Payson may have locked in its share of Blue Ridge water just in time.
That’s because the flow of the Colorado River and its tributaries will likely drop by 20 percent in the next few decades, according to a U.S. Geological Survey analysis of 19 different climate models.
That decrease could spell big trouble for the 25 million people who use water from the river and its reservoirs, which also irrigate 2.5 million acres of farmland. The number of people in the region is expected to increase to 38 million by 2020.
The metastudy by the USGS underscores several other, recent studies suggesting that rising average global temperatures have already shifted storm tracks north, resulting in a big drop in average rainfall in the southwest in the late spring — especially April.
This year already supports that finding, with flows this week in the Salt River as it enters Lake Roosevelt running at less than half of normal.
The USGS study of a host of climate computer models concluded that average temperatures in the region will rise by 5 degrees F in the course of this century, which will dry out the soil, cause a big increase in dust storms, reduce the vital snowpack and spur an earlier and less useful spring runoff.
Moreover, tree ring data suggests that the plentiful rains of the 20th Century on the watershed actually made it the wettest century in the region since the last Ice Age some 10,000 years ago. So, when planners divided up the flow of the Colorado River among seven states, they dramatically overestimated the average flows.
As a result, by 2070 the river will not produce enough water to provide the full, promised allotment at least 40 percent of the time.
That’s bad news for Arizona, whose rights to water from the Colorado River are generally behind other states like California — which monopolizes most of the flow of the river.
Therefore, many Arizona cities will likely have to scramble for enough water to keep up with projected growth.
However, Payson will likely start receiving 3,000 acre-feet of water from the Blue Ridge Reservoir atop the Rim within about two years. That flow will double the town’s water supply, making it one of the few cities in Arizona will enough water to serve all its projected future needs.
The latest USGS attempt to sort through the results of 19 different climate models offered a sobering look at the region’s climatic future.
The USGS researchers generally discounted several climate models that predicted a much more severe 45-percent decline in Colorado River runoff.
However, the likely decline in runoff of 5 to 20 percent by 2050 provided plenty to worry about.
Rising temperatures would reduce rainfall and dry out the soil so effectively that the average conditions would resemble the worst droughts of the 20th century, including the Dust Bowl era from 1953 to 1956 and the more recent, severe drought from 1999-2004.
The dry soils will soak a much larger share of the reduced rainfall. For instance, in 2005 the region received normal rainfall — but the drought years that came before dried out the soil so much that the actual runoff remained only 75 percent of normal.
Moreover, the dried out soils will generate a big increase in dust storms. Those storms in turn deposit more dark, sun-absorbing dust on normally bright, reflective snow on mountain tops. This covering of dust absorbs heat, causing a quicker, earlier snowmelt, which ultimately reduces the average flow of the river.
Ironically, that warming trend may effectively return the region to more normal conditions, which the sopping wet 20th Century interrupted.
The study of growth patterns in tree rings going back thousands of years revealed severe droughts in the 11th, 14th, 15th and 16th centuries — some lasting as long as 24 years. By contrast, the longest drought in the more recent historical record lasted just 14 years.
One study of water levels in Alpine Lakes suggested the 20th Century may have represented the wettest extended period in 5,000 to 12,000 years. Another study showed that 5,000 to 9,000 years ago the tree line was more than 200 feet higher than it is today — additional evidence that the 20th Century was unusually wet.
As the climate heats up and shifting storm tracks affect the depth and timing of the snowpack, the climate will likely generate all kinds of complicated feedback effects, the researchers concluded.
Just the timing of the snowmelt will have a big impact on how much water flows down the river — and ends up in the chain of giant reservoirs on which many of the 38 million people in the region will depend by 2020. One study suggested that early snowmelts yield 7 percent less runoff overall than later snowmelts.
The rise in temperatures and decline in runoff will spur all sorts of complications, like an increase in pine beetle infestations in dehydrated trees.
Already, infestations caused by the recent drought years are killing most of the trees in a three-million-acre swath of Colorado forests.
The death of so many trees, in turn, causes an increase in flooding and sedimentation in streams, reducing overall, long-term yield. For instance, studies show a 60-percent increase in the phosphorus content in streams where bark beetles have killed many of the trees in the watershed.
A separate study by scientists from the USGS and the University of California Los Angeles concluded that the changes have already started to take place with a sharp drop in perennial vegatation and grasslands in the past 20 years in Arches and Canyonlands National Parks in Utah.
The shift has resulted in a sharp increase in dust storms, modified to some degree by the moses, lichens and cyanobacteria that grow in the soil and form a protective crust.