STOCKHOLM (Reuters) - With a steady stream of bleak predictions that "water wars" will be fought over dwindling supplies in the 21st century, battles between two Sumerian city-states 4,500 years ago seem to set a worrying precedent. But the good news, many experts say, is that the conflict between Lagash and Umma over irrigation rights in what is now Iraq was the last time two states went to war over water. Down the centuries since then, international rivals sharing waters such as the Jordan River, the Nile, the Ganges or the Parana have generally favored cooperation over conflict. So if history can be trusted, things may stay that way. "The simple explanation is that water is simply too important to fight over," said Aaron Wolf, a professor at Oregon State University. "Nations often go to the brink of war over water and then resolve their differences." Since the war between Lagash and Umma, recorded on a stone carving showing vultures flying off with the heads of defeated Umma warriors, no wars have been fought and 3,600 international water treaties have been signed, he said. Yet politicians regularly warn that water shortages caused by surging populations and climate change could trigger conflicts this century in a world where a billion people in developing countries lack access to clean drinking water. "Fierce competition for fresh water may well become a source of conflict and wars in the future," U.N. Secretary-General Kofi Annan said in 2001. The English word "rival" even comes from the Latin "rivalis" meaning "someone sharing a river." Other experts say international "water wars" are unlikely. "I don't really expect wars over water because ... the benefits of collaboration are so great," said Frank Rijsberman, head of the International Water Management Institute (IWMI). And still others say water might be one factor in future conflicts. Achim Steiner, executive director of the U.N. Environment Programme (UNEP), says this is particularly true in border regions where countries share rivers. "I am not somebody who believes that our third world war will be over water, but I think the potential for conflict will grow as we are faced with water scarcity," he told Reuters. SCARCITY WORSENS? Rijsberman led a U.N.-backed report in August that said one in three people lives in a region where water is scarce and that demand could almost double by 2050 -- led by farming which absorbs 74 percent of all freshwater used by humans. Planting extra crops to produce biofuels and global warming -- which could bring more erosion, droughts and floods -- could add new pressures, the report said. But it added that there was enough water to go around, with better planning. "If there is a war between two countries the 15th reason could be water but the first 14 reasons will have absolutely nothing to do with water," said Asit Biswas, head of the Third World Center for Water Management in Mexico City. "But if I want to get in the media the easiest thing is to say that a water war is about to break out in the Middle East," he said. "The last war over water was thousands of years ago." A problem, he said, was that water was often viewed as a commodity like oil, which cannot be re-used. Water in the Colorado River, for instance, can get used seven times for hydropower, drinking water or irrigation. The academics' view is not shared everywhere. "If we don't address the water issue in the Middle East in a coherent way there will be a war. There's scarcity and when it comes to water it's a matter of life," said Shadad Attilik, a Palestinian who conducts water negotiations with Israel. He said vital aquifers in the Gaza Strip were being polluted and causing health problems. "If you see a Palestinian with yellow teeth you know he comes from Gaza," he said. Experts note that violence over water often breaks out within countries -- over rivers, lakes, oases or wells. In Kenya, dozens of people died early this year in fighting between nomadic tribes over scant water and grazing rights. Tamil Tiger rebels were accused of shutting off sluices in Sri Lanka in August in their separatist war with government forces. Steiner said countries most vulnerable to water scarcity included already conflict-prone Chad, Sudan and Somalia, as well as Ethiopia, parts of Pakistan, south India and China. "We must work very fast in the next few decades to ensure that nations have a shared approach to deal with water scarcity," he said, calling this a priority for UNEP. HOW WOULD YOU WIN? In the five decades to 1999, Wolf's research (http://www.transboundarywaters.orst.edu/) indicates there have been no wars and just 37 military acts over water between states -- 30 of them involving Israel and its neighbors. Among signs of cooperation, Israel and Jordan held secret talks about managing the Jordan River from the 1950s, even when they were technically at war. The Indus River commission kept going despite wars between India and Pakistan. Among military acts, Israel in the 1960s destroyed Syrian construction on the headwaters of the Jordan River which was part of a project to divert waters for an "all-Arab" water plan. But those predicting future "water wars" should also consider another problem: how do you secure victory? "If you conquer territory to gain control over a river you still have to provide water to people living there," said Anders Jaegerskog of the Stockholm International Water Institute. "It's very difficult to imagine how you win a water war."