244 S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253
1984; Myneni et al., 1986; Whitfield, 1986; Goudri- aan, 1988; Kurata et al., 1988; Gijzen and Goudriaan,
1989; Yang et al., 1990a, b have been done in the past. However, only a few studies dealing with the effect of
row orientation on solar radiation interception by the row canopy can be found among them. Allen 1974
conducted a study on direct-beam radiation penetra- tion into a wide-row crop canopy the dimensions of
the canopy were: width of inter-row W
ir
= 60 cm, row height H
r
= 80 cm, row width W
r
= 40 cm of field sorghum and concluded that there were 37
and 44 daily interceptions for east–west E–W and north–south N–S row orientations, respectively 17
August, 40
◦
N. Iwakiri and Inayama 1974 made nu- merical calculations on the characteristics of direct so-
lar radiation penetration into cucumber row canopies the dimensions of the canopy were: W
ir
= 100 cm,
H
r
= 150 cm, W
r
= 80 cm grown in a plastic green-
house based on the understanding of the canopy geometric structures through experimental measure-
ments. Their results showed that E–W row canopy gave a higher percent sunlit leaf area FlFt: where
Ft indicates leaf area index LAI and Fl the sunlit LAI on the winter solstice and the vernal equinox,
but a lower value on the summer solstice than the N–S row canopy 35
◦
N. Mutsaers 1980 carried out computer simulations to investigate the effect of row
orientation, season and latitude on light absorption by row crop canopies the dimensions of the canopy
were: W
ir
= 50 cm, H
r
= 100 cm, W
r
= 50 cm. His re-
sults showed that at latitudes of 35–55
◦
N, daily direct light absorption is highest with N–S row orientation
during the summer months and with E–W orientation for the rest of the year. Kurata et al. 1988 utilized
fisheye photography to analyse the effect of row ori- entation on the direct solar radiation interception by
tomato row canopies and obtained results similar to those of Iwakiri and Inayama 1974. For example,
in winter E–W orientation gave a higher value of daily canopy absorptance ratio of daily integral of
intercepted direct solar radiation to daily integral of incident direct solar radiation than N–S orientation,
but in summer E–W orientation showed lower daily canopy absorptance than N–S orientation 35
◦
N. However, these analyses seem to have diffi-
culty in giving satisfactory explanations to the above-mentioned practical case being conducted in
the lean-to greenhouses in China. Furthermore, almost all the research on solar radiation penetration into crop
canopies conducted so far was based on mathematical models, and no report was found in the literature on
direct measurement of the solar radiation reaching the leaves’ surfaces of the row crops. This is mainly
because of the very complicated geometric structures of the crop canopies, which makes it almost impos-
sible to conduct complete measurements inside them. Moreover, in respect to solar radiation environments
of a row crop canopy cultivated in greenhouses, no work dealing with both the effects of row orienta-
tion and the greenhouse structure has been reported. The above-mentioned analyses made by Iwakiri and
Inayama 1974 and Kurata et al. 1988 came from research on greenhouse row canopies, but the effects
of greenhouse structures were not included in their results.
In the present study, model row crop canopies, for which some simplifications on the geometric struc-
tural parameters were made, were put into a model greenhouse, and experimental measurements were
conducted under artificial direct solar radiation. The purpose of this study was to investigate the effect of
row orientation on direct solar radiation interception by the row canopies.
2. Materials and methods
2.1. The model lean-to greenhouse The lean-to greenhouse is E–W oriented and its
north wall, north roof and east-, west-gable walls are made of opaque materials. A 1 m wide area from the
north edge on the floor is used as a path and the remain- ing 5 m wide area on the south part of the floor is used
for cultivation. The model greenhouse was constructed to a ratio of 115, with a floor area of 40.0 cm × 160.0
cm width × length, Fig. 1. The inside surfaces of the north wall, north roof and east-, west-gable walls were
painted matt black to eliminate the influence of reflec- tion inside the greenhouse. PVC-film was used as the
covering materials for the transparent south roof.
2.2. The model crops and canopies Geometrical structural parameters of the model
crops and canopies have to be decided before they
S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253 245
Fig. 1. A schematic diagram of cross-section of the model lean-to greenhouse and the model E–W row canopy 1 : 15 inside. Values in parentheses are actual structural parameters of the greenhouse unit: cm.
are made artificially. The basic parameters include: LAI, the average leaf dimensions width and length,
leaf angle, row height H
r
, row width W
r
and width of inter-row W
ir
, see Figs. 1 and 2. It is almost impossible to simulate the detailed structures of a
crop canopy completely because of their complexi- ties. Therefore, the following hypotheses were raised
when the model crops were made: 1 The shape of the leaves is round and the areas of all leaves,
Fig. 2. A bird’s-eye view of the model row crop canopies used in the experiment. Plants with big circles indicate the measuring ones and the black small circle on each model plant indicates the top leaf unit: cm.
regardless of their vertical positions on the plant, are the same; 2 The internodes between every two
vertically neighbouring leaves are the same, and the difference between their azimuthal directions is 90
◦
. Based on the above-mentioned hypotheses and
by referring to those of greenhouse cucumber row canopies Iwakiri and Inayama, 1974; Liu et al., 1987;
Yang et al., 1990a, model crops and canopies were made manually as described below.
246 S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253
2.2.1. The model crops The leaf was 1.5 cm in diameter and which gave
a leaf area of 1.77 cm
2
. The length of the petiole was 0.5 cm. The stem height of the plant was 6.0 cm,
and 12 leaves were attached on it. Consequently, the dimensions of a single plant became: 4.0 cm
width × 4.0 cm length × 6.0 cm height.
All model leaves were made of green paper about 0.2 mm thick, the stems were made of wooden bars
with a diameter of about 1.8 mm, and the petioles were made of copper wires with a diameter of about 0.3 mm.
2.2.2. The model canopies Planting density of the model canopies was 5.50 cm
distance between rows, i.e., W
r
+ W
ir
× 3.33 cm in- tervals between plants, which gave a LAI of 1.16
Figs. 1 and 2. For E–W row canopy, there were 5 rows × 22 plantsrow = 110 plants, which occupied
the floor area of 77 cm length × 26 cm width; and for N–S row canopy, there were 14 rows × 7
plantsrow = 98 plants, which occupied the floor area of 77 cm length × 23.8 cm width. Both E–W and
N–S row canopies were located at the central part of the cultivation area of the greenhouse. Figs. 1 and 2
show the side view and the bird’s-eye view of the arrangements of the canopies, respectively.
As shown in Fig. 2, five plants in E–W row canopy, four plants in N–S row canopy on the central parts of
the canopies were selected for measurement, respec- tively. Light sensors were set on the upper 10 leaves of
each measuring plant to detect the radiant flux density reaching it.
2.3. The light sensor Solar cells KYOCERA, PSC1010, each with a de-
tecting area of 1.0 cm width × 1.0 cm length, were used as the light sensor in this investigation. The so-
lar cell’s diagonal line 1.41 cm was nearly equal to the diameter of the model leaf 1.5 cm and it cov-
ered about 57 surface area of the model leaf Fig. 2. The solar cell can detect a light spectral range of
400–1100 nm.
2.4. Experimental treatments and observations The model canopies were arranged to be E–W and
N–S oriented Fig. 2 and with a leaf inclination of
◦
horizontal. The other parameters related to the canopy geometrical structures were kept to be constant
as those mentioned above. Measurement days were assumed to be from the
winter solstice to the summer solstice at an interval of 1 month, and the greenhouse locations were supposed
to be 35, 45 and 55
◦
N. Results of the other half of the year were symmetric to the measured half. On every
assumed measurement day, the time from the sunrise to noon was divided into several measurement time
points at an interval of one hour. Results in the af- ternoon were roughly considered to be symmetric to
those in the morning. Table 1 gives some meteorolog- ical parameters of the main measurement days.
A slide projector 300 W was used as the light source. The distance between the slide projector and
the model greenhouse was about 12 m, and the light beams reaching the model canopies could be regarded
as parallel in this condition.
To simulate the solar position at each measurement time point, a stand whose attitude could be changed
three dimensionally was designed and made manually. Instead of changing the position of the slide projector,
the attitude of the stand, on which the model green- house was mounted, was adjusted manually according
to the angles calculated beforehand to fit the solar po- sition seen from it. The stand could give a precision of
about 0.5
◦
in both latitude and azimuth adjustments. To measure the direct light reaching outside the
model greenhouse, a solar cell of the same type KY- OCERA, PSC1010 was set parallel to the floor just
above the north wall and at the ridge height level of the model greenhouse Figs. 1 and 2. The voltage out-
puts of all solar cells, including those on the measur- ing plants in the model canopies and the one outside
the greenhouse, were recorded by a hybrid recorder YOKOGAWA, Model 2500.
2.5. Definitions related to the quantity of solar radiation intercepted by the canopies
To make the results easily understood, several con- cepts were defined to represent the amounts of direct
solar radiation accepted by a leaf surface, a single plant and the whole canopy. The term ‘normalized leaf ir-
radiance’ refers to the ratio of direct solar radiation reaching the surface of a leaf at a certain time to that of
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
