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The Colorado River Basin likely will lose about 9 percent of its annual runoff by mid-century because of a warming climate, further squeezing Utah and its neighbors in a region that already expects to struggle getting water to its growing population, according to a U.S. Interior Department report released Monday.

Bureau of Reclamation scientists calculated likely regional temperature and precipitation models based on a range of possible carbon dioxide emissions, then used the mean of the results to predict an 8.5 percent reduction in water supply. The report actually predicts a 2.1 percent increase in precipitation for the Upper Colorado Basin — of which Utah is a part — but temperatures 5 to 7 degrees warmer than today's are expected bring more rain than snow, and enough more evaporation to sap the supply.

In the Lower Colorado Basin, precipitation is expected to decline 1.6 percent.

"Water is the lifeblood of our communities, rural and urban economies and our environment," Interior Secretary Ken Salazar said in a statement, "and small changes in water supplies or the timing of precipitation can have a big impact on all of us."

The report, a requirement of the Secure Water Act of 2009, projects wetter conditions in the Northwest and north-central United States, and further drying out in the Southwest. Even where moisture increases, winter rains — in place of snows — may decrease springtime and summer runoff for irrigation.

The region's smaller reservoirs may have to alter operations to keep water available late in the summer, Bureau of Reclamation hydraulic engineer Katrina Grantz said. But even in parts of the Interior West, where more rain falls in the future, the increased evaporation is likely to reduce river flows.

"Temperature is a huge driver," she said.

Salazar said affected areas should use the report as a baseline for planning.

Utah taps about 1 million acre-feet of Colorado River watershed water a year, with some of that piped to the urban Wasatch Front. Under an interstate compact, the state has rights to another 400,000 acre-feet, though about half of that is reserved for the Ute and Navajo tribes. A planned pipeline from Lake Powell to St. George would bite into the remainder, as would population growth that's expected to need another 40,000 acre-feet — each acre-foot being about what it takes to supply a family for a year.

That leaves a lot of uncertainty for potential water applications in eastern Utah's oil shale fields, or other development, even without a drying river system.

"You've got lots and lots of competition for a little bit of water," said state Division of Water Resources Director Dennis Strong.

Urban Utah has room to change its habits without hurting its quality of life, Strong said, though it could mean more decorative rocks and fewer sprawling lawns.

Two-thirds of Utah's municipal water is applied to landscaping instead of in-home uses, a fact that accounts for the state more than doubling the per-capita water consumption seen in most of the East. And about 80 percent of Utah's total water supply goes to farms, which increasingly sell some of their water rights to municipal systems as homes displace fields.

"We've got a lot of grass and trees," Strong said. "We can change that if we have to. We can change it slowly."

The state's goal is a 25 percent reduction in individual use by 2050 through conservation, a goal that may get simpler if scarcity raises the price of water. Meeting that goal likely provides for 60 percent of expected population growth without any new water projects, Strong said. The rest will have to come from the Colorado and Green rivers, the Bear River, and from conversion of agricultural rights.

Some environmental advocates fear that a natural system already damaged by dam construction in the past century will bear the brunt of reduced supply in this one. River managers tend to base decisions on resource uses — farms, cities, hydropower — while neglecting national parks carved by the rivers, said David Nimkin, Southwest region director for the National Parks Conservation Association.

The result in the Grand Canyon, for instance, is a loss of sand for recreational beaches and endangered fish habitat.

The parks have a legal responsibility to protect their resources, Nimkin said, and he hopes water managers will factor them in to climate-caused adaptations.

"What is often not adequately considered," he said, "are the impacts on ecological resources."

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Flows hurting parks

Even before significant climate change, the Southwest's use of the Colorado River and its tributaries has treated the region's riverine national parks harshly, according to a report released Tuesday by the National Parks Conservation Association.

Dams in the basin — including the big hydropower projects at Lake Mead and Lake Powell on the Colorado and Flaming Gorge on the Green — have leveled off floods that benefit the native fish, wildlife and vegetation.

"The natural systems were really defined by this seasonal flow and fluctuation," said David Nimkin, regional director for the association. Now, fluctuations come throughout the summer, with air-conditioning and other needs, and are leveled off from their historic peaks.

For instance, according to data compiled by the association, Flaming Gorge Dam has cut peak flows through Dinosaur National Monument from 10,900 cubic feet per second (cfs) to 5,300. Base flows the rest of the time have roughly doubled, from 500 cfs to 1,000.

Downstream of Glen Canyon Dam, where the Colorado enters the Grand Canyon, peak springtime flows have dropped from 79,000 cfs pre-dam to 30,800 now. The yearlong base flow, though, altered by hydropower needs throughout the summer, has shot up from 3,000 cfs to, at times, 25,000.

This, along with alterations in sediment movement, create wide-ranging threats to fish, wildlife, recreation and cultural sites, according to the report, which is available at