Invoking rising temperatures to explain drought in Colorado River
by Allen Best
The drought in the Colorado River Basin in the 21st century has been phenomenal. There was that harrowing year of 2002, which broke records for low streamflows going back 150 to 300 years. Even now, the levels of water in the two big reservoirs on the river, Powell and Mead, continue to decline.
And then there’s California, epic in its drought.
But here’s something to confuse the storyline. Precipitation in these droughts alone fails to explain the droughts, as we conventionally think about them. California’s Sierra Nevada last winter got 40 to 90 percent of normal precipitation. That would suggest a drought, yes, and a memorable one. But this drought was defined as the worst in 1,200 years.
Declining reservoir levels in the Colorado River Basin this century similarly can’t be explained simply by lack of precipitation. There have been some good snow years, too. Yet Lake Mead has fallen from 91 percent of capacity in 2000 to just 35 percent of capacity now.
What’s going on? Global warming, say scientists. The story is of rising temperatures. In the Sierra Nevada last winter, the average temperature was above 32 degrees Fahrenheit, the first time in 120 years of recorded history. This, in turn, influences —and reduces—runoff.
The increased warmth results in more moisture sublimating directly out of the snowpack into the atmosphere, more water in creeks, lakes, and reservoirs evaporating, and plants requiring more water because they transpire moisture at higher levels into the atmosphere.
“Repeat after me: precipitation is not runoff,” instructed Brad Udall, a climate research scientist at the Colorado Water Institute, at a recent presentation.
A hydrologist by training, Udall has been boring down at the intersection of water and climate change in recent years. He pays particular attention to the Colorado River, where he once was a river guide in the Grand Canyon. But he points to the Rio Grande as probably the most stressed river in the West.
“Climate change is water change. The two go hand in hand,” he said in a lecture at the Chautauqua Community House in Boulder, Colo. “Heat drives the water cycle.”
The Intergovernmental Panel on Climate Change has reported all kinds of observations that are consistent with climate change. Seven of 10 were water-recycle related. For example, sea-surface temperatures have gone up by about 0.5 degrees Fahrenheit since 1950. Water vapor in the atmosphere has increased by about 5 percent, consistent with projections.
Greenhouse gas emissions, he said, exacerbate existing drought sequences. One report, by Benjamin Cook and others, found that probability of a drought lasting 35 years or longer in both the Central Plains and the Southwest exceeds 80 percent in the 21st century.
Udall, working with Jonathan Overpeck, a climate scientist at the University of Arizona, has been studying runoff in the Colorado River, to understand the broader story of supply and demand, droughts, and falling reservoirs.
The current 15-year drought in the basin, he says, has had only 40 percent of the precipitation decline associated with a similar drought of the 1950s.
Why isn’t the water in the river? It’s because of rising temperatures.
“You have to invoke temperatures to explain the current drought,” Udall said.
It’s 1.6 degrees C warmer already, and climate models show that every degree C increase in warming will see about a 6.5 percent decrease in flows.
“Over the course of a century, you are looking at 20 percent to 30 percent losses (in river volume) due to temperature, and some models show up to 80 percent,” he said.
The Rio Grande looks to have the largest threat to flow reductions in the United States, perhaps 50 percent. It has a small collection basin in the San Juan Mountains of Colorado before flowing south into arid New Mexico and along the border of Texas and Mexico.
Climate change will slam the Rio Grande. One study projects annual reservoir storage in the basin will drop 55 percent by century’s end.
Udall also had this to say:
- Dams are not the entire answer, or maybe even much of an answer, to the need for storage. “There is a logical limit to what you can do to dams,” he said. “It’s like building more highways in Denver or building more interstates.”
The United States has 98,000 dams, including 8,000 big ones, and while they store water, they have adverse consequences: they inundate riparian areas, they water water, they change the river flows, and they hold back sediments.
Udall sees aquifers as a preferred place to store water. Arizona already has a robust aquifer recharge program, and water districts in the southern part of metropolitan Denver have also started recharging aquifers.
- Rain barrels probably make sense. In Colorado, it’s still illegal to capture rain falling off your roof. That’s because Colorado’s prior appropriation doctrine requires that all water diversions, no matter how tiny, be adjudicated through courts. This astounds people, as the barrels don’t capture all that much water. The value of rain barrels, said Udall, is that they can cause people to think about water.
- Desalinization can be done, but is extremely expensive. Cloud-seeding may offer just a sliver of increased snowpack. There are no magic bullets.
- Undergoing severe drought, Australia upended its entire water allocation system. Will Colorado and other Western states ever upend their prior appropriation doctrine? Probably not, Udall said.