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Common practice and conventional wisdom hold that fluctuations in air temperature control interannual variability in snowmelt and subsequent river runoff. However, recent observations in the Upper Colorado River Basin confirm that net solar radiation and by extension radiative forcing by dust deposited on snow cover exerts the primary forcing on snowmelt. We show that the variation in the shape of the rising limb of the annual hydrograph is controlled by variability in dust radiative forcing and surprisingly is independent of variations in winter and spring air temperatures. These observations suggest that hydroclimatic modeling must be improved to account for aerosol forcings of the water cycle. Anthropogenic climate change will likely reduce total snow accumulations and cause snowmelt runoff to occur earlier. However, dust radiative forcing of snowmelt is likely consuming important adaptive capacity that would allow human and natural systems to be more resilient to changing hydroclimatic conditions.

Plain Language Summary We address the question, do air temperatures or absorbed solar radiation explain the year-to-year variability of the rate of snowmelt and therefore the shape of the way the streamflow rises in the melt season? Our analysis shows that absorbed solar radiation, which varies with the amount of wind blown dust deposited into the snowpack, causes the streams to rise more quickly in years with more dust, whereas the rate at which the streams rise does not depend on air temperature. Forecasts of snowmelt runoff must account for the variability in dust deposition.