, Sweet corn 
28. 93.08.02 v 
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24 CA 
% S NM 
22 N J^ : 
  
Temperature('c) 
> 
  
  
0.6 
0.5 
0.4 
  
  
Solar radiation (kw/r ) 
0.3 
08:00 10:00 12:00 
  
14:00 16:00 
Local time(hr.) 
Fig. 5 Daily changes of leaf surface temperature 
for sunlit and horizontal parts of leaves 
at four azimuthal directions and solar radiation. 
Wind direction 
  
  
Fig. 6 Daily mean surface temperature for sunlit 
and horizontal parts of leaves at eight 
azimuthal directions. 
The mean leaf temperature and standard 
deviation of eight azimuthal directions for each hour 
are calculated in Table 2. The mean leaf 
temperatures of soybeans for each hour are higher 
than those for corn except at 08:30 and 13:30. This 
is explained by the fact that the lower is the location 
within the canopy for corn, the lower is air 
temperature in the daytime, and the lower part of 
leaves within the canopy is affected by the low air 
temperature and results in a low leaf temperature 
that is included in the canopy temperature for corn. 
Also, the standard deviations for soybeans are 
larger than those for corn except at 11:30. This is 
due to the fact that leaves are affected strongly by 
wind and air temperature above the canopy 
because the leaves of soybeans are located in the 
upper part of the canopy. Those facts, i.e., that the 
760 
the soybean field, in spite of nearly 
leaf temperature for soybeans has a large deviation 
in azimuthal directions and is higher than that of 
corn in the daytime, were confirmed by the 
measurements in 1994. 
The results of leaf temperature are summarized 
as follows; ( leaf temperature for soybeans is 
higher than that for corn, when solar radiation is 
high, ® when solar radiation is low, leaf 
temperature for soybeans is lower than that of corn, 
@ leaf temperature for soybeans has large 
variations for each azimuthal direction and at each 
hour, and @ the variation of daily mean leaf 
temperature for azimuthal directions for soybeans is 
large. The characteristics of leaf temperature must 
influence the canopy temperature. 
Table 2 The mean leaf temperatures of eight 
azimuthal directions and standard 
deviations for each hour. 
  
  
Crop Time(JST) 08:30 09:30 10:30 11:30 12:30 13:30 14:30 15:30 
Corn Meantemp(C).25.3; 25.4, 025.0 256-261 246 234 215 
Stand. dev((C)| Hit 208 203 =09 =02 T10 —11 —04 
Soybeans Meantemp.(C) 24.6 25.5 265 263 265 240 23.6 219 
Stand. dev((C). 50.3 ELSE LILEO2 13-517 13 17 
  
3.3 Comparisons of canopy temperature 
Daily changes of canopy temperature and 
maximum canopy temperature were compared 
between the corn fields and the soybean fields. 
3.3.1 Daily change of canopy temperature: Daily 
changes of canopy temperature are shown in Fig. 7. 
The vegetation cover ratios for the corn fields and 
the soybean fields were 70.1% and 83.4% on 
August 30, 1992 (Fig. 7(a)), 28.4% and 26.3% on 
June 30, 1993 (Fig. 7(b)), and 81.9% and 49.1% on 
July 24, 1993 (Fig. 7(c)), respectively. 
According to Fig. 7(a), there is a large difference 
of canopy temperature between the corn field and 
the soybean field when solar radiation is high, and 
a small difference when solar radiation is low. 
There is no difference of canopy temperature 
between the corn canopy and soybean canopy at 
night. 
In an early growth stage, on June 30, 1993 (Fig. 
7(b), canopy temperature of a the corn field that 
had approximately the same vegetation cover ratio 
as a the soybean field was slightly higher than that 
of the soybean field or the nearly same as the 
soybean field in the daytime. On July 24, 1993 
(Fig. 7(c)), however, the vegetation cover ratio of 
the corn field became more than that for the 
soybean field, and the canopy temperature of the 
corn field became lower than that of the soybean 
field. This phenomena that the canopy 
temperature of the corn field was lower than that of 
the same 
vegetation cover ratio at a latter growth stage in 
1992 (Fig. 7(a)). 
Though those results are for the daytime, the 
vegetation cover ratio insignificantly influences the 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996 
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