IMPACT OF DUST ON SNOW ALBEDO, SNOWMELT, AND RUNOFF IN THE COLORADO RIVER BASIN

The allocation of Colorado River runoff to more than 25 million people in 7 states and 2 countries is overcommitted by more than 10% of its historical mean. Climate models project runoff losses of 7 to 20% from the basin in this century due to human-induced climate change. Recent work has shown however that by the late 1800s, decades prior to allocation of the river’s runoff in the 1920s, a five-fold increase in dust loading from anthropogenically disturbed soils in the southwest US was already decreasing snow albedo and shortening the duration of snow cover by several weeks.

From in situ radiation measurements in the mountains of the Upper Colorado River Basin (UCRB), we have found that dust absorption in snow reduces snow albedo from its fresh snow value of ~0.85 down to 0.4-0.5 and in years with extreme dust loading, down to ~0.3. At-surface radiative forcings range from 25 to 110 W/m², compared with the globally-averaged anthropogenic greenhouse gas forcing of about 3 W/m². These radiative forcings increase the already dominant net solar radiation in melting snow and shortening snow duration by 25-50 days.

We have extrapolated these results to the UCRB using the Variable Infiltration Capacity model with post-disturbance and pre-disturbance impacts of dust on albedo to assess the impact on runoff from the UCRB across 1916 to 2003. We find that peak runoff at Lees Ferry, AZ has occurred on average three weeks earlier under heavier dust loading and that increases in evapotranspiration from earlier exposure of vegetation and soils may decrease runoff by more than ~1.0 billion cubic meters (bcm) or ~5% of annual average runoff.

Toward understanding dust’s impact on snowmelt runoff in the context of water management, we investigated the interannual variability of rates of change in runoff during the snowmelt runoff relative to the interannual variability in air temperature (through positive degree days) and dust radiative forcing across 6 years of data in the San Juan Mountains, Colorado. We find that the temperature variation explains none of the variation in runoff anomalies whereas runoff anomalies are strongly dependent on the variation in dust radiative forcing.