THE INFLUENCE OF OPENING SIZE ON SNOW EVAPORATION IN THE FORESTS OF THE ALBERTA FOOTHILLS

In situ snow evaporation was measured in circular openings, from 0H (full forest) to 5H (where H is the height of the surrounding trees; approximately 20 m), cut in the lodgepole pine (Pinus contorta Dougl.) forest of the Alberta Foothills. Additional measurements were also made in a large 40H ir-regular clearing. The ratios of snow evaporation measured in the circular clearings to that measured in the large irregular opening were 0.24, 0.23, 0.28. and 0.41 for the 0H, 1H, 3H, and 5H, respectively. The aerodynamic approach yielded a good estimate of average daily snow evaporation in the large opening, with a computed value of 1.01 mm versus a measured one of 1.07 mm. Snow evaporation in the OH opening was enhanced over that attributable to turbulent transport by nighttime radiative transfer from the canopy, which kept the snow wanner than in the other openings and prevented condensation. Vertical transfer efficiencies, the slope of the curve relating snow evaporation to the product of horizontal wind speed and vertical humidity gradient, were similar in all clearing sizes in spite of possible fundamental differences in wind fields created by the various canopy-opening interactions. Comparison of data from smaller openings with data from a previous study by A.J. West suggests that differences in snow evaporation between openings can be indexed to opening size.