From Arizona Central (Ian James):
Scientists with the U.S. Geological Survey found that the loss of snowpack due to higher temperatures plays a major role in driving the trend of the river’s dwindling flow. They estimated that warmer temperatures were behind about half of the 16% decline in the river’s flow during the stretch of drought years from 2000-2017, a drop that has forced Western states to adopt plans to boost the Colorado’s water-starved reservoirs.
Without changes in precipitation, the researchers said, for each additional 1 degree C (1.8 degrees F) of warming, the Colorado River’s average flow is likely to drop by about 9%.
The USGS scientists considered two scenarios of climate change. In one, warmer temperatures by 2050 would reduce the amount of water flowing in the river by 14-26%. In the other scenario, warming would take away 19-31% of the river’s flow.
“Either of the scenarios leads to a substantial decrease in flow,” said Chris Milly, a senior research scientist with USGS. “And the scenario with higher greenhouse-gas concentrations decreases the flow more than the scenario with lower greenhouse gas concentrations.”
The findings, which were published Thursday in the journal Science, refine previous estimates and indicate the impacts of warming will likely be on the high end of what other scientists calculated in previous research…
Looking at trends over the past century, the researchers examined recorded measurements from 1913-2017 and found the average temperature across the Upper Colorado River Basin increased by 1.4 C (2.5 F) and the river’s flow decreased by about 20%.
They estimated that more than half of this lost flow was attributable to higher temperatures. That equates to a loss of roughly 1.5 million acre-feet of water per year…
Milly and fellow USGS scientist Krista Dunne zeroed in on their estimate by pinpointing the reflectivity of snow, known as albedo, as a key element in the river’s sensitivity to warming.
They used measurements of albedo across the Upper Colorado River Basin recorded over decades by instruments called MODIS (short for Moderate Resolution Imaging Spectroradiometer), which orbit the Earth aboard two NASA satellites.
Milly and Dunne focused on the role of snow cover as a “protective shield” for water in the river basin.
Milly likened the flowing river to the leftovers of the “meal” of snow and rain that falls across the basin, after evaporation has “eaten” its share.
“The more that’s consumed by evaporation, the less that’s left for the river and the people downstream,” Milly said.
And the amount consumed by evaporation is driven by how much energy the basin absorbs in the form of sunlight. The snow cover in the Rocky Mountains reflects back to the sky and space a significant fraction of the sunlight.
As the world gets hotter with the burning of fossil fuels, more of the precipitation falls as rain instead of snow. And the snow melts away earlier in the year. As the snow cover in the mountains is progressively lost, the river basin absorbs more energy.