Snow water equivalent along elevation gradients in the Merced and Tuolumne River basins of the Sierra Nevada

摘要

We used daily remotely sensed fractional snow-covered area (SCA) at 500 m resolution to estimate snow water equivalent (SWE) across the Upper Merced and Tuolumne River basins of the Sierra Nevada of California for 2004 (dry and warm) and 2005 (wet and cool). From 1800 to 3900 m, each successively higher 300 m elevation band consistently melts out 2-3 weeks later than the one below it. We compared two methods of estimating SWE from SCA: (1) blending the fractional SCA with SWE interpolated from snow-pillow measurements; and (2) retrospectively estimating cumulative snowmelt based on a degree-day calculation after the snow disappeared. The interpolation approach estimates a lower snowmelt volume above 3000 m and a higher snowmelt contribution at elevations between 1500 and 2100 m. Snowmelt timing from the depletion approach matches observed streamflow timing much better than snowmelt estimated by the interpolation method. The snow-pillow sites used in the interpolation method do not cover the highest elevations and melted out several weeks before the basin itself was free of snow. Middle elevations (2100-3000 m) contributed 40%-60% of the annual snowmelt in both basins, the lower elevations (1500-2100 m) 10%-15%, and elevations above 3000 m the remaining 30%-40%. The presence of snow in the highest elevations highlights their critical buffering effect in accumulating snow every year. Variability in lower-elevation snow illustrates its susceptibility to climate variability and change.