The exchange efficiency of heat and mass between a forest community and the atmosphere varies with leaf quantity and forest canopy structure. However, there are no examples investigating the relationship between leaf quantity, forest canopy shapes and exchange efficiency. We had evaluated that forest canopy surface had been smoother in the defoliate season than in the foliate season in one deciduous forest. As the next step, the seasonal change of exchange efficiency was estimated in this forest and compared with the smoothness of canopy shape in this study. A bulk coefficient is used widely as an exchange efficiency index. However, this varies with wind velocity and isn't suitable for this study. Therefore, we used the coefficient A using in the Thornthwaite-Holtzmann model as our exchange efficiency index. The coefficient A is derived from the parameters depending on the land surface condition, such as roughness length and zero plane displacement.
Our measurement was conducted in a forested study basin located at central Japan, whose leaf area indices were estimated as 4.42 and 2.70 in the foliate and defoliate seasons, respectively.
Explaining the evaluation of canopy surface smoothness, the canopy surface elevations had been measured for every 2.5m square at 144 points and analyzed their dispersion. This result had indicated that the size of swelling and hollow shapes on the canopy surface were smaller vertically and larger horizontally in the defoliate season than in the foliate season. Thus, it had been concluded that canopy surface was smoother in the defoliate season than in the foliate season in this study basin.
Net radiation (Rn), wind velocity (u2), dry and wet bulb temperature differences (TD1-TD2, TW1-TW2) and soil heat flux (G) were measured at the weather measuring tower. In addition, precipitation rate and discharge rate from this basin were also observed. Annual data in 1992 was used in this study.
Thornthwaite-Holtzman model was expressed as Eq. (1):
lE = Rn - G - pCpAu2(TD1-TD2) (1)
where lE is latent heat flux, p is the atmospheric density, and Cp is the constant pressure specific heat.
Eq. (2) is used to obtain the value of A in this study, utilizing Eq. (1) and a calculated monthly evapotranspiration rate lEsb deprived with the precipitation and discharge rates with the short-period water balance method.
A = (Rn - G - lEsb )/ pCpu2(TD1-TD2) (2)
Although having some variation, A is calculated to be around 0.12 and 0.077 in the foliate and defoliate seasons , respectively. This result indicates that exchange efficiency in the foliate season was around 1.5 times as much as that in the defoliate season.
The forest canopy this study was conducted had been judged to be smoother in defoliate season than in foliate season in this basin from the dispersion analysis of canopy elevation heights. It is thought that as the canopy shape changed to be smoother, the vertical wind velocity might become so small that the thermal exchange efficiency decreased.