The worst drought in this century is forcing most southern African nations to revise economic outlays and seek vast imports of food.

An unprecedented lack of rain in the past two years across a broad swath of the region from the Atlantic to the Indian oceans has scorched the earth, leaving fields barren and rivers dry.

South Africa, which has been the food basket that many food-poor black African nations have had to turn to in recent years, has suffered a startling reversal in its agricultural fortunes as a result of the drought.

It is seeking to import 1.5 million metric tons of cornmeal, the staple of southern African diets. Just two years ago after a bumper harvest of 14 million tons, the white-ruled country exported 5.4 million tons, much of it to its neighbors despite their political differences. This year's crop is estimated at only one-third of the 1981 record.

South African millers are having to mix yellow and white corn even though Africans traditionally do not like the yellow "mealie-meal." Superstition has it that yellow corn affects virility but superstitions are going by the boards in the face of hunger.

Zimbabwe, the major black African country with a regular food surplus, has had to turn to western donor nations for food aid for the first time. It is seeking 15,000 tons of oil and 150,000 tons of wheat, valued at more than $25 million.

Other countries that normally have a food deficit have been hit even harder.

Mozambique, which has a regular grain shortfall of 400,000 tons, will have to import an additional 300,000 tons this year.

Aranda da Silva, minister of internal commerce, estimated that a third of Mozambique's 12 million people is suffering from the drought in the southern part of the country.

"Hunger is a striking reality in these areas," a government report said, adding that about 500 cattle a day were dying for lack of pasture and water.

Less than 15 inches of rain fell in key agricultural regions of Maputo province, about half of the normal amount, the report said. In many drought-stricken areas it has scarcely rained for two years, da Silva said.

The small amount of irrigated agriculture in the country has been severely affected by the drop in the level of rivers.

The Limpopo River, the largest in the country and the source for a vast irrigated rice plantation at Chokwe, is at its lowest level in years. During the normal peak of the rainy season in February the maximum level of the river was only about four feet, compared with an average of 20 feet.

The fickle pattern of the rains also played havoc with agriculture. Northern Mozambique and northern Zambia received normal rains but the drought-afflicted southern regions are the major food producing areas

However, Malawi, which borders the two countries, benefited since the rains came just in time to save the corn crop, and as a result it has 60,000 tons available to export to its neighbors. It is the only southern African country untouched by the drought.

Malawi has given more than 200 tons of food to Mozambique, even though relations between the two countries are sometimes strained. Unlike in famine-plagued Ethiopia, however, there is little likelihood of a large death toll in southern Africa.

"I don't think anyone has to die in southern Africa, if governments can respond by providing relief and pumping incentives into the agricultural sector," a western food aid specialist said.

He praised their response so far. There could be pockets of severe illness caused by food distribution problems in remote areas, such as the snowbound mountain regions in Lesotho, he added.

Even if precipitation returns to normal with the scheduled start of the rainy season in November, there will be serious long-term economic effects from the drought, an official of the U.S. Agency for International Development said.

The water table will be down and the vast loss of livestock will have a "quite striking" impact on the structure of herds for several years, he said.

The drought could also cause a change in food patterns in southern Africa with a switch to traditional dry land crops like sorghum, millet and beans to provide insurance if the rains fail again. Those crops would have survived the drought much better than corn, the staple food, the official said.

U.S. AID is proposing a $14 million program for research on semiarid crops for 10 southern African nations as a result of the famine.

"South Africa is hands down the biggest loser in value of crops," the western food aid official said. South African economists have estimated that crop failure will cost the country more than $800 million in lost export earnings.

The government, however, has the wealth to mount a relief program "to absorb the shock," he said. It also has a sophisticated agricultural sector that will be able to rebound if the rains return to normal.

On the other hand, the people in Mozambique will be "the hardest hit," the expert said, citing "weak food distribution, a crumbling agricultural input system and bad economic politics," a reference to government attempts to apply Marxist principles to agriculture.

"Mozambique does not have the resources, infrastructure or policies in place to rebound," he said. In addition, the food shortages are compounded by a war against rebels that Mozambique says are backed by South Africa.

Mozambique this year has become the major recipient of U.S. free food aid with assistance valued at almost $10 million. It will doubtless receive additional American food as a result of a $25 million emergency program for southern Africa announced by the State Department this month.

However, Donald Kimmel, the Washington director of the U.N. Food and Agriculture Organization, complained that, in general, major donor countries are not responding to appeals from drought-affected countries. One reason he cited was recession-related efforts by western governments to cut their budgets.

The U.S. budget for free food assistance for Africa has been trimmed more than 16 percent in the past two years, according to a report published by AID.

The amount of food the money will buy has also decreased, since the budgeted amount includes shipping charges which have gone up in the last two years.

In addition, there are more mouths to feed as African populations are growing at an average rate of about 3 percent, double the rate of increase in food production.

A report this month by the U.N. Food and Agriculture Organization said only five subsaharan countries have increased their production more than their population growth. In seven African countries production has fallen by more than 20 percent per capita in the past 12 years.

Zimbabwe has also ceased to be a source of supplies for its neighbors after exporting to 12 African countries in the last two years as a result of a record marketed corn harvest of about 2.1 million tons in 1981.

Prime Minister Robert Mugabe also gave 25,000 tons each to Mozambique and Tanzania, which were key allies in his guerrilla war for independence. Those halcyon days are gone, however, and the country has postponed any new sales commitments.

This year's marketed production is estimated at 700,000 tons and another year of drought could force Zimbabwe to import food since most of the 1.4 million-ton reserve will depleted by the next harvest.

Almost half of the country's 7.5 million population is expected to receive food aid from the government at a cost of about $100 million, according to Finance Minister Bernard Chidzero. Estimates of the total drought-related costs to the country range as high as $500 million, equivalent to last year's balance of payments deficit.

About 100,000 cattle died last year and the Cold Storage Commission, the government's beef marketing authority, projects that as many as 250,000 will die in 1983 because of the disastrous lack of rainfall, particularly in southwestern Matabeleland, the main cattle-producing region.

Next: Sahel after the drought