Seasonal Variations of Heat Balance Components
over a Japanese Red Pine Forest in Snowy Northern Japan
Kazuyoshi Suzuki1, Takeshi Ohta1, Hiroshi Miya1, and Satoshi Yokota2
We observed meteorological elements and heat fluxes over a red pine forest for more than one-and-
a-half years. There was significant seasonal variation in the global solar radiation, air temperature,
and albedo above the forest canopy. During the spring thaw, the albedo above the forest decreased
and reached a minimum value after the snow disappeared. Due to seasonal variation in the canopy
structure, the ratios of insolation and wind speed at the forest floor compared to those above the
forest canopy also varied. Under dry canopy conditions, the peaks of each heat balance component
occurred at different times. The net radiation peaked in June, the latent heat flux peaked in July, and
the sensible heat flux peaked in May. When the forest floor was snow covered, the Bowen ratio was
more than 1.0 and the latent heat flux was virtually constant. The maximum ratio of sensible heat
flux to net radiation also occurred during this period. This showed that the dry forest canopy was a
significant heat source for the surrounding atmosphere. Potential evaporation from the forest canopy
of total evaporation from the dry canopy (E) to Ep is the evaporation efficiency. The seasonal change
in evaporation efficiency was similar to that in the air temperature above the canopy. When the daily
air temperature was above 10C, the evaporation efficiency and saturation deficit were negatively
correlated. However, below 10C this relationship disappeared. Snow cover also affected the rela-
tionships between evaporation efficiency and meteorological conditions. With snow cover, the evap-
oration efficiency increased as the air temperature decreased or the wind speed increased. In non-
snow-cover conditions, however, evaporation efficiency decreased as the air temperature decreased
or the wind speed increased. This was caused by properties of the snow cover. Furthermore, the
relationship between the evaporation efficiency and the global solar radiation depended on the air
temperature. If the air temperature was below 10C, the global solar radiation increased the evapora-
tion efficiency.
1
Faculty of Agriculture, Iwate University, 3-18-8 Ueda, Morioka, Iwate 020-8550, Japan
2
Kokusai Kogyo Co. Ltd., 5-1-23 Miyagino-ku, Sendai, Miyagi 983-0852, Japan
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