The black hoe blade of Antonio Aurelio dos Santos lifted high in the air and then swung straight down into the sand. Aurelio stood back. He had cut a fat, deep square in what looked like pure beach sand, and now a tiny pool of water formed at the base of the square.

Aurelio's son, a quick, skinny, barechested boy, poured dry cow manure into the hole, and Aurelio piled dry sand atop the manure. Then he stuck a sweet potato stem in the top. Every foot of the small sand bed around his low, stagnant stream was planted this way. In the Brazilian Northeast, which is staggering this season through its longest drought of the last hundred years, planting straight into the sand is one of the few ways a farming family survives.

The cyclical despair and mounting frustration of northeastern Brazil, a huge patchwork of semiarid lands that have beaten down stubborn small farmers since the end of the 18th century, can be read this year in the 62 acres Aurelio shares with his brother Pedro. They have 22 children between them, and until last March their sweet potatoes were their only crop to survive the drought.

Their beans died. Their corn died. Their pasture dried up, and three of the cows starved to death. The Aurelios lived on sweet potatoes and whatever their meager government emergency money could buy.

To the north and west of their farm, where there were not even stagnant streams to moisten the land of hundreds of thousands of northeasterners, men abandoned their farms by the truckload to swell the shantytowns of Rio de Janeiro and Sao Paulo, looking for work. Thousands of peasant families stormed into towns of the interior, demanding food. Grocery stores locked their doors, supermarkets were ransacked, and one town in the north was invaded by flagelantes, as the drought refugees are called here, three times in a week.

In the second half of March it started to rain. It rained for almost three weeks straight, and the rains came down so hard that earth dams broke and rivers spilled over their banks. On the Aurelios' farm, the stream flooded and destroyed every plant along its sandy shores.

Then it stopped raining. All over the Northeast, farmers hurried out to the damp earth to plant their crops, but there was no more rain. The wild scrub flourished, and healthy weeds lunged out over the backlands, but the corn and beans died again, and most of the small family farms still had no food.

They have a special name for this condition in the Northeast. It is called, with all the bitterness the words can carry, "green drought."

In Brazil, which is churning ahead of the rest of the continent with its factories and its gleaming airports and its capital sprung straight up out of the red dust, the Northeast stretches out like a great dry reproach, one of the most dismal and poverty-plagued expanses of land in all South America. With about 15 million people spread over 25,000 square miles of dusty earth and scrub brush -- the sertao, as Brazilians call the semiarid backlands -- the Northeast has been subject to sporadic and sometimes catastrophic droughts for as long as anybody can remember.

In the great 19th century drought that ended in 1879, 500,000 people are reported to have died from hunger or the cholera and smallpox that infested their crowded refugee camps.

That drought, which was the greatest disaster that had ever befallen the Northeast, lasted three years. This one is heading toward its fourth year, and a report from the Brazilian Space Institute's atmospheric sciences division has predicted that the drought will continue until 1985, with the most critical period extending from this year to 1983.

The Northeast has enough health care, transportation, and emergency money now to keep the dry farms and crop failures from killing many people outright -- nine deaths have been attributed to the drought since 1979. But with more than 700,000 people being forced off their land and into low-paying government emergency work projects, hungry peasants continuing to invade towns, and whole families living on what one magazine called the "drought diet" -- beans and rice, twice a day -- it is obvious to almost everybody in Brazil that the Northeast is in desperate trouble, despite the years of effort and millions of federal dollars that have been poured into it.

"The reason the drought is so strong, so terrible as a social phenomenon, is because it reaches a group of the population that lives on a bare subsistence," said Clovis Cavalcanti, a sociologist who heads the Joaquim Nabuco Research Institute in the coastal city of Recife.

A visitor poised between Antonio Aurelio's sweet potato rows could hear in the distance the sputtering engine of the only thing that saved his family from green drought this summer. Down where the stream widened, a half-horsepower pump sucked water into tubes, up the banks, and then out over a planted patch on the opposite shore. The Aurelios and seven nearby families bought the pump together five months ago with the aid of a new government-sponsored credit program.

Until the pump arrived, the Aurelios -- lifelong farmers who were born on the land they now work -- had no way to get the water from their stream to the dry cropland 200 yards away. When it rained, the crops grew. When it did not rain, the Aurelios went hungry.

In 1981, a hundred years after the drought that killed so many, that is the way millions of northeasternsmall farmers still live -- shut off from the private bank credit the big ranchers get easily, excluded from the government reservoirs built in the last two decades, kept alive by marginal subsistence farming that fails whenever the rains hold back, and attached with a now-legendary fervor to the unpredictable sertao.

"We might have died here," Pedro Aurelio said. "But we would not leave. This is our land, and we will stay on it."

The Brazilian government began trying to figure out how to cope with the northeastern drought problem as early as 1880, and from the start it was a formidable challenge. Much of the soil is hard and will not hold water, so that moisture evaporates quickly in the heat. Even in years when there is enough rain, it sometimes falls all at once in a tremendous deluge, or else it delays just long enough to ruin the harvest.

But there is just enough rain -- just enough good years between every bad one -- that the northeastern farmers have never evolved the kind of drought-resistant farming techniques that usually develop in an area where it hardly ever rains. The staple sertao plants -- corn, beans and sweet potatoes -- need regular water to grow, and even farmers with a permanent water supply are usually unfamiliar with basic canal irrigation.

For a long time the answer was thought to be reservoirs -- big artificial lakes that filled with the good years' rains. But the first reservoirs benefited almost no one but large landholders with cattle ranches. Then, in the 1960s and 1970s, following the creation of the special Northeast development agency called SUDENE, the government began an ambitious program of large public reservoirs surrounded by small colonists who were to farm the land with government technical help.

The plan looked splendid on paper, but by the time the colonization projects were finished they had ruined far more farmers than they had helped. Every time the technicians descended on a river to prepare it for a reservoir and colonization, they first dislodged all the homesteaders and small farmers who had settled there -- generally six times the number of people who were finally allowed to come back and colonize, according to the English researcher Anthony L. Hall.

With those projects now widely considered an expensive flop, SUDENE is trying a new approach. The Aurelios' water pump is part of an effort to help small farmers make the most of the land and water they already have. Agronomists are encouraging farmers to substitute some of their old staples for drought-resistant crops such as sorghum, or high-profit crops such as tomatoes and grapes. With advice from Israeli, African, French, and Indian experts, SUDENE has also set up a few farmers with experimental systems such as solar-powered water pumps and elaborate irrigation tubes that shoot tiny jets of water onto the base of each plant.

"The essence of our philosophy is, 'How can we live with the drought?' " said Leonidis da Silva, deputy director of SUDENE. "In Europe they live with the snow. In the United States they live with cyclones. We have to learn to live with the drought."