S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253 247
Table 1 Meteorological parameters of the main measurement days
Latitude Date
Sunrise time
a
Sunrise hour angle
b
Solar altitude at culmination 35
◦
N winter solstice
7:10 61.1
31.6 vernal equinox
6:00 90.0
55.0 summer solstice
4:49 119.1
78.5 45
◦
N winter solstice
7:42 55.9
21.6 vernal equinox
6:00 90.0
45.0 summer solstice
4:17 124.3
68.5 55
◦
N winter solstice
8:32 46.3
11.6 vernal equinox
6:00 90.0
35.0 summer solstice
3:26 134.1
58.5
a
The true solar time.
b ◦
for south, positive for eastwards.
the solar cell outside the greenhouse at the same time. The ‘normalized plant irradiance’ means the average
of the normalized leaf irradiance of the 10 leaves, on which the solar cells were set, on the measuring plant.
The ‘normalized canopy irradiance’ means the average of the normalized plant irradiance of all the measuring
plants five in E–W row canopy and four in N–S row canopy, see Fig. 2. Consequently, the term ‘normal-
ized daily leaf irradiance’ means the ratio of daily in- tegral of the direct solar radiation reaching the leaf to
that of outside the greenhouse, ‘normalized daily plant irradiance’ and ‘normalized daily canopy irradiance’
mean the average of normalized daily leaf irradiance of the 10 leaves, and the average of the normalized
daily plant irradiance, respectively.
Among the above-mentioned definitions, the nor- malized daily leaf irradiance was calculated with the
following equation:
LEAF
d
= R
t
1
t
A
in
t A
ou
t J
h
t dt
R
t
1
t
J
h
t dt
where t and t
1
were the starting hour of the measure- ment and the noon 12:00, A
in
t, A
ou
t were direct solar radiation Wm
2
reaching the surface of the leaf and that of solar cell outside the greenhouse at time
t, respectively. J
h
t was direct solar radiation Wm
2
on horizontal outside plane at time t, which was nu- merically calculated using Bourger’s equation Kurata
and Okada, 1984, assuming the air transmissivity of 0.7.
3. Results
Fig. 3 shows the seasonal variations of normalized daily canopy irradiance as a function of latitude. The
following characteristics can be recognized from this figure: 1 E–W and N–S row canopies showed dif-
ferent patterns of seasonal variation in the normal- ized daily canopy irradiance. For E–W row canopy,
the highest value of normalized daily canopy irradi- ance was observed on 21 January at 35
◦
N, 21 Febru- ary at 45
◦
N and 21 March at 55
◦
N, respectively. And the values of the rest of the months were decreased
gradually from this peak point, except for those from March to June at 35
◦
N. For N–S row canopy, the low- est values of normalized daily canopy irradiance were
found on the winter solstice for all the three latitude regions, while the highest value was found on the sum-
mer solstice at 45 and 35
◦
N, and 21 April at 35
◦
N. 2 At 35
◦
N, normalized daily canopy irradiance of E–W row canopy showed higher values than that of
N–S row canopy from the winter solstice to about the end of February, while the results became the oppo-
site from March to the summer solstice. 3 However, at 45 and 55
◦
N, values of normalized daily canopy ir- radiance of N–S row canopy were higher than those
of E–W row canopy throughout the year. 4 Further- more, the larger differences between E–W and N–S
orientations in the normalized daily canopy irradiance could be seen from late spring to the summer months
than that from the winter to early spring months re- gardless of latitude.
Fig. 4 shows the comparison of diurnal variations of normalized canopy irradiance between E–W and N–S
248 S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253
Fig. 3. Seasonal variations of normalized daily canopy irradiance of E–W and N–S row canopies as a function of latitude.
orientations at different seasons and latitudes. On the winter solstice, higher values of normalized canopy ir-
radiance during the early morning hours for E–W row canopy, and around 10:00 a.m. for N–S row canopy
than those of other time points could be seen at ev- ery latitude. But values between row orientations were
quite similar and one or two cross points of the curves could be found. Moreover, a more gentle variation of
Fig. 4. Diurnal courses of normalized canopy irradiance as functions of row orientation and latitude.
normalized canopy irradiance at 35 and 45
◦
N than that at 55
◦
N from the early morning to noon was ob- served. During the vernal equinox and the summer sol-
stice, on the other hand, values of normalized canopy irradiance were increased generally from morning to
noon. And higher values of normalized canopy irradi- ance could be seen at most time points of the day in
N–S row canopy than in E–W row canopy. Further-
S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253 249
more, no obvious differences could be found at noon in the normalized canopy irradiance between E–W and
N–S orientations in every season and latitude. For both the row orientations, the lower values of normalized
canopy irradiance in the early morning on the vernal equinox and summer solstice were obviously caused
by the shade-effect of opaque north wall, north roof and the east-, west-gable walls of the lean-to green-
house during that time period Li et al., 1994, 1995; Li, 1995.
As an example, daily variations of the normalized plant irradiance at 45
◦
N are shown in Fig. 5. Similar results were also obtained at 35 and 55
◦
N. The loca- tions of the south and the north plant in the canopies
shown in this figure are as follows see Fig. 2. The south plant
The north plant In E–W canopy
1 5
In N–S canopy 1
4 that is, the south plant was located at the south edge,
and the north plant at the north edge of the canopy, respectively. Moreover, each corresponding pair of
plants has the same attitude in the canopy Fig. 2. For the south plant on the winter solstice, higher value
of normalized plant irradiance from 8:00–9:00 a.m. could be seen in E–W row canopy than that in N–S
Fig. 5. Diurnal courses of normalized plant irradiance in different locations of the canopies at 45
◦
N.
row canopy, but at most points from 9:00 a.m. to 12:00 p.m., higher value could be seen in N–S row canopy
than that in E–W row canopy. While for the south plant on both the vernal equinox and summer solstice,
higher values of normalized plant irradiance could be read in N–S row canopy than that in E–W row canopy
throughout the day. For the north plant on the win- ter solstice and vernal equinox, similar values of nor-
malized plant irradiance were obtained under both the row orientations and two cross points could be recog-
nized around 9:00–11:00 a.m.; however, higher values in N–S row canopy than in E–W row canopy could be
found throughout the day on summer solstice. In ad- dition, the differences of normalized plant irradiance
between row orientations became bigger from winter to summer regardless of the plant location.
Figs. 6 and 7 show the comparisons of the normal- ized daily leaf irradiance between row orientations of
the south and the north plant, respectively. The hor- izontal axis of the figures indicates the leaf number.
That is, number 1 means the top leaf of the plant, and numbers 1–10 represent the 10 leaves on which the
solar cells were mounted. The locations of the two plants, and the azimuthal directions of the top leaves
were shown in Fig. 2. The vertical axis of the figures indicates the normalized daily leaf irradiance.
250 S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253
Fig. 6. Comparisons of normalized daily leaf irradiance of the south plant under E–W and N–S row orientations.
In Fig. 6, the normalized daily leaf irradiance of N–S row canopy was higher than that of E–W row
canopy in most cases. And this difference between row
Fig. 7. Comparisons of normalized daily leaf irradiance of the north plant under E–W and N–S row orientations.
orientations, especially those of the lower leaves i.e., those with bigger numerals on the horizontal axis, be-
came more remarkable in spring and summer than in
S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253 251
winter. This suggested that the south plant in E–W row canopy was affected more by its neighbouring plants
than that in N–S row canopy. In addition, the regular fluctuation following the leaf number in values of nor-
malized daily leaf irradiance was obviously caused by the differences in their azimuth directions. For exam-
ple, leaf number 2, 6 and 10 on the south plant have a azimuthal direction to south see Fig. 2, consequently
comparatively higher normalized daily leaf irradiance was observed from these leaves in most cases Fig.
6. Some exceptions about this, e.g., lower values of leaf number 6 than leaf number 5 at 35 and 45
◦
N for E–W row plant, and at 55
◦
N on the winter solstice for both E–W and N–S plants, further suggested that the
amount of direct solar radiation accepted by a certain leaf is affected by many factors and under certain cir-
cumstances, shade-effect from the surrounding plants or leaves was also an important factor affecting values
of normalized daily leaf irradiance.
Comparisons of normalized daily leaf irradiance be- tween the north plants under E–W and N–S row ori-
entations are shown in Fig. 7. On the winter solstice, leaf numbers 2 and 3 of the N–S row plant gave higher
normalized daily leaf irradiance than that of E–W row plant, but the comparison between the rest of the leaves
showed the opposite results in general in every lati- tude region. On the vernal equinox, leaf numbers 2, 3,
6, 7, 8 at 35
◦
N, and 2, 3, 7 at 55
◦
N of the N–S row plant had higher normalized daily leaf irradiance than
that of E–W row plant. And on the summer solstice, higher values of normalized daily leaf irradiance were
shown in most leaves on N–S row plant than that of E–W row plant. Furthermore, compared with the re-
sults for the south plant Fig. 6, the regular fluctua- tion in normalized daily leaf irradiance following the
leaf number i.e., azimuthal direction was not so ob- vious. This was especially true for the cases on the
winter solstice and vernal equinox. Instead, leaves on the upper nodes of the plant i.e., leaf numbers 1, 2,
3 generally gave higher normalized daily leaf irradi- ance than those on lower nodes. Similar results were
obtained on the plants in the central part of the row canopies under both row orientations data not shown.
This suggested that, for those plants which located in the central or northern parts of the canopy, the leaf
position has more important effect on the amount of direct solar radiation reaching it than its azimuthal
direction.
4. Discussion
