An Effect of Canopy Snow on Energy Balance
above a Coniferous Forest
Yuichiro Nakai1, Tomoki Sakamoto1, Tomomi Terajima1,
Kenzo Kitamura1, and Tomoki Shirai1
To evaluate an interactive effect of snow with forest on energy exchange between forest surface and
atmosphere, the energy balance above a forest was measured continuously between midwinter and
snowmelt season of the winter 199697 in Sapporo, northern Japan.
The forest was a dense, 23-year-old, coniferous plantation. The trees were two species of spruce
and a fir species, all 23 years old with an average height of 6.4 m, a mean trunk diameter of 0.09 m,
and a density of 0.24 stems m-2. The leaf area index was estimated to be 6.0 m2 m-2. The total stand
area of 46,000 m2 was bordered by deciduous forests extending over an area of 1,300,000 m2. The
study site had frequent snowfalls and the canopy was frequently covered with snow.
Snowcover on the canopy was daily monitored using a photo camera above the canopy, and a ratio
of the snowcover to the total area of the canopy was determined as an index for the canopy snow.
Turbulent fluxes above the canopy were measured using eddy and band-pass covariance method.
The effect of the canopy snow on turbulent energy exchange was examined using an expression of
the evaporative efficiency for the canopy layer in a double-source model that combined the energy
balance and the bulk turbulent transfer both for the canopy and for the forest floor.
The diurnal courses of the measurement showed that turbulent fluxes were sensitive with the snow-
cover on canopy. This means that latent heat fluxes dominated above the snow-covered canopy and
that sensible heat flux prevailed above the snow-free and dry canopy. The evaporative efficiency
for the canopy layer changed dynamically with the canopy-snow condition even during a short term
and could be positively related to the extension of the snow-covered canopy. A relationship was
empirically determined between the evaporative efficiency and the fraction of the canopy-snow
area. The snowcover season can be typified into the following two types of energy balance: (1)
latent heat flux from the canopy dominated and the Bowen ratio was positive but low for midwinter
under the snow-covered canopy; (2) for the snowmelt season, sensible heat flux from the dry cano-
py prevailed and the Bowen ratio was much larger than for midwinter.
1
Hokkaido Research Center, Forestry and Forest Products Research Institute, Sapporo 062-8516, Japan
24
Previous Page
