Rising temperatures explain drop in reservoirs better than drought
by Allen Best
GRAND JUNCTION, Colo. – The Colorado River originates in Rocky Mountain National Park in Colorado, gathering water from tributary rivers that arise near Winter Park and Breckenridge, Vail and Crested Butte.
A little more than halfway on its 1,450-mile route to the Pacific Ocean, the Colorado River gets blocked by a giant slab of concrete called Hoover Dam. This dam creates Lake Mead, the primary water source for Las Vegas.
Since 2000, water levels have declined in Lake Mead and the other major Colorado River impoundment, Lake Powell. The usual explanation is drought. Certainly, there have been some very snow-deficient winters, and at one point the Southern Nevada Water Authority decided that its two tunnels into Lake Mead might not be enough if the reservoir declined further. So, a 3-mile tunnel was engineered to come in at the very bottom of the reservoir.
That tunnel, completed at a cost of $817 million, was unplugged in late September, giving Las Vegas a resource in case the reservoir empties. Engineers compared the challenge of the work to construction of the Eisenhower-Johnson Tunnels for Interstate 70 in Colorado. Those two-mile-long tunnels are at over 11,000 feet in elevation.
Speaking at a recent conference sponsored by the Colorado Mesa University Water Center, Doug Kenney warned against thinking that the drought will end.
“There’s a lot of thinking that when the drought ends, the reservoirs will come back,” observed Kenney, a research fellow in western water policy at the University of Colorado-Boulder’s Getches-Wilkinson Natural Resources Law Center.
Kenney also pointed out that over the last 15 years, the good years and bad years of snow have more or less evened out. The total precipitation has declined only a few percentage points from the longer-term average.
Drought is only third on the list of what explains the declining reservoir levels in the Colorado River Basin, he observed. The larger story is that demand has now outstripped supply. Las Vegas, for example, exceeded the population of Manhattan about a decade ago and now has two million people.
But there’s also a second reason why the levels have been declining, said Kenney. Temperatures in the Colorado River Basin have already been rising, causing greater evaporative losses, both in the soil and from reservoirs.
These rising temperatures have broad implications: hay, corn, and cotton crops need more water, and soils dry out more readily. “The warming climate affects the water cycle in ways that are problematic for the basin,” he said.
Dagmar Llewellyn, of the U.S. Bureau of Reclamation, said that rising temperatures predicted by climate models will increase demands for water used by agriculture and municipal lawn watering.
But hotter temperatures will also increase evaporation of existing reservoirs, such as Elephant Butte, on the Rio Grande in New Mexico, which already loses a quarter of its annual storage to evaporation.
Better storage mechanisms will be needed as the climate warms, she said, and suggested that recharge of a partially depleted aquifer underlying Albuquerque might be one answer.