TELL AL-SAMEN, Syria The Euphrates River is close by, but the water does not reach Abdelrazak Aween. Here at the heart of the fertile crescent, he stares at dry fields. The Syrian government has promised water for Aween's tiny village. But upstream, in Turkey, and downstream, in Iraq, similar promises are being made. They add up to more water than the Euphrates holds. So instead of irrigating his cotton and sugar beets, Aween must siphon drinking and washing water from a ditch 40 minutes away by tractor ride. Just across the border, meanwhile, Ahmet Demir, a Turkish farmer, stands ankle deep in mud, his crops soaking up all the water they need. Now, across a widening swath of the world, more and more people are vying for less and less water, in conflicts more rancorous by the day. From the searing plains of Mesopotamia to the steadily expanding deserts of northern China to the cotton fields of northwest Texas, the struggle for water is igniting social, economic and political tensions. The World Bank has said dwindling water supplies will be a major factor inhibiting economic growth, a subject being discussed at a weeklong international conference in South Africa starting Monday about balancing use of the world's resources against its economic needs. Global warming, some experts suspect, may be adding to the strain. Droughts may be extended in already dry regions, including parts of the United States, as wetter areas tend toward calamitous downpours and floods like those ravaging Europe and Asia this summer. In general, the world's climate may be more prone to extremes, with too much water in some areas and far too little in others. Both the United Nations and the National Intelligence Council, an advisory group to the CIA, have warned that the competition for water is likely to worsen. "As countries press against the limits of available water between now and 2015, the possibility of conflict will increase," the National Intelligence Council said in a report last year. By 2015, according to estimates from the United Nations and the U.S. government, at least 40 percent of the world's population, or about 3 billion people, will live in countries where it is difficult or impossible to get enough water to satisfy basic needs. "The signs of unsustainability are widespread and spreading," said Sandra Postel, director of the Global Water Policy Project in Amherst, Massachusetts. "If we're to have any hope of satisfying the food and water needs of the world's people in the years ahead, we will need a fundamental shift in how we use and manage water." An inescapable fact about the world's water supply is that it is finite. Less than 1 percent of it is fresh water that can be used for drinking or agriculture, and demand for that water is rising. Over the last 70 years, the world's population has tripled while water demand has increased sixfold, causing increasing strain especially in heavily populated areas where water is distant or is being depleted or is too polluted to use. Already, a little more than half of the world's available fresh water is being used each year, according to one generally accepted estimate. The proportion could climb to 74 percent by 2025 based on population growth alone, and would hit 90 percent if people everywhere used as much water as the average American. Water tables are falling on every continent, and specialists warn that the situation is expected to worsen significantly in years to come. On top of the shortages that already exist, the outlook adds to the tensions and uncertainty for countries that share water sources, like Turkey and Syria, where Aween is among those still waiting and hoping for the Euphrates to be brought to his door. The stories of Aween and Demir illustrate how the intensifying fight for water can make or ruin lives. Until last year, Demir, 42, a father of nine in Turkey, was living an itinerant life as a smuggler and a migrant laborer. But on a recent scorching afternoon, he stood sunburned and content, his striped pants rolled above his knees, bare feet squishing in Euphrates mud. "It seems like we have all the water we need," Demir said, leaning on his shovel and running a hand through his close-cropped hair. What has changed in this swath of southern Turkey is the arrival of irrigation. It is part of one of the world's largest water projects, a $30 billion plan by the Turkish government to spread the Euphrates' gifts across a vast and impoverished region of the country. By now, Aween, the Syrian, might have been celebrating, too. Under Syria's irrigation program, ambitious in its own right, water from the Euphrates should have reached Aween's door, less than 80 kilometers, or 50 miles, from Demir's. But strong doubts are emerging about whether the vast scope of Turkey's project will leave enough water for its neighbors downstream - so much so that Syria appears to have put the brakes on its development plan. "We're still waiting," Aween, 40, said on behalf of his two wives, three children, and 17 brothers and sisters, who all live in a hamlet that bears the family name. He wore a loose, Arab-style outer garment and cheap plastic sandals as he hitched a rusty tanker trailer to a sputtering tractor, his water bearers. "But the water hasn't come." The trouble over the Euphrates can be expressed in a simple, untenable equation. As best as anyone can determine, the river, in an average year, holds 35 billion cubic meters of water. But the separate plans drawn up by Turkey, Syria and Iraq for building dams and irrigating fields would, taken together, consume nearly one and a half times more water than the river holds. Each country has acknowledged the impossibility of marrying its irrigation programs with others. But none have shown any willingness to scale back. Trying to accommodate fast-growing populations and to head off a migration to the cities, each country is still clinging to its irrigation dreams. On both sides of the Turkish-Syrian border, snapshots of those dreams unfold on summer dawns, in fields that used to be good for little but grazing, but where new irrigation canals are now delivering Euphrates water in regular supply. Thirsty crops like cotton and sugar beets have begun to thrive. Farm incomes have tripled. Young women tend prized plants with special care, shepherding the water down each muddy row. Young boys cavort in irrigation ditches that provide relief from the intense midday heat. Now that there is water, Demir said, it would not be such a bad thing if his four sons, ranging in age from 4 months to 22 years, decided to stay and work the land, something he could not have imagined only a year ago when farming was far harder. But in a world in which so much depends on having water, he bristled at the idea of sharing it. "If I used any less, the others would use more," he said. "I use what I need, and as for the rest, it's their business."