Clearing up confusion about the future of Colorado River flows
June 27, 2013
Brad Udall
Understanding how climate change will affect the flow of the Colorado River
in 2050 is important to water managers and the 30 million people who rely on it for
water. But an unsettling range of estimates, from a decrease of 6 percent to a steep
drop of 45 percent by then, has made planning for the future difficult.
But now a new report involving government agencies and universities,
including CU-Boulder, explains why those estimates differ and summarizes what is
known about the future of the river—key information for decision makers, says
Brad Udall, director of CU’s Getches-Wilkinson Center for Natural Resources, Energy
and Environment.
CUT 1 “People wanted to know which one of these projections was right. Was it the
one that said minus 6 percent at the year 2050 or was it the one that said almost a
50 percent decline by 2050? (:10) So we hoped to shed some light on what maybe is
not so much what the real number, but can we narrow it somewhat so that people
can get a better idea of what the best studies are telling us about the future of the
river? (:21)
Udall, who contributed to the study as director of the CIRES Western Water
Assessment, a NOAA-funded program at CU-Boulder, says after reviewing all the
flow studies, the author of the 45 percent decrease study realized his study was not
valid and pulled it from the report resulting in a new analysis that supports more
modest estimates of changes in future flows.
CUT 2 (409) “Now most people’s current guesses mid-century, we’re looking at 10
to 20 percent decline in the future Colorado River flow. (416) And I think that helps
decision maker begin to understand the nature of the problem, the size of the
problem.”
The reason the results of the studies differed is due to which climate model is
being used for a particular study, says Udall.
CUT 2 “There is a whole hierarchy of models that go into these studies – from a
global climate model to what we call land surface model. And all models, all these
choices that you make along the way, really do matter and you can put some
quantification on them. (:13) It helps the water community that relies on these
studies to begin to understand that there is no one number here, that there is a
range and that these choices all do influence what the scientific community comes
up with.” (:27)
Udall says researchers for this study did something that no else has ever
done when trying to forecast the future flow of the Colorado River. They looked at
what would happen if you warmed the basin from 1 degree Fahrenheit to 2 degrees
on up to 7 degrees Fahrenheit but kept precipitation at a constant level or kept the
temperature the same but decreased precipitation levels.
CUT 3 “And based on our studies, if you keep precipitation the same that translates
to anywhere from 12 percent to 40 percent decline in runoff just due to warming.
(:10) What’s nice about that is you don’t have to deal with any of the rigmarole of
global climate models or anything else to tell you wow, that warming really is going
to drive up evaporation and reduce run off.” (:20)
One aspect that most climate models have not taken into account, says Udall, is how
much human consumption and plant evaporation will impact runoff.
CUT 4 “Demand is going to increase. Evapotranspiration – that is to say is either
water that gets evaporated or water that runs through plants and is evaporated out
through the leaf surface of the plant – that demand goes up and human demand will
go up as well. (:16) We didn’t look at that and that is another squeeze that this basin
and other of the world’s water basin are going to face as it warms.” (:24)
Other aspects Western water managers must take into account, says Udall, is that
the early 19th century is the basis for water allocation in the basin, but was a period
of unusually high flow, so the river is already overallocated. Moreover, tree ring
record suggests that the Colorado has experienced sever droughts in the past of 60
years to 80 years and will do so again.
-CU-