Predicting the effect of a forest canopy on ground snowpack accumulation and ablation in maritime climates

To investigate the processes controlling snow accumulation and ablation beneath forest canopies in maritime climates, continuous measurements of snow water equivalent (SWE) and snowpack outflow were made during the winters of 1996/97 and 1997/98 at a site in the transient snow zone of the Southern Oregon Cascades. Measurements were taken with large weighing Iysimeters (-25 m2) below a mature canopy and in an adjacent harvested site. Analysis of the observations shows that approximately 60% of annual snowfall was intercepted by the canopy (up to an apparent maximum above 30 mm), with approximately 72 percent removed as meltwater drip and 28 percent removed as large snow masses. Apparent sublimation rates from the intercepted snow were less than 1 mm per day. These data, along with measurements of standard micrometeorology , were used to develop and test an energy balance model of snow interception and canopy effects on ground snowpack ablation. The model represents intercepted snow load as a function of Leaf Area Index, adjusted for the effects of temperature. Removal of intercepted snow by sublimation, melt -water drip and mass release is governed by available energy , the vapor pressure deficit, and the liquid water content of the intercepted snow. The profiles of wind and radiation through the canopy are modeled explicitly. Turbulent heat exchange at the surface is calculated via the bulk aerodynamic method and corrected for atmospheric stability via a modified Richardson's number approach. The model was calibrated to the 1996/97 observations and validated using the 1997/98 data. Model predictions of snow accumulation at both sites are highly sensitive to the rain/snow threshold temperature while predicted ablation agrees well with observations.