At the mighty Iguacu Falls, whose cataracts dwarf those of Niagara, the flow of water has slowed to a trickle. In the far south of Brazil, farmers have begun praying for rain, calling on both Catholic priests and Indian medicine men for aid.

Three years after the disastrous "black frost" that destroyed millions of coffee trees and sent coffee prices rocketing worldwide, Brazilian farmers are confronting another natural calamity. A drought that agronomists here call "the worst in 50 years" has plunged Brazilian agriculture into crisis and raised the possibility of new disruptions in the world's commodity supply patterns.

Throughout the area that Brazilians refer to as "the breadbasket," a vast and fertile region that extends from the grassy plains of Goias state to the gentle, rolling hills that mark Brazil's border with Uruguay, the story is the same. The lush green fields of years past are replaced by cracked, parched soil and withered plants.

In the state of Parana alone, crop losses have been estimated at over $1.2 billion. In Rio Grande do Sul, where damages are reported to amount to more than $850 million, some areas have had no rainfall at all since November, and what water is left in reservoirs is being rationed. Electric power output is down, forcing the closing of shops and factories.

"In some ways, this drought has been even worst for us than the frost of '75," says Deny Sohwarts, a Parana state assemblyman who is leading a campaign to have the government declare a "state of calamity" in the drought area. "Then, it was pretty much just coffee that was affected. Now, it's everything: coffee, soybeans, rice, corn, beans, wheat, cotton."

In some areas, farmers are simply plowing their crops under.

"It's a nightmare," says Eduardo de Souza, a small soybeans and corn producer. "My yield is so poor that it's not even worth the effort it would take me to harvest it."

The devastation caused by this southern hemisphere summer of high temperatures and virtually no rainfall is likely to have important consequences for American farmers and consumers. Nearly two-thirds of the $12 billion that Brazil earned in exports last year came from agricultural products, making Brazil second only to the United States as an agricultural exporter.

As a result of this year's disastrous harvest, Brazilian government officials expect that their country will be unable to export soybeans, rice and corn to traditional customers in Europe, Asia and Latin America and will be forced to import several million tons of these products. Imports of wheat, the only basic agricultural product in which Brazil is not usually self-sufficient, are not likely to rise, putting even more pressure on market prices.

Among the crops most seriously affected are coffee and soybeans - the two top foreign-exchange earners for this nation of 115 million people. The Brazilian government has ordered the suspension of all soybean exports in an effort to meet the needs of its domestic market, an action that followed earlier official curbs on corn and rice exports.

Although government statistics indicate a drop in this year's coffee crop, the full extend of the damage may not be known until next year, when coffee trees planted to replace those destroyed in the 1975 frost are supposed to begin producing. Growers, however, are already pessimistic about their prospects.

"Nobody is planting anything, not to even coffee, says Geraldo Toffilli, head of a farmers' cooperative in Garca, an important coffee-growing area west of here. "I have seen nothing but dried-out fields and two-year-old coffee plants that have died of thirst, simply withered away."

Some foreign observers, though, think these warnings are exaggerated.

"Coffee prices have taken a tumble lately, so there's a lot of wishful thinking going on," says one U.S. economic analyst. "You can almost hear them telling themselves maybe it'll get as bad as it was in '75."

There is no doubt, however, about the seriousness of the damage to the soybean crop, which last year earned Brazil $2.1 billion in foreign exchange. Official figures say that this year's harvest will amount to only 8.5 million tons, a drop of nearly 4 million tons from the record in 1977 harvest.

"I feel kind of sorry for the Brazilian soybean grower," says an American economic observer here. "The drop in their output as a result of the drought is enough to put them in a real money squeeze, but it's not yet enough to create the kind of shortage in the world market that would send prices at the Chicago Exchange up into the stratosphere."

"The financial situation of the grower in this area is lousy," adds Amadeu Piovesan, president of the Piqueri Valley agricultural cooperative in Parana, where many farmers switched from coffee to soybeans after the 1975 frost. "With soybeans in a good situation in recent years, everybody made big investments in machinery and land, and now they're in trouble."

To alleviate the situation, the Brazilian government has made available to farmers approximately $500 million in emergency credit. But, says Marie Stadler de Souza, president of the Parana Agricultural Federation, "the question is not money, it's rain."