A Comparison of Snowpack Mass and Energy Dynamics Across a Canopy Discontinuity and Small-Scale Elevational Gradient

Abstract:

The processes governing the accumulation and melt of seasonal snowcovers in large open areas and under continuous forest canopies are well understood, however our understanding of spatiotemporal variations in snowcover processes in discontinuous forests is limited. The objective of this study was to quantify the snowcover mass and energy-balance dynamics across a canopy discontinuity and from a valley bottom to hilltop location to improved the understanding of hydrologic fluxes in complex terrain. Energy balance components were calculated using both the mass and energy balance snowmelt model (SNOBAL) and the 1-dimensional Simultaneous Heat and Water Balance (SHAW) Model, driven with micrometeorological from 7 contrasting sites. Results indicate that the canopy gap accumulated 52% more snow water equivalent (SWE) than the forest canopy sites. Net energy flux in the gap was approximately half of the open location and twice the sub-canopy locations. Net radiation was highest at the north and center points of the gap, smallest at the south point in the gap, and intermediate at the forested locations. Net turbulent (sensible and latent) energy fluxes were typically small positive values in the gap, and slightly negative in the forest, due to cold-air drainage and low wind speeds at all valley bottom sites, and colder air temperatures at the forested sites. Soil heat flux was a relatively minor component of the energy balance at the gap sites, but was surprisingly large at the forested sites due to relatively warm soil temperatures and slightly cooler snow temperatures. A sensitivity study, multi-model comparison, and comparison to other similar studies suggests that the computed spatiotemporal fluxes are reasonable for this specific site. The combination of mass and net energy flux differences between the open and gap sites resulted in the open site melting out 60 days prior to the south gap site. The mass and energy flux differences similarly resulted in the forested sites melting out 15 days prior to south gap site. This work demonstrates that distinctive micrometeorological conditions occur in both discontinuous canopies and in valley bottom locations that can produce large spatiotemporal variations in peak SWE and ablation timing.