Canopy Interception and Sheltering Effects on Macroscale Snow Sublimation

Sublimation of snow cover directly affects snow accumulation, impacting ecosystem processes, soil moisture, soil porosity, biogeochemical processes, wildfire, and water resources. Available energy, the exposed surface area of a snow cover, and exposure time with the atmosphere vary greatly in complex terrain (e.g., aspect, elevation, forest cover), with latitude, and with continentality. It is therefore difficult to scale up results from site specific short term studies. Using the 32-km North American Regional Reanalysis, the 4-km Parameter-elevation Regression on Independent Slopes Model, with 30-m terrain and forest cover data, meteorological variables are downscaled to simulate sublimation of canopy intercepted snow and sublimation from the snowpack. There is generally good agreement between model input (e.g., temperature, humidity, precipitation, wind speed) and independent observations reported by previous investigators as well as between modeled and observed SWE/snow covered area. Winter totals of observed vapor flux from above canopy eddy-covariance systems agree well with simulated sublimation in Colorado (mean absolute error 9.2%) while a consistent bias is evident at the New Mexico site (mean error +23.2%). Macroscale simulations of sublimation over the Salt River Basin in Arizona for a wet and dry year indicate that: (1) total sublimation is highly variable in response to variability in both sublimation rate and snow cover duration; (2) total canopy sublimation is similar for both years while ground sublimation is considerably greater during the wet year; (3) sublimation is a relatively greater contribution to the snow water budget during the dry year (28% vs. 20% of total snowfall); (4) at high elevations, ground sublimation is less in open areas than forested areas during the dry year, while the reverse is evident during the wet year as snowpack lasted longer into spring. While a reduction in leaf area index leads to a reduction of total sublimation due to less interception in both years, ground sublimation increases during the dry year, possibly due to less sheltering from solar radiation and wind. This reduction in sheltering results in a large decrease in snowpack duration (i.e., ten days in spring) at mid-elevations for the wet year, leading to a decrease in ground sublimation. This results in a 500 meter difference in the elevation of maximum sublimation reduction upon reduced leaf area index between the two years. Forest cover properties can vary considerably on short and long time scales through natural (wildfire, bark beetle infestation, drought) and anthropogenic (land management practices) processes. Therefore, understanding how small scale changes impact snow sublimation at larger spatial scales, and how this varies temporally, is critical from ecosystem function and water resources perspectives.