A new paper by researchers at the University of Washington, Cooperative Institute for Research in Environmental Science (CIRES), NOAA and other institutions across the West investigates and explains why those estimates differ and summarizes what is known about the future of this iconic Western river—key information for decision makers.
“We know, for example, that warmer temperatures will lead to more evaporation and less flow,” said co-author Bradley Udall, who contributed to the study as director of the CIRES Western Water Assessment, a NOAA-funded program at the University of Colorado Boulder. Udall is now director of CU-Boulder’s Getches-Wilkinson Center for Natural Resources, Energy and Environment at Colorado Law. “Although projections of future precipitation aren’t as clear, it’s likely that we’re going to see a reduction in overall flow in the Colorado,” Udall said.
The study is published this week in the Bulletin of the American Meteorological Society.
While the paper does not narrow the existing range of estimates, it provides context for evaluating the current numbers. The 6 percent reduction estimate, for example, did not include some newer climate model runs, which tend to predict a drier West. And the 45 percent decrease estimate relied on models with a coarse spatial resolution that could not capture the effects of topography in the headwater regions. The new analysis, thus, supports more moderate estimates of changes in future flows.
“The different estimates have led to a lot of frustration,” said lead author Julie Vano, a University of Washington doctoral researcher in civil and environmental engineering. “This paper puts all the studies in a single framework and identifies how they are connected.”
In evaluating recent scientific papers that estimate future flow of the Colorado River, she and her co-authors identified several reasons for different flow estimates. Among them are differences in:
- Climate models and future-emissions scenarios used. Models run with higher future greenhouse gas emissions, for example, typically produce warmer and often drier climates, and smaller Colorado River flows.
- The models’ spatial resolution, which is important for capturing topography and its effects on snow distribution in the Colorado River’s mountainous headwaters. Models with coarser resolution tend to overestimate the sensitivity of runoff to climate change.
- Representation of land surface hydrology, which determines how precipitation and temperature changes will affect the land’s ability to absorb, evaporate or transport water.
- Methods used to downscale from the roughly 200-kilometer resolution used by global climate models to the 10- to 20-kilometer resolution used by regional hydrology models.
Paper authors include leaders in Western water issues, ranging from specialists in atmospheric sciences to hydrology to paleoclimate: Bradley Udall at CIRES’s Western Water Assessment and the University of Colorado Boulder; Daniel Cayan, Tapash Das and Hugo Hidalgo at Scripps Institution of Oceanography, UC San Diego; Jonathan Overpeck, Holly Hartmann and Kiyomi Morino at the University of Arizona in Tucson; Levi Brekke at the U.S. Bureau of Reclamation; Gregory McCabe at the U.S. Geological Survey in Denver; Robert Webb and Martin Hoerling at the NOAA Earth System Research Laboratory in Boulder; and Kevin Werner at the National Weather Service in Salt Lake City.
The research was funded by NOAA through its Regional Integrated Sciences and Assessment program and by the Pacific Northwest Climate Impacts Research Consortium.