Study Shows that Dirty Snow Reduces Colorado River Runoff
Research has revealed that dust accumulating on mountain snowpacks in the Upper Colorado River basin significantly reduces runoff, negatively impacting human activities in a large area.
The 1,450 mile-long Colorado River is the only major source of surface water in the Southwestern U.S. At least 27 million people in parts of seven U.S. states and two Mexican states rely on the river for irrigation water, municipal water supply, and industrial uses. The demand is so great (and withdrawals so poorly regulated) that legal entitlements to Colorado River water exceed the river's flow. As Jonathan Waterman laments in his recently published book Running Dry: A Journey From Source to Sea Down the Colorado River, the Colorado is being "sucked dry."
Among the many entities dependent on the Colorado River and impacted by changes in the total flow and the timing of the peak volume are a number of National Park System units, including Grand Canyon National Park, Canyonlands National Park, Glen Canyon National Recreation Area and Lake Mead National Recreation Area.
Given that even slight changes in the river's flow can be very important, and that the governing variables are very imperfectly understood, there should be plenty of interest in the results of a recent study that investigated the hydrologic effects of human-produced dust deposits on mountain snowpacks in a large area of the Colorado River basin.
This was a scientifically rigorous study, make no mistake about that. Co-sponsored by the National Science Foundation and the US Geological Survey, it was conducted with the assistance of the National Snow and Ice Center, the Center for Snow and Avalanche Studies, the USGS Southwest Biological Center, the University of Colorado-NOAA Western Water Assessment, and the University of Washington. A team of scientists led by Tom Painter, a snow hydrologist with NASA's Jet Propulsion Laboratory, employed a sophisticated hydrological model to examine the hydrologic effects of human-produced dust deposits on mountain snowpacks over the Upper Colorado River basin during the period 1915 to 2003.
The model enabled the scientists to compare today's water balance and river flow conditions with those that existed in the mid- to late 1800s before grazing and farming removed natural vegetation and exposed disturbed soils to serious wind erosion. This wind erosion put a great deal more dust in the air -- lake sediment studies suggest about five to six times the pre-settlement amount -- and much of that dust settled on mountain snowpacks. In simpler terms, the snow in the mountains has gotten a whole lot dirtier over the past century and a half.
Snowpacks with dust accumulations have a lower albedo than clean snow, meaning that they do not reflect sunlight as well. Because dirty snow absorbs more heat, it melts faster, evaporates faster, and exposes underlying vegetation sooner (adding increased transpiration to the water loss equation). Scientists have long speculated about the runoff implications of dirty snow, but this study was the first to carefully measure dust-on-snow impacts on snowmelt rates and basin runoff in a comprehensive way.
The study's findings, which were recently reported in the authoritative Proceedings of the National Academy of Sciences under the title "Response of Colorado River Runoff to Dust Radiative Forcing in Snow," are not reassuring. The peak spring runoff from the Upper Colorado River Basin now comes as much as three weeks earlier than it did in the mid- to late 1800s, and the annual runoff has apparently decreased by more than 5 percent since then.
Whether the missing 5 percent of the Colorado River's annual flow can be restored is an intriguing question. Accomplishing a return to the era of the relatively clean mountain snowpack would necessitate stabilizing disturbed soils and minimizing soil-disturbing activities within a vast region that is very heavily invested in grazing and farming.
If the task seems daunting, the potential rewards are gigantic. The missing water amounts to 35 billion cubic feet a year, or enough to keep Los Angeles supplied with water for 18 months. That's a lot of economic development, jobs, and tax revenues hanging in the balance.