One of these mega-droughts likely will strike again someday. And water managers and scientists need to be prepared, according to a new National Academy of Sciences report.

The report looked at studies that examined tree-ring data to assess drought conditions over the past several hundred years. Severe droughts were noted in the late 1500s and the mid-1800s before scientists began installing gauges along the Colorado River, on which most Western states rely heavily as a water source, Connie Woodhouse, a University of Arizona researcher involved with the report, said during a Wednesday tele- conference.

While models all project a warmer climate for the Colorado River basin, there is less agreement on the future of precipitation.

"We're not seeing a coherent message from models about precipitation," Woodhouse said.

Even without that agreement, a warmer climate alone can reduce water supplies, she said. Snowpack will melt earlier, and less of that runoff will make it to the Colorado River.

The report made no specific recommendations on how to deal with future crises. But it did say future droughts will force water managers and leaders to make tough decisions over allocations - such as how to balance urban needs with those of the agricultural community.

"Our hope would be that communities and the decision makers will have planned before crises occur," said Ernest Smerdon, a report author and emeritus researcher at the University of Arizona.

He stressed that given population growth, water conservation will not be enough.

"The technological and conservation options, although useful and necessary, will not in the long run be a panacea," Smerdon said.

Another challenge in dealing with this issue is that the original Colorado River Compact was signed in 1922, a particularly moist period in the basin's history. This poses questions about whether the long-term allocations, made during a bountiful period of the river's history, were realistic.

The report also mentions government projections made in 2003 that places such as Salt Lake City and Denver may not have enough water to meet all their needs by 2025.

"I am not predicting doom," Smerdon said of the committee's report, but "there are critical issues that have to be addressed."

Dennis Strong, director of the Utah Division of Water Resources, said the report serves as a reminder that Utahns live in a desert amid climate variability.

State officials have water resources identified to handle Utah's needs for the next 50 years. But Strong cautioned that global warming has not been taken into account in the current plan. If global warming becomes more apparent, water managers will have to modify the plan.

More draconian measures may be needed if a severe drought settles in across Utah, he said.

"We have those very 'drop dead, we don't want to do this options' that will allow us to have water," he said.

Examples include dramatic restrictions on lawn watering or switching to native plants that don't need irrigation.

In an extreme, extended drought, government leaders could potentially limit population growth in certain areas. But if water supplies become too scarce, people may simply choose to move to wetter destinations, he said.

These changes won't happen overnight, so leaders will likely have time to react to changing circumstances, he said.

The report is available at www.nationalacademies.org.

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* GREG LAVINE can be contacted at glavine@sltrib.com or 801-257-8620.