Thousands of drainage canals that allowed the Mayan Indians to grow crops in what otherwise would have been a swamp have been discovered in the abandoned rain forests of Guatemala and Belize by airborne radar.

The network of drainage canals helps to explain how the Mayans fed a population of between 2 million and 3 million at the peak of their civilization, around 800 A.D. The canals provide the first extensive evidence that the Mayans rotated crops on the same land, season after season.

"The extent of these drainage canals is much greater than anybody would have guessed," said the University of Arizona's Dr. T. Patrick Cubert, one of two archeologists who analyzed the radar images. "You can see a network of these canals all over the Mayan lowlands of Guatemala and Belize."

The idea of using airborne radar to penetrate the Mayan rain forests came from two other archeologists, Drs. Bruce Dahlin of Catholic University and Richard E. W. Adams of the University of Texas at San Antonio, who is now on a year's leave at Cambridge University in Britain.

"The type of radar we had in mind had the potential of getting through the canopy of trees right to the ground," Culbert said by telephone yesterday. "If you fly over there today, you can't see anything but trees."

Numerous flights of a Convair 990 carrying synthetic aperture radar, developed by the Jet Propulsion Laboratory in Pasadena, Calif., uncovered what looked like a network of narrow roadways under the rain forests of Guatemala and Belize. The roadways were in what appeared to be unnaturally uniform grid patterns, all interconnected and emerging from the swampy surface of the Mayan rain forests.

Taking the aerial radar images with them, Adams and Culbert set out for Guatemala and Belize in February to see if they could find in the swamps what the radar had seen from 28,000 feet. Traveling by dugout canoe and on foot, the archeologists located numerous drainage ditches that had been hidden in the rain forests for more than 1,000 years.

The typical canal was a half-meter deep and no more than three meters wide.

Culbert said the canals carried water from thousands of fields no larger than 150 square meters, which apparently were cultivated to grow maize (corn) and cacao.

According to Adams, the archeologists found evidence to believe that "as much as one-third of the grid patterns [seen by the radar] are ancient canals." Adams estimated that as many as 11,185 square miles of canal systems exist beneath the rain forests of Guatemala alone.

Culbert said it is their guess that the canals thrived between 600 and 800 A.D., when as many as 1.5 million Mayans lived in what is now Guatemala and Belize.