In this April 16, 2013 file photo, a "bathtub ring" marks the high water mark as a recreational boat approaches Hoover Dam along Black Canyon on Lake Mead, the largest Colorado River reservoir, near Boulder City, Nev. Scientists say global warming may already be shrinking the Colorado River and could reduce its flow by more than a third by the end of the century. (AP Photo/Julie Jacobson, file)
Global warming is shrinking river vital to 40,000,000 people
March 01, 2017
Global warming is already shrinking the Colorado River. It is the most important waterway in the American Southwest. Rising temperatures could reduce the flow by more than a third by the end of the century. This is according to two scientists.
The river's volume has dropped more than 19 percent during a recent drought. It has gripped the region since 2000. A shortage of rain and snow can account for only about two-thirds of that decline. This is according to the hydrology researchers. Brad Udall teaches at Colorado State University. Jonathan Overpeck teaches at the University of Arizona.
Their study was published in the journal Water Resources Research. The scientists concluded that the rest of the decline is due to a warming atmosphere. It has been induced by climate change, they believe. The change is drawing more moisture out of the Colorado River Basin's waterways, snowbanks, plants and soil. The reduced moisture is because of evaporation and other means.
Their projections could signal big problems for cities and farmers. They live across the 246,000-square-mile basin. The area spans parts of seven states and Mexico. The river supplies water to about 40 million people. In addition, it supplies water to 6,300 square miles of farmland.
The Colorado River and its two major reservoirs, Lake Mead and Lake Powell, are already overtaxed. Water storage at Mead was at 42 percent of capacity Feb. 22. Powell was at 46 percent.
Water managers have said that Mead could drop low enough to trigger cuts next year in water deliveries to Arizona and Nevada. They would be the first states affected by shortages. This could occur under the multistate agreements and rules that govern the system.
But heavy snow in the West this winter may keep the cuts at bay. Snowpack in the Wyoming and Colorado mountains provides much of the Colorado River's water. It has ranged from 120 to 216 percent of normal. This was as of Feb. 23.
For their study, Udall and Overpeck analyzed temperature, precipitation and water volume in the basin. They looked at data from 2000 to 2014. They compared it with historical data. That included a 1953-1967 drought. Temperature and precipitation records date to 1896. River flow records date to 1906.
Temperatures in the 2000-2014 period were a record 1.6 degrees Fahrenheit above the historical average. Meanwhile, precipitation was about 4.6 percent below, they said.
The researchers used existing climate models. They said that much decline in precipitation should have produced a reduction of about 11.4 percent in the river flow. But not in the 19.3 percent that occurred.
They concluded that the rest was due to higher temperatures. The temperatures increased evaporation from water and soil. The temperatures sucked more moisture from snow. And, they sent more water from plant leaves into the atmosphere.
Martin Hoerling is a meteorologist. He works at the National Oceanic and Atmospheric Administration. He was not involved in the study. He questioned whether the temperature rise from 2000 to 2014 was entirely due to global warming. Some was likely caused by drought, he said.
Udall said warming caused by climate change in this century will dwarf any warming caused by drought. He noted that during the 1953-1967 drought, the temperature was less than a half degree warmer than the historical average. That was compared with 1.6 degrees during the 2000-2014 period.
Udall said climate scientists can predict temperatures with more certainty than they can precipitation. Studying their individual effects on river flow can help water managers.
Rain and snowfall in the Colorado River Basin would have to increase 14 percent over the historical average through the rest of the century to offset the effect of rising temperatures, he said.
"We can't say with any certainty that precipitation is going to increase and come to our rescue," Udall said.
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CRITICAL THINKING QUESTION
Why don't warmer temperatures produce more snow runoff?
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