Why drought alone does not explain Lake Mead’s expanding bathtub ring
by Allen Best
It’s been a good runoff this year in the Rocky Mountains, the Colorado River still swelling in size last weekend as it flows out of Rocky Mountain National Park, flexing muscle as the Eagle, the Gunnison and other tributaries augment its flows on the way toward Utah.
So why is Lake Mead continuing to decline? The giant storage reservoir, located near Las Vegas, in May dropped to its lowest level since 1937, the year after Hoover Dam was completed.
“Drought” is the usual answer. “Over-appropriation” is another. But an increasing body of evidence points to a more complicated story: the water is literally going up into the air.
That’s the thesis of Brad Udall, a senior water and climate scientist at the Colorado Water Research Institute. “Climate change is water change,” he said in a presentation at the Martz water conference held last week at the University of Colorado-Boulder.
The Colorado River Basin has had horrendous droughts this century. The summer of 2002 toppled records hundreds of years old. There have been big snow years, too. Taken altogether, the period from the 2000 to 2015 had lower snowpack, meaning drought.
Still, this drought was only a little worse than those of earlier periods, such as in the late 40s and the 1950s, said Udall. Yet the water flowing down the river is significantly less.
“What is making these droughts so much worse?” he asked. Rising temperatures, he answered. Rising temperatures increase the evapo-transpiration rate. Things get hotter, and more water evaporates. It gets hotter, and planets need more water.
“It’s really pretty simple,” he said.
This has important implications for the Colorado River and its tributaries, as well as the roughly 40 million people who derive at least part of their water from the basin. Colorado River Basin waters also irrigate 5.5 million acres of farmland and, thorough hydroelectric turbines at Hoover, Glen Canyon and other dams, has generating capacity of 4,200 megawatts.
Those statistics came from Lawrence MacDonnell, senior fellow at the Getches-Wilkinson Center for Natural Resources, Energy, and the Environment. He painted a picture of a river system being asked to do more than it is capable of. “Our allocation exceeds our supply,” he said in explaining the various compacts and entitlements governing withdrawals form the river by the states, Mexico and Native American tribes.
The various dams on the river have the capacity to store four times the Colorado River’s annual flow, but the flows have been decreasing.
“We have climate scientists telling us that we will have a reduced supply of water in the future, even as there are others who say they will use more water from the Colorado River Basin,” he said. “Where is the reality?”
MacDonnell pointed to conservation as critical. “Some impressive work has been done, particularly by cities to manage urban water demand through conservation,” he said.
As agriculture remains the dominant use of water in all states, MacDonnell sees reduced ag water as inevitable. “No doubt about it, there will be some retirement of agriculture,” he said.
A “structural deficit” is often mentioned in Colorado River discussions. The deficit is inherent in the allocations of water out of Lake Mead, said Amy McCoy, of the University of Arizona. Again, the idea is that more water is assumed in allocations than actually exists.
While deliveries have been made, Lake Mead is being drawn down by 1.2 million acre-feet per year. Measured on the walls of Hoover Dam, that means a decline in the water level of 12 feet per year.
A giant slice of water from the Colorado is transferred to Phoenix-Tucson areas via the Central Arizona Project. Unlike most areas of the West, agriculture in that delivery project has lowest priority. But curtailment of water deliveries would also affect Native American tribes and even cities.
Taylor spoke of the inevitability of curtailed water deliveries, a shrinking of the circle, and the need to “share the burden, share the water.”
In a way, that’s what Udall was also talking about. He alluded to a Bureau of Reclamation report from earlier this year that predicted substantially the same amount of water in the Colorado River through the 21st century.
“Do you believe that? I don’t,” he said. Again, he drew attention to the 16 years of drought that can only partially be explained by reduced snowfall and other precipitation.
What is needed, he suggested, is a proper appreciation of risk when making decisions about water—including, as climate scientists have warned, that much hotter temperatures will mean even less water in the Colorado for the 40 million people who depend upon it.
Nobody really knows whether the changing climate will produce more water or less. The growing evidence is that the climate is already changing and, as Udall demonstrated, the upshot is less water.