The opium fields of Southeast Asia have produced less than half their normal harvest for the second year in a row, narcotics agents here report, but they warn that the shortage has driven prices so high that farmers are likely to vastly increase their plantings next fall.

Two years of drought -- not the slow progress of antiopium efforts by the government -- account for the reduced crops of the last two years.

"If the rains do come, it could be a bumper crop" next year, as much as three times the 250 tons harvested this year, one narcotics official said.

The reduced harvest has cut slightly the volyme of opium's refined form, heroin, that is being smuggled into the United States from here, but heroin experts predict that exports will pick up again in 1981.

An estimated 1.5 tons of southeast Asian heroin entered the United States in 1978, about a third of the total that came to the United States from all sources. One agent in Thailand said the drought may have curbed Asian heroin's tonnage to the United States by 5 to 10 percent.

Most of this regions's opium is grownin the so-called Golden Triangle the mountians zone where the borders of Thailand, Laos and Burma converge. In many parts of the region, modern-day warlords and their private armies exercise the only effective control.

Burma's fields are the biggest producers, accounting for perhaps two thirds of recent harvests. Most of the remaining third is grown in Laos, whose five-year-old communist government has permitted continued cultivation. Thailand is the smallest producer of the three.

Primitive hill tribes that inhabit the tiangle have cultivated opium for generations. It thrives in the cool mountain air, generates high profits for low volume and transports easily. About half the crop is smoked or eaten in the village -- opium is considered one of the rewards of old age -- while the rest is sold to traders, who refine it into heroin in laboritories hidden along the Thai-Burmese border.

Control of the opium trade is at the root of virtually every shift in the maze of alliances and conflict that overlays the various armed groups in the triangle. Opium profits arm and feed the soldiers and buy villas for the warlords and, occasionally, American educations for their children.

Among the major groups are remnants of the old Nationalist Chinese Army that fled into norther Burma in the late 1940s to escape communists; the Shan United Army, fielded by the Shan minority group in its separatist struggle against the Burmese government; the Thai and Burmese Communist parties; and the armies and plice of the area's three central governments.

Efforts to stamp out opium go against the grain of a longstanding political and military status quo. Warlords can raise substantial firepower in protest, or they might bribe officials.

Moreover, some groups have obtained special privileges in return for bearing arms against the communists, The Meo (Hmong) hill tribe in Laos, for instance, was armed by the U.S. Central Intelligence Agency during the 1960s and 1970s. In return, their opium dealings were overlooked, if not assisted, by the C.I.A. and the then Laotian government, acording to many accounts.

Today, under diplomatic pressure, the three governments have moved publicly to attack the opium trade, but their policies sometimes conflict.

Burma, often using the 27 helicopters provided by the United States for narcotics suppression, sends troops to chop down poppies in remote fields and to raid heroin laboratories.

In february, for instance, Burmese forces attacked a heroin refining center close to the Thai border at Lao Lo Chai. It was the most successful raid in a year and captured 11 laboratories and 15 pounds of heroin, the Burmese said.

Suppression officials in Bangkok say the Burmese Army has suffered many casualties in destroying poppy fiellds. Troops do not worry about alienating the opium farmers because in the Burmese government's view, most growers are loyal to secessionist groupsand can be considered enemies.

Thailand, in contrast, does not destroy opium plants, although growing them is illegal.

"Opium is their sole cash crop in most places. If you took it away they'd have nothing," said one American working in narcotics control.Crop destruction might also lead to armed revolt, it is feared.

Thailand reasons that people must fitst learn to grow new types of crops, then the laws against opium can be enforced.

In the 1960s, Thai King Bhumibol Adulyadej started crop replacement programs among the northern villages. In 1972, the U.N. fund for drug abuse control began similar projects with the Thai government.

The idea is to promote coffee, kidney beans, potatoes and other cash crops. Program officials say these crops can be more profitable than opium.

The program offers loans for seed and fertilizer, and advice from extension workers. After the harvest it buys produce at guaranteed prices

Officials say the program has virtually eradicated opium cultivation in a few villages. But it is still too small to affect overall production, reaching only 38 of the estimated 500 opium-producing communities in Thailand. r

Yet foreign specialists note with satisfaction that the program has done well enough to prompt the Thai police to destroy a few fields. In January, several acres of poppies were uprooted near a village where substitution crops were introduced six years ago.

"That was a historic occasion," said one American. For 1980, the Thai government has agreed to enforce "opium-free zones" around about 10 villages.

Plans call for U.S. crop programs in 75 villages by 1983. The World Bank, West German government and U.S. Agency for International Development are also considering crop replacement programs here.

Opium is considered off-limits to the police as long as it remains in the village, but fair game once it passes into the hands of traders. U.S. Drug Enforcement Administration agents stationed here assist in narcotics intelligence.

The days have long passed when Thai military and police officers could finance political parties with their opium earnings, as they did in the 1950s. But narcotics officers still frequently complain of political constraints in their work.

One Thai official suggested his government has delayed moving against Khun Sa, leader of the Shan United Army, because his troops helped police remove border districts.

Thai narcotics officiers say publicly that Khun Sa's group operates in Burma and is the Burmese government's responsibility.

Opium cultivation continues in Laos, although today farmers are supposed to sell their crop to the government, which is discussing supply contracts with pharmaceutical companies in the Soviet Union and Eastern Europe. Visitors to Laos have reported the authorities there are also making efforts at crop replacement.

Bangkok narcotics agents, however, report increasing evidence that large quantities of Laotian opium are finding their way into the Golden Triangle's underworld network.

There is little coordination among the three governments and at times they distrust one another.

Despite all of the constraints, however, the fight against opium appears to be making slow progress.

Still, the events of the past two years have underlined to suppression officers how long the road will be. Nature's withholding of water has had far more effect than the efforts of thousands of people and millions of dollars.