On a late autumn day, newlyweds fresh from the marriage palace stroll with delight beside the cascading fountains that line the city's gigantic new administrative square.

In the ancient city of Bukhara, Uzbeki children idly play in the warm rush of water that spill across city parks from the community's complex irragation system.

Across Central Asia, the Soviet government is pushing back the desert on waves of water brought hundreds of miles from the mountains that lie along the Chinese and Afghan borders in the southeast.

They are digging massive canals that double as freightways, laying pipes, installing pump complexes, and patiently leaching acrid salts from the parched earth, irrigation the desert in dozens of separate projects to expand the Soviet Union's shortage-prone agriculture.

The costly effort is remaking the face of this arid region of the globe, where for thousands of years humans clustered around such fabled oases as Samarkand, Bukhara and Tashkent. eking out a marginal existence through summers of heat as high as 150 degrees Farenheit, and harsh, dry winters marked by bitter winds that sweep the scant snow from the land. Total rainfall in the desert areas never ranges above a foot annually, and averages much lower.

In ancient times, some civilizations spread handmade irragation systems into the desert, but time and invasion effaced them. Now, from the air, the vistas are impressive: endless red-and-brown sand dunes and barren rock outcrops suddenly give way to watered green zones whose borders proclaim the end of the arid lands.

The greening of the deserts is powered by a host of canals, including the world's longest, the Kara-kum, bringing water from the Amu-Darya, one of the Central Asia's principal rivers, westward across almost a thousand miles of the 140,000-square-mile Kara-Kum ("black sands") desert to and beyond Ashkhabad in Soviet Turkmenia.

The irrigated lands of the Kara-Kum stretch like a verdant zone along the length of the canal, and immense controlling reservoirs provide desert peoples the rarity of an occasional swim.

Other major canals help water such important agricultural areas as the rich Fergana Valley in the Pamir Mountains shared by Kirghizia, Tadzhikistan and Uzbekistan; parts of the Kyzyl-Kum desert in Uzbekistan, and the Kara-Kalpak autonomous republic lying in the flat delta country of Uzbekistan's Turan lowland, where the Amu-Darya flows into the Aral Sea from the south.

The spread of water has bought boom times to the Central Asian republics, revolutionizing ancient life-styles and fattening both the coffers and aspirations of the republics' leaders.

The republics of Central Asia and neighboring Kazakhstan to the north now provide 95 percent of the raw cotton, 40 percent of the rice, 25 percent of the vegetable and melon crops and many of the grapes grown in the U.S.S.R., according to a recent report by Violet Conolly, a prominent Western specialist on the Soviet economy.

"All is dependent on irrigation." she notes.

Water is king. And the kingdom created out of irrigation canals, pumps and pipes grows yearly. Yet, there is deep trouble comming and the Soviets knows it. What they haven't decided is how to preserve the kingdom.

Everywhere, there is disturbing evidence that just as in the American Southwest, intensive irrigation and the inevitable accompanying waste of preicous water already has inflicted possibly irreversible ecological calamity in the name of industrial and agricultural development.

The Aral Sea, a unique, landlocked body of water in western Uzbekistan and Kasakhstan, has dropped more than six feet in recent years because of irrigation diversion from its principal sources, the Amu-Darya and the Syr-Darya, Central Asia's other major river. Soviet scientists warn that the Aral, once rich with important fish like the sturgeon, may dry up in the next few decades unless new sources can be found.

The Caspian, the world's largest inland sea on the boundary between Europe and Asia, also is drying up, the waters of its principal source, the river Volga, thickened and poisoned with pollution that has brought drastic drops in the valuable beluga sturgeon and salmon catches.

Leakage and overflows from badly designed, badly built irrigation systems have created vast brakish marshes where cultivated fields might stand.

Some scientists predict that continued shrinking of the seas will bring major new temperature fluctuations in the already harsh climate-higher highs and lower lows, combined with such bizarre events as salt rains and saltification of fields more than a hundred miles from the present shores of the Aral and Caspian.

Yet, the drive for more industry and agriculture presses forward, with the current five-year economic plan calling for ambitious new desert reclamation and irrigation projects as well as new industry.

Where will more water come from to prevent the collapse of an entire sea, and fuel the wave of prosperity?

Powerful central government bureaucrats are pushing a plan to tap the rivers of Siberia, half a continent away. They want to build a vast new water of the north-flowing Ob River south to Kazakhstan and Central Asia, a distance of 1,500 miles.

Their drive to begin the project, first suggested in czarist times, has touched off a rarely seen regional conflict between the "northerners" of Siberia and the "southerners" of Central Asia who are at deep odds over the proposal.

The conflict boils down to a clash of self-interest between two important region of the country, with a multi-billion ruble project that could take more than 20 years to complete as the fulcrum.

The Central Asians, fearful of the future, are pressing the canal scheme while the Siberians oppose it, saying they fear it may bring disastrous enviromental consequences for their region.

A. Babayev, director of the Turkmenian Academy of Sciences' Desert Institute, favors draining the 44,000 square mile Aral. "We would get excellent fertile lands in its zone," he asserted in an article. He said new lands would yield 1.5 million tons of cotton a year, more than enough to make up for the lose of Aral Sea fishing and shipping.

Other specialists warn the drained seabed would be a salt desert for decades, creating salt storms and sending clouds of harmful dried salt on the wind to ruin other fertile fields.

The proposed route for the canals would carry them near the Tyumen oil fields, among the most important energy production fields in the country. According to an article in Trud last December, Tyumen oil and gas men, hydrologists, ecologists, and even veterinarians, declare the plan "unacceptable" on grounds it would bring widespread flooding and marsh-building, stop river navagation on some of tributaries, disrupt fish spawning, cause climate changes and mean the destruction of huge forest and reindeer preserves.

I. Rusinov, director of the West Siberian Institute for Hydraulic Engineering and Land Reclamation, also declared in Trud, "A solution to the problem of improving Central Asia's water resources at the expense of the Ob should not mean that Siberia's own needs are forgotten." He proposed moving the diversion point farther north.

Although only three percent of the Ob's water would be diverted from its flow, some Soviet Arctic experts have warned that the reduction of fresh water flowing into the Kara Sea and Arctic Ocean of even that relatively small amount could have grave enviromental effects. Geographer S. L. Vendrv complained that such schemes unfairly threaten resources of western Siberia in favor of other regions.

Writing in Pravada last July, Soviet Academician N. Nekrasov and another scientist, N. Razin, warned that "the scientific research so far carried out is manifestly inadequate to provide acceptable answers to these urgent questions."

While the debate simmers, the project seems to crawl toward reality. Gosplan, the state planning committee, is now in a preliminary cost-study of the canal system. The government two months ago "endorsed" the project, according to the Tass official press agency. A Tass spokesman said this means no final decision was made but that planning continues.