infrared photos of each layer from a vertical position
1.5m above the canopy, and the leaf area in the
photo was scanned and calculated by a computer in
the laboratory.
(3) Number of leaves for each azimuthal direction :
There is a specific azimuthal direction of leaves for
each species, which is affects the canopy surface
temperature. Distribution of plant leaves for
azimuthal directions was measured on September 2,
1993, and classified into one of eight different 45°
intervals. Number of leaves for each azimuthal
direction was determined for the mean value of
each 10 plants.
2.3.2 Comparisons of leaf temperature:
(1) Difference of surface temperatures for the
upper-most leaves : Surface temperatures for the
upper-most leaves, that were artificially stretched to
a horizontal orientation, for both crops were
measured with the IR thermometer at the height of
im above the crop. The measurements of 10 leaf
temperatures for each crop were taken at one-hour
intervals from 08:30 to 15:45 on August 16, 1993,
while the weather was partly cloudy and partly
clear. Both crops were planted close to each other,
so soil water and meteorological factors such as air
temperature, wind speed and solar radiation were
approximately the same.
(2) Difference of leaf temperatures for each
azimuthal direction : On September 2, 1993 of a
clear day, temperatures for sunlit and horizontal
parts of leaves at eight azimuthal directions were
measured with the IR thermometer at one-hour
intervals from 08:30 to 15:45. The measurements
were taken for sunlit and horizontal parts of leaves,
that were located from the No.1 layer (the
upper-most) to the No.4 layer within the corn
canopy, and only the No.1 layer (the upper-most )
within the soybean canopy. The leaf temperatures
of 10 plants were averaged for each azimuthal
direction.
2.3.3 Comparisons of canopy temperature: Canopy
temperatures for the corn field and the soybean
field were measured with an [IR thermometer that
was mounted 2m above the top of the vegetation
and scanned over the canopies with a 45° viewing
angle during measurement periods. Also, the
temperature measurements of a horizontal reference
plate (painted black) were used to correct readings
of the IR thermometer for skyward longwave
radiation (Wang et al., 1994). The following items
were compared between the corn field and the
soybean field.
© Daily change of canopy temperature and &®
Maximum canopy temperature.
2.3.4 Measurements of other meteorological factors: The
data of air temperature and solar radiation were
obtained during measurement periods as Table 1
by a thermocouple thermometer 1.5m above the
ground and with a pyrheliometer.
3. Results and discussion
3.1 Comparison of canopy structures
3.1.1 Distribution of fresh weight with height:
Distributions of fresh weight within canopies of both
corn and soybeans are given in Fig. 1. Corn canopy
has a A-type structure in which there is much
vegetation in the lower part of the canopy, but the
soybean canopy has a V-type structure. 50% of
the total fresh weight within the corn canopy was
located in the lower one-third of the plant height,
and 60% within the soybean canopy in the upper
one-third. Similar results were reported by Shaw
and Weber (1967) and Nakaseko and Gotoh
(1984). These canopy characteristics affect canopy
temperature in both fields.
'93.08.05
K. 396
'93.09.08
Sweet corn
1
Soybean
1 - 1
40 30 20 10 0 10 20 30 40
(96) (96)
758
Fig. 1 Distributions of fresh weight within
canopies of both corn and soybeans.
3.12 Distributions of leaf area facing skyward with
height: Temperatures of leaves facing skyward
within the canopy and soil surface temperature are
sensed by an IR thermometer when measurements
are taken from above the canopy, and the weighted
mean temperature of leaves facing skyward and
soil surface temperature is called the canopy
temperature of canopy. Therefore, distribution of
leaf area facing skyward within the canopy is
important for canopy temperature measured by the
IR thermometer.
The ratio of leaf area facing skyward to the area
of total vegetation cover above ground is shown in
Fig. 2. The corn canopy has 41% of the leaf area
facing skyward in the upper-most layer, and the
soybean canopy has 72%. Up to the second layer,
the ratio is 53% for the corn canopy and 96% for
the soybean canopy. This is important for canopy
temperature measured by IR thermometer, because
leaf temperatures of lower parts within the canopy
strongly influence the measured composite surface
temperature of the corn canopy. On the other hand,
only the upper-most part of leaf temperatures
influences the measured canopy temperature for the
soybean canopy.
Transpiration from leaves of each layer is
different because there are air temperature and
vapor pressure profiles within canopies, that is, the
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
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