The CAP

The Central Arizona Project system diverts Colorado River water to cities, farms and tribes in Central and Southern Arizona.

Warmer weather has reduced streamflows in recent years in the Upper Colorado River Basin that supplies Arizona and six other states with drinking water, a new study concludes.

Looking at more than a century of records, the study drew a link between temperatures and runoff. It shows overall that in years where spring temperatures are unusually cool or unusually warm, stream flows end up being more or less, respectively, than one would expect based only on the amount of wintertime precipitation that falls.

In a finding that has ominous overtones for future water supplies in the West, the study shows that in years when river flows have been particularly low, temperatures have been very warm.

The finding matches what many experts have said in recent years: That our warming temperatures are exacerbating the impacts of the West’s ongoing drought on river flows. Computer models have also predicted that spring temperatures can influence streamflow, but this is the first study to show the link between the two based on historical river runoff records dating back 110 years, said Connie Woodhouse, a University of Arizona professor who was the study’s lead researcher.

The study’s findings mean that if temperatures keep warming over the coming decades as many climate researchers have projected, there will be less runoff in the Colorado, said Woodhouse, a professor of geography and development and of dendrochronology.

The spring-summer runoff totals in the Upper Basin, in particular, are all-important to Arizona and the Southwest. The Colorado River serves close to 40 million people living in the seven basin states. The runoff is the leading force driving how much water gets released from Lake Powell at the Arizona-Utah border to drinking-water reservoir Lake Mead at the Arizona-Nevada border.

In the last six years, the drought and low runoff have twice brought the region to the edge of shortages in the river that could have triggered cutbacks in deliveries of Central Arizona Project water to Arizona farmers. (The CAP diverts Colorado River water via canal to central and Southern Arizona.) But the region and state were bailed out by heavy rainfall and snowfall in 2011 and by the wettest May in recorded history last year.

The study examined temperature, rainfall and Colorado River runoff records dating from 1906 to 2012. That was the last year for which the researchers could obtain runoff totals from the U.S. Bureau of Reclamation, said Woodhouse. The runoff totals were calculated at Lee’s Ferry just below Glen Canyon Dam and above the Grand Canyon.

Over the entire period studied, the amount of Upper Basin wintertime precipitation explained about 65 percent of the variability in individual years in the amount of streamflow.

“From the mid-1980s on, that’s where we saw a much greater number of years with less flow than we might expect given the precipitation, and (a lot of that) was explained by warmer weather,” she said.

The study didn’t try to determine why warmer weather reduces streamflows, “but if you think about it, warmer temperatures in March and April can cause more melting of snowpack in high mountains and more evaporation of surface water. That’s not what we researched but it’s what other studies are finding,” Woodhouse said.

In comparing the legendary drought of the 1950s in the West to the region’s ongoing drought of the 2000s and 2010s, the researchers found that the 1950s was the driest and coolest period in the historic record, Woodhouse said. By contrast, the 2000s have brought the least dry drought on record but also the hottest drought on record. In both cases, the river flows have been similar, she said.

The study was published online this week in the journal Geophysical Research Letters.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter: @tonydavis987.

Tony graduated from Northwestern University and started at the Star in 1997. He has mostly covered environmental stories since 2005, focusing on water supplies, climate change, the Rosemont Mine and the endangered jaguar.