NASA Funded Study Shows Desert Dust Cuts Colorado River Flow

WASHINGTON -- Snow melt in the Colorado River basin is occurring
earlier, reducing runoff and the amount of crucial water available
downstream. A new study shows this is due to increased dust caused by
human activities in the region during the past 150 years.

The study, led by a NASA scientist and funded by the agency and the
National Science Foundation (NSF), showed peak spring runoff now
comes three weeks earlier than before the region was settled and
soils were disturbed. Annual runoff is lower by more than five
percent on average compared to pre-settlement levels.

The findings have major implications for the 27 million people in the
seven U.S. states and Mexico who rely on the Colorado River for
drinking, agricultural and industrial water. The results were
published in this week's Proceedings of the National Academy of Sciences.

The research team was led by Tom Painter, a snow hydrologist at both
NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the
University of California at Los Angeles. The team examined the impact
of human-produced dust deposits on mountain snowpacks over the Upper
Colorado River basin between 1915 and 2003. Studies of lake sediment
cores showed the amount of dust falling in the Rocky Mountains
increased by 500 to 600 percent since the mid-to-late 1800s when
grazing and agriculture began to disturb fragile but stable desert soils.

The team used an advanced hydrology model to simulate the balance of
water flowing into and out of the river basin under current dusty
conditions and those that existed before soil was disturbed.
Hydrologic data gathered from field studies funded by NASA and NSF
and measurements of the absorption of sunlight by dust in snow were
combined with the modeling.

More than 80 percent of sunlight falling on fresh snow is typically
reflected back into space. In the semi-arid regions of the Colorado
Plateau and Great Basin, winds blow desert dust east, triggering
dust-on-snow events. When dark dust particles fall on snow, they
reduce its ability to reflect sunlight. The snow also absorbs more of
the sun's energy. This darker snow cover melts earlier, with some
water evaporating into the atmosphere.

Earlier melt seasons expose vegetation sooner, and plants lose water
to the atmosphere through the exhalation of vapor. The study shows an
annual average of approximately 35-billion cubic feet of water is
lost from this exhalation and the overall evaporation that would
otherwise feed the Colorado River. This is enough water to supply Los
Angeles for 18 months.

"The compressed mountain runoff period makes water management more
difficult than a slower runoff," Painter said. "With the more rapid
runoff under dust-accelerated melt, costly errors are more likely to
be made when water is released from and captured in Colorado River reservoirs."

Prior to the study, scientists and water managers had a poor
understanding of dust-on-snow events. Scientists knew from theory and
modeling studies that dust could be changing the way snowfields
reflect and absorb sunlight, but no one had measured its full impact
on snowmelt rates and runoff over the river basin. The team addressed
these uncertainties by making systematic measurements of the sources,
frequency and snowmelt impact of dust-on-snow events.

"These researchers brought together their collective expertise to
provide a historical context for how the Colorado River and its
runoff respond to dust deposition on snow," said Anjuli Bamzai,
program director in NSF's Division of Atmospheric and Geospace
Sciences in Arlington, Va. "The work lays the foundation for future
sound water resource management."

Painter believes steps can be taken to reduce the severity of
dust-on-snow events in the Colorado River basin. He points to the
impact of the Taylor Grazing Act of 1934 for potential guidance on
how dust loads can be reduced. The act regulated grazing on public
lands to improve rangeland conditions. Lake sediment studies show it
decreased the amount of dust falling in the Rocky Mountains by about
one quarter.

"Restoration of desert soils could increase the duration of snow
cover, simplifying water management, increasing water supplies and
reducing the need for additional reservoir storage of water. Peak
runoff under cleaner conditions would then come later in summer, when
agricultural and other water demands are greater," Painter said. "It
could also at least partially mitigate the expected regional impacts
of climate change, which include reduced Colorado River flows,
increased year-to-year variability in its flow rate and more severe
and longer droughts," he added. "Climate models project a seven to 20
percent reduction in Colorado River basin runoff in this century due
to climate change."

Other institutions participating in the study include the National
Snow and Ice Center in Boulder, Colo.; U.S. Geological Survey
Southwest Biological Center in Moab, Utah; University of Washington
in Seattle; Center for Snow and Avalanche Studies in Silverton,
Colo.; and the University of Colorado-NOAA Western Water Assessment
in Boulder.

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http://www.nasa.gov

Source: NASA