Unloading on intercepted snow in conifer forests

Abstract

Snowfall interception is particularly important to the hydrology of forested cold regions. Unloading of intercepted snow controls the snow available for interception loss due to sublimation from that held in the canopy. This thesis seeks to determine the factors that affect the magnitude and timing of unloading at the forest-stand scale. A field program was established that measured interception and unloading at a forest-stand scale using a series of hanging lysimeters and a 7 m tall spruce tree suspended, in-situ, on a load-cell.

Meteorological conditions including snowfall, wind speed, air temperature, and incoming radiation were recorded above and below the forest canopy. Unloading did not behave as described by current unloading models. It was observed to be triggered by occurrences of wind gusts or melt conditions within the canopy but no trends were found in the measurements that could be used to predict the onset of unloading from gusts or air temperature alone. An association between intercepted snow sublimation and unloading was found and this relationship was further found to be an exponential function of air temperature. An expression based on this empirical model can be used to calculate unloading as a function of sublimation rate in hydrological models or to calculate unloading directly as a function of canopy snow load and air temperature.