and Singapore, although the idea is not popular.
This year's bicentennial celebration of the Chakri monarchy and the founding of Bangkok has failed to spark a new wave of foreigners. Promotion was late, and the purpose of the celebration was not made clear. But it should make Thailand's historic stability better known, helping to draw potential tourists who might be wary of a visit while fighting continues along the Thai-Vietnam border.