Quantifying the impact of declining snow on river discharge in a warming climate

Abstract:

Mountain snowpack reduction resulting from global warming may impact water supply in regions such as the Western United States (WUS), where snowmelt serves as a major source of river discharge. In addition to the earlier melt, the changes in river discharge volumes may result from the snowmelt-radiation feedback, specifically the change in albedo and hence net solar radiation associated with the transition of snow cover to vegetation or bare ground as the snow melts. However, it remains uncertain to what degree does this snowmelt-radiation feedback controls changes in river discharge across the heterogeneous hydroclimatic regimes of the Western U.S. We examine warming-induced streamflow changes and their drivers for the HUC-8 river basins of the WUS for the period 1915 to 2018. We evaluate the role of the snowmelt-radiation feedback on runoff volumes using output from the Variable Infiltration Capacity (VIC) model implemented at the 1/16^th degree spatial resolution. We classified the 616 WUS UC-8 basins west of the Continental Divide by their elevation, mean winter and mean annual temperature, summer to winter precipitation ratio, primary vegetation type, and the fraction of forest cover. We define the snowmelt feedback coefficient as the correlation between springtime net radiation change and annual runoff change. Our preliminary results demonstrate a significant negative snowmelt feedback impact on runoff, especially at watersheds with low runoff coefficients, which have greater evapotranspiration. At Upper Colorado River Basin (UCRB), the correlation between snowmelt feedback impact on runoff and runoff coefficient is 0.59, at a significant level of 0.01.