20. EXPERIMENTAL STUDY OF STATISTICAL
CHARACTERISTICS OF PLANT CANOPY RADIATION REGIME
189
As indicated earlier, global radiation
and net radiation
measurements are simultaneously recorded above the forest at the height of
about 12 meters, with a Reemann pyranometer, a LI-COR quantum sensor
and a Reemann net radiometer, respectively.
The data acquisition system consists of Delta-T Devices, a multi-channel
data-logger and a Notebook PC U865x-25. The mean values, standard
deviations and flux density distribution functions were calculated during a
preliminary data processing phase. To calculate the flux densities in the
PAR, the sensitivity of LI-COR quantum sensor provided with the
instrument certificate was used. A conversion coefficient of 2.174
was
used to convert the PAR flux densities expressed in
to kW
(Sulev, Ross, 1996). Both the Reemann pyranometer and the net radiometer
were calibrated against the Sun, using a normal pyrheliometer, i.e., their
sensitivities were linked to the International Pyrheliometric Scale.
Simultaneously with these radiation measurements, air temperature and
humidity at a height of 150 cm inside and outside the forest, wind speed
above the canopy, and ground surface temperature beneath the forest were
acquired.
3.
RESULTS AND DISCUSSION
Measurements above the canopy at a height of about 12 meters enable to
determine the daily course of incoming radiation fluxes, the fraction of the
PAR in the global fluxes, and also the albedo of the plantation. An example
of these data is presented in Fig. 2 and 3. The variability of these curves is
caused by the presence of slight
clouds. The relative part of global PAR
PAR (Q)/Q exhibits a minimum of about 0.45 around noon, and increases to
0.5 at solar heights less than 10°. For direct solar radiation, the relation
increases with solar height and reaches a maximum value of about 0.38
around noon. The relative part of PAR in the reflected radiation depends
slightly on solar height and has a maximum of 0.10 for a solar height
between 15 and 30°. The albedo for global radiation Ak has an obvious daily
variation with a minimum value of about 0.24 at noon. The albedo in the
PAR region is much less (about 0.04 at noon) and also increases when solar
height decreases. The asymmetry of the curves is mainly due to azimuthal
non-homogeneity of the willow plantation structure, and possibly also to
changes in the plant structure and optical properties of leaves during the day.
CHARACTERISTICS OF PLANT CANOPY RADIATION REGIME
189
As indicated earlier, global radiation
and net radiation
measurements are simultaneously recorded above the forest at the height of
about 12 meters, with a Reemann pyranometer, a LI-COR quantum sensor
and a Reemann net radiometer, respectively.
The data acquisition system consists of Delta-T Devices, a multi-channel
data-logger and a Notebook PC U865x-25. The mean values, standard
deviations and flux density distribution functions were calculated during a
preliminary data processing phase. To calculate the flux densities in the
PAR, the sensitivity of LI-COR quantum sensor provided with the
instrument certificate was used. A conversion coefficient of 2.174
was
used to convert the PAR flux densities expressed in
to kW
(Sulev, Ross, 1996). Both the Reemann pyranometer and the net radiometer
were calibrated against the Sun, using a normal pyrheliometer, i.e., their
sensitivities were linked to the International Pyrheliometric Scale.
Simultaneously with these radiation measurements, air temperature and
humidity at a height of 150 cm inside and outside the forest, wind speed
above the canopy, and ground surface temperature beneath the forest were
acquired.
3.
RESULTS AND DISCUSSION
Measurements above the canopy at a height of about 12 meters enable to
determine the daily course of incoming radiation fluxes, the fraction of the
PAR in the global fluxes, and also the albedo of the plantation. An example
of these data is presented in Fig. 2 and 3. The variability of these curves is
caused by the presence of slight
clouds. The relative part of global PAR
PAR (Q)/Q exhibits a minimum of about 0.45 around noon, and increases to
0.5 at solar heights less than 10°. For direct solar radiation, the relation
increases with solar height and reaches a maximum value of about 0.38
around noon. The relative part of PAR in the reflected radiation depends
slightly on solar height and has a maximum of 0.10 for a solar height
between 15 and 30°. The albedo for global radiation Ak has an obvious daily
variation with a minimum value of about 0.24 at noon. The albedo in the
PAR region is much less (about 0.04 at noon) and also increases when solar
height decreases. The asymmetry of the curves is mainly due to azimuthal
non-homogeneity of the willow plantation structure, and possibly also to
changes in the plant structure and optical properties of leaves during the day.