Abstract

Canopy distribution and density are important determinants of snowmelt patterns in forested catchments. Canopy influence on interception, during accumulation, and radiation, during ablation, result in significant differences in snow distributions between open versus under-canopy locations and in forest gaps of varying sizes. These differences pose a challenge to accurate measurement and prediction of hydrologic fluxes in forested catchments at all scales. We present results from instrumental observations and surveys of snow accumulation and melt from a Sierra Nevada red-fir forest at multiple scales including 20 × 20 m plots around individual trees, synoptic surveys at hill-slope scales, and physically based watershed-scale models. Our measurement results show that snow meltout dates can vary up to 45 days in the vicinity of individual trees and a snow water equivalence variance of 35% between under-canopy and open locations at the hill-slope scale. Repeat early April surveys were conducted approximately 2 weeks after peak snow accumulation in three successive years. These measurements, collected at over 200 locations, show that percent differences between under-canopy and open snow water equivalence are similar despite 60% differences in total precipitation between the three years. Modeled radiation of the snow ablation period using a digital elevation model alone shows a poor agreement with measured snow water equivalence distributions. However when we use field measurements of incoming shortwave radiation, canopy height, leaf-area index (based on Licor LAI-200 measurements), and canopy density (based on hemispherical photographs) as initial conditions the simulations of snowmelt (using the SNOWPACK model) at both plot and stand scales improve. These results support our assertion that strategically placed, spatial measurements in embedded sensor networks can provide the distributed inputs needed to accurately describe snow accumulation and ablation, including the significant role of vegetation at multiple scales.

Citation

Kirchner, P.B., Bales, R.C., Musselman, K.N., Molotch, N.P. (2009): Multi-scale observations and modeling of the snowpack in a forested Sierra Nevada catchment. Fall meeting, American Geophysical Union, December 2009. 90(52). Abstract C23D-08..