Drought in Colorado River Basin best explained by temperatures -

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.

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.

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Taxonomy

• Drought
• Climate Change
• Water Losses
• Evaporation