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panicles. It is important to understand the effects of
the canopy's geometric structure on the remotely
sensed surface temperature.
The study was performed for two different types
of canopy structure, soybean (pla nophile) and corn
(erectophile) (Blad, et al. 1972). The canopy
structures, such as involves distributions and
orientations of leaves, and temperatures of leaves in
the canopies, were measured for both crop fields in
order to understand the relationship between
canopy structure and canopy temperature.
2. Materials and Methods
Experiments were conducted at Hokkaido
University Farm during crop seasons from 1992 to
1994. The radiative surface temperatures were
measured with an IR thermometer (OPTEX HR-1P),
which had a 4° field of view, a stated accuracy of
+0.5°C, aresolution of +0.1C, and a spectral band
of 10.5 — 12.54 m. The three items, comparison of
canopy structures, comparison of leaf temperatures,
and comparison of canopy temperatures, for fields
of corn and of soybean were analyzed.
2.1 Items for analysis
2.1.1 Comparisons of canopy structures: De Wit (1965)
distinguished four major kinds of plant canopies by
the arrangement of leaves: planophile, erectophile,
plagiophile, and extremophile. Corn belongs to the
erectophile type of canopy and soybean to the
planophile type. The arrangement of leaves in a
canopy, that is, spatial distribution, leaf area facing
Skyward, and azimuth distribution, influences the
canopy temperature measured by the IR
thermometer. The following canopy structures of the
corn fields and the soybean fields were measured.
(D Distribution of fresh weight with height, ®
Distribution of leaf area facing skyward with height ,
and © Number of leaves for each azimuthal
direction.
212 Comparisons of leaf temperatures: Leaf
temperatures of both corn and soybeans that
affected canopy temperature were compared. It is
known that each crop has a characteristic leaf
temperature; therefore, leaf temperature differs by
species and variety of crop and affects the canopy
temperature. The following measurements were
performed for leaf temperature of corn and
soybeans.
«D Difference of surface temperatures for the
upper-most leaves and @ Difference of leaf
temperatures for each azimuthal direction.
2.1.3 Comparisons of canopy temperatures: Canopy
temperatures of the corn field were compared to
those of the soybean field. The following analyses
were performed.
® Daily change of canopy temperature and ®
Maximum canopy temperature.
757
EE Sr a RE LE
2.2 Cultural practices
Measurements of canopy structures were
performed in 1993, measurements of leaf
temperature in 1993 and 1994, and measurements
of canopy temperature from 1992 to 1994.
Agricultural practices and measurement periods for
canopy temperature are shown in Table 1.
The rows for both the corn field and the soybean
field in those years were oriented north-south. In
1992, thinning was performed twice on July 3 and
24 in the soybean fields, and the resulting plant
spacing was 40 x 20 cm.
Table 1 Study conditions in fields.
Crop Soybean Corn
Year 1992 1993 1994 1992 1993 1994
Vari Saporro midori Ezinisiki Sirohana Hany Hany Peter corn
arie Taisetu Bantamnu36 Banntamu36
Seeding Date May 17 May 18 May 18 May21 May 14 May 17
20x 10 (em)
(5/17~7/03)
Plant 20x20 (cm): 25x25 (25x25 45 x 30 40 x 30 40 x 30
Spaces (7/04—7/24) (cm) (cm) (cm) (cm) (cm)
40 x 20 (cm)
(7/25—9/22)
Area of Field 11x8.5(m) 18x8(m) 13x8(m) 9x8(m) 11x8.5(m) 13 x 8 (m)
Period of Jun.26 — Jun. 12— Jun.30— Jun.26— Jun.12~ Jun. 30—
Measuring Sep.21 Aug.27 Sep.22 Sep.21 Aug.27 Sep.22
2.3 Measurement methods
2.3.1 Comparisons of canopy structures:
(1) Distributions of fresh weight with height : On
September 5 and 8, 1993, the distributions of fresh
weight at different plant depths within canopies
were measured, while plant heights of corn and
Soybean were 165cm and 70cm, respectively.
During measurement days the heights for both
crops were approximately the maximum. The canopy
was divided into 5 layers for corn, 0— 31cm, 31—
60cm, 61~90cm, 91 ~120cm and 121—165cm, and
4 layers for soybean, 0~ 15cm, 16 ~30cm, 31 ~
45cm and 46 ~70cm. The above-ground fresh
weight per layer was determined from samples in
1m?.
(2) Distributions of leaf area facing skvward with
height : The thermal radiance emitted from leaf
areas facing skyward within the canopy was sensed
by IR-thermometer, while the canopy temperature
was measured from above the canopy by
IR-thermometer. Ratio of leaf area facing skyward
of each layer to area of ground cover with crop was
measured on September 5 and 8, 1993. The sum
of the ratios for leaf area facing skyward equals the
percent ground covers of the vegetation. The
percent ground covers for corn and soybeans were
80.6% and 95.996, respectively, while the
measurements were performed. The layers adopted
are the same as those of fresh weight, that is, 5
layers for corn and 4 layers for soybeans. Ratio of
leaf area facing skyward was determined using
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
