Influence of canopy structure and direct beam solar irradiance on snowmelt rates in a mixed conifer forest

Abstract

Sub-canopy snow ablation rates were measured for three years at forested research plots in the Sierra Nevada, California with a network of 24 automated snow depth sensors and monthly snow density surveys. Snow ablation rates, in mm SWE day−1, specific to each depth sensor location were estimated as the seasonal maximum SWE divided by the number of days from peak SWE to snow disappearance. Estimates of sub-canopy direct beam solar irradiance and sky view factor (SVFθ) derived from hemispherical photographs were used to explain the spatial distribution of snow ablation rates. Cumulative direct beam irradiance during the observed snowmelt periods explained the most variability in snow ablation rates for the most cloud-free melt season (58% in 2008; 4 cloudy days; at 15 sensor locations snowmelt duration ranged from 39 days to 88 days and direct irradiance ranged from 96MJm−2 to 603MJm−2) and explained the least ablation variability for the cloudiest melt season of the study (29% in 2009; 23 cloudy days; at 12 sensor locations snowmelt duration ranged from 45 days to 79 days and direct irradiance ranged from 121MJm−2 to 410MJm−2). Conversely, sky view factor (SVFθ) explained the most variability in snow ablation rates under cloudier conditions (i.e. 87% in 2009) and the relationships were strongest when developed over the entire hemisphere (i.e. SVF90°, which ranged from 0.17 to 0.31). Combined, the two metrics studied here (sub-canopy direct beam irradiance and SVFθ) may be used to explain much of the observed plot-scale variability in SWE ablation at finer time scales relevant to snow and hydrological model applications.