This session dealt with the current state of the science around measuring snowpack and forecasting streamflow. It was a treat. It’s not often that so many experts currently working on this problem are assembled in one location to present overviews of their current research and thinking.
Jeff Deems (Research Scientist, NOAA Western Water Assessment and National Snow and Ice Data Center) started things off with a bit of a primer on snow albedo and the effects of dust on snowmelt and runoff. “Solar radiation is the big deal,” he said. He added that air temperature has an effect as well but it is much less than solar radiation. Dust decreases the snow albedo and increases the effects (melting) of solar radiation.
The dust that accumulates on the snow surface comes mainly during the spring and therefore impacts the melt season, he said. The snowpack melted off 28 days earlier than the historic norm in 2005, in 2009 it was 50 days earlier and last season the melt occurred 43 days ahead of the average. In the Colorado Rockies, “snow all gone,” is occurring 1 to 2 months earlier now.
Deems said that silt samples from high mountain lakes in the San Juans indicates a six-fold increase in dust in the mid-1800s. There was a drop in the increase of dust directly after the Taylor Grazing Act of 1934 was enacted but dust levels have climbed steadily since the late 1940s.
Desert soils in their natural state are armored by crusts, he said. These crusts are composed of biotic materials and shield the soil from wind erosion. Grazing animals and vehicle traffic break down the crusts leading to a 50-fold increase in sediment production when disturbed, according to Deems. Invasive species, primarily annuals, also add to the disturbance as less ground cover is often present in drought conditions, he said.
In the Colorado River Basin his team has estimated a 5% annual runoff decrease or 800,000 acre-feet. To put that into perspective the total is more than two times the annual Colorado River Compact allocation for Southern Nevada.
Deems hopes to see some sort of dust mitigation strategy put in place in northeastern Arizona and northwestern New Mexico that would lessen the amounts of wind-borne silt making it’s way to the Rockies. [Click on the thumbnail graphic to the right for the satellite view of the April 29, 2009 dust event in Four Corners.] During the question and answer period afterward he was asked if any measures are in place and if they are getting results. His answer was no, but another panelist suggested that the Taylor Grazing Act was a good example of what could be done, citing the drop in dust accumulation after its enactment.
Noah Molotch (University of Colorado) explained his team’s work on using remote satellite sensing to measure snowpack. Their approach is to estimate snowpack by calculating backward from “snow all off” to arrive at a snowpack total. This is based on the assumption that snow persists where the accumulation is deepest.
Mark Williams (University of Colorado) emphasized that remote sensing can include a much larger area for measurement rather than the point values inherent in the NRCS’ Snotel network. They are, “able to measure snow-water equivalent across the landscape.”
Williams is working to get dust measuring devices across the southwest to gather data. A new site at Telluride is going to measure uranium and vanadium components in order to monitor the recently licensed uranium mill at Paradox.
David Clow (USGS) and his team are using chemistry and Principle Component Analysis to identify the increase in dust deposition along the Rockies. They collect and analyze snow samples from 57 sites ranging from Montana to New Mexico. The analysis shows an increase in calcium in the samples from the Colorado Rockies indicating an increase in dust over time, he said.