Dust-on-snow events may not sound like a big deal, but they have a major impact on the Colorado River Basin watershed, which serves approximately 27 million people. Fresh snow reflects over 80% of sunlight back into space, according to UCLA atmospheric scientist Tom Painter. But when dark particles like dust are sprinkled on the snow, that number plummets as the dark particles absorb sunlight and warm the surrounding snow. And the amount of dust on snow has been increasing, with worrisome effects.
When Southwestern deserts are dry, eastward-blowing winds deposit dust on the snowy peaks of the mountains in the Colorado River Basin. But since the mid-1800s, grazing, agriculture, and other anthropogenic disturbances have increased dust emissions between five- and seven-fold. Greater dust emissions create a darker snow load in the spring, which melts more quickly over a shorter time period. So in Lee’s Ferry, Arizona, which divides the upper and lower Colorado River Basins, peak spring runoff occurs an average of three weeks earlier as a result of the recent five-fold increase in dust. That means there are three more weeks of snow-free conditions during which water evaporates rather than running through the river system, resulting in a decline of about five percent of total annual runoff each year. Overall, Painter says, airborne dust has caused runoff to decrease by over 35 billion cubic feet. The quicker, more compressed rush of runoff also creates a greater challenge for water management than slower, more regular melting. Minimizing soil disturbances and keeping dust emissions down could therefore reduce tensions over water in the communities served by the Colorado River Basin.