Everything's going Zia's way now, but how firm is country's foundation?

General Muhammad Zia ul-Haq has certainly been fortunate.

Since seizing power five and a half years ago, he has had four years of extremely good weather, producing bumper crops. He has reaped the policy benefits of former Prime Minister Zulfikar Ali Bhutto, who sent waves of workers to the oil-rich Gulf. Workers now send home $2.2 billion a year - critical foreign exchange for the coffers of Pakistan's military regime.

World prices for rice and cotton, Pakistan's major exports, were favorable until 1981.

And the 1979 Soviet invasion of neighboring Afghanistan has proved both a curse and a blessing to General Zia. Above and beyond the implicit threat to his own borders and the economic implications of housing nearly 3 million Afghan refugees, Afghanistan has bestowed a modicum of legitimacy on his martial-law regime. No longer dismissed as a pariah by the geopolitically conscious Western world, General Zia has stridden across the stage of political expediency - reaping new aid commitments to shore up his armed forces and cushion his country's fragile economy.

He will receive $3.2 billion in economic and military assistance - including 40 F-16s - from the Reagan administration over the next five years. He has signed a three-year agreement with the International Monetary Fund, which will bring him $1.4 billion in foreign exchange. And last summer he rescheduled Pakistan's enormous national debt, which costs this nation over $700 million yearly.

He has adroitly sidestepped US congressional restrictions on military assistance to countries believed to be developing nuclear arms - even at a time of mounting evidence that Pakistan is on the threshold of entering the nuclear club.

But Western officials see General Zia presiding over a structure whose foundation could weaken and perhaps collapse. The country's impressive growth rate of 6 percent a year is based on a fragile premise. Political violence is on the rise. Only one of the country's religious parties - the militant Jamaat-i-Islami - still supports the military regime. And a magic embellishes the name of the late Zulfikar Ali Bhutto, whose own excesses appear forgotten today.

Even among supporters of the 1977 military takeover there appears to be waning enthusiasm for one-man rule. From the manicured lawns of the Karachi Yacht Club to the poorest North-West Frontier village, one hears over and over, ''How much longer will it last?''

The enigmatic General Zia has done little to establish a power base. His only constituency, he tells foreign visitors, remains the armed forces. Nearly 80 percent of the Army of 450,000 comes from the stark agricultural villages of the sweeping Punjab, long a source of resentment in the North-West Frontier, Baluchistan, and Sind. Resentment has grown with persistent reports of corruption within the Army, and the number of officers assuming key civilian positions.

General Zia himself is largely accepted as an honest, pious man. But he remains largely unknown to businessman and peasant. He has won the sympathy of Western ambassadors by his humility and charm. But he has also perplexed them - as he has many in this nation of 84 million, stretching from the Himalayas to the Arabian Sea.

''I see no major threat to him at the moment,'' a Western diplomat said. ''But he can't govern indefinitely without popular support. He's sitting on a volcano. Look at recent history. No one thought that the Shah would fall. One small incident could touch things off. We've seen twice in Pakistan's independent history of only 35 years that mobs can go to the streets in this country, and governments do fall.''

There have been persistent reports, which cannot be verified, of at least three attempts this year alone on General Zia's life. Officials refuse to discuss the incidents, but acknowledge a wave of car bombings, attempts to assassinate other government officials, political arson, and general sabotage. And although neutral observers say there is no evidence that the incidents are linked, the government has put the onus squarely on the shoulders of the Kabul-based terrorist group headed by Bhutto's two sons.

The government has ordered firearms licenses issued to any ''law abiding'' citizen, to repel the escalating number of ''terrorist attacks,'' an ominous portent in a nation where Islamic extremists are said to be stockpiling arms.

But as in much of the third world, the greatest vulnerability this nation's military rulers face is a downturn in the economy. Though this is a developing country, there are certain signs of consumer wealth.

The best wines and French perfumes are available in the cities, and in the chaotic village bazaars one can find the newest video cassette recorders, color televisions, and nylon stockings. It is a remittance economy, for better or for worse, and villagers for the first time have money in their pockets.

But they still lack a viable infrastructure and openly complain that they are being largely ignored today, as they were by Pakistan's democratically elected governments.

From precipitous mountain to vast desert expanse, only 25 percent of Pakistan's villages have electricity. Less than 20 percent have drinkable water. The country has only 10 percent of the roads considered necessary. Of the nation's 7,000 doctors, only 1,000 are in the tens of thousands of villages, even though 70 percent of the population is rural.

The man charged with turning around the equation is Dr. Mahbub ul-Haq, now Pakistan's minister of planning and one of the country's most pre-eminent development experts.

His ambitious five-year development program, to be launched in July 1983, is notable for its shift toward agriculture, energy, and social services, and away from the industrial sector. But the program is as costly as it is ambitious. Dr. Mahbub ul-Haq looks toward the private sector and toward channeling the billions of dollars in foreign remittances each year away from consumption into production.''

The strongest ties of these workers are to their families and to their land, and this can be turned into a national investment, a national asset,'' he said.

But the outlook for private sector financing is far less certain, and Mahbub ul-Haq acknowledged the difficulty of enticing private investment into industrial schemes. The Zia government continues to sit astride the nationalized structure bequeathed by Bhutto.

''There are vast and formidible problems,'' one Western economist said. ''Therefore, when you look at what are pretty impressive growth rates - 6 percent for the gross domestic product, 4 percent for agriculture, and 12 percent in the manufacturing sphere - don't be deceived by the figures. There's a lot of fragility underneath.''

Nearly 3 million refugees on his border are another economic drain, but it is also Afghanistan that has catapulted General Zia ul-Haq of Pakistan onto the international stage.

He has taken the lead in attempting to forge a solution that would allow the Soviet Union to withdraw its 100,000 troops.

''Quite honestly, given the opportunity, I'd do exactly as he's done,'' said one sympathethic Western ambassador. ''He needs a solution, and he's preoccupied with Afghanistan and Iran. It's quite enough to have the Soviets squeezing his eastern border, an India hostile to the West, an Iran intent upon exporting revolution. . . .He is truly boxed in.''

Meeting privately with Soviet leader Yuri Andropov in Moscow at the time of the Brezhnev funeral, General Zia is said to have received no quarrel from the new Soviet Communist Party chief on the scheduled arrival in Rawalpindi of the first of the 40 F-16s. He was also not pressured to begin direct negotiations with Babrak Karmal's Afghanistan regime. It was recognized that talks have a long way to go, but progress has been registered, beginning with the United Nations discussions in Geneva in June.Kremlin leaders are believed to be seriously studying a four-point negotiating brief that would center on the withdrawal of Soviet forces, the return of refugees, international guarantees of noninterference in Afghan affairs, and self-determination for the people of Afghanistan. Implicit in the latter is that Afghanistan would be a moderate, nonaligned country, not hostile to the Soviet Union, China, or Pakistan.

Government officials smart when it is suggested that Pakistan may make concessions unacceptable to the West to get an Afghanistan settlement. ''What would we gain by unilateral concessions?'' one high official asked. Unbalanced accommodation with the Soviet Union would carry far too high a price. It would disrupt Pakistan's close alliance with China, estrange it from the Saudi Arabian regime, which, according to high Western sources, has already made the $111 million down payment on Pakistan's F-16s, has agreed to underwrite $500 million in military equipment, and is giving the country $1 billion in economic aid.

It would also wreak untold damage on Pakistan's valued relationship with the West, now closer than for many years.