Canopy Reflectance as Influenced by Solar Illumination Angle
Purdue University
Purdue e-Pubs LARS Technical Reports
Laboratory for Applications of Remote Sensing
1-1-1981
Canopy Reflectance as Influenced by Solar Illumination Angle J. C. Kollenkark V. C. Vanderbilt C. S. T. Daughtry M. E. Bauer
Follow this and additional works at: http://docs.lib.purdue.edu/larstech Kollenkark, J. C.; Vanderbilt, V. C.; Daughtry, C. S. T.; and Bauer, M. E., "Canopy Reflectance as Influenced by Solar Illumination Angle" (1981). LARS Technical Reports. Paper 14. http://docs.lib.purdue.edu/larstech/14
This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information.
AgRISTARS Supporting Research
SR-PI-04039 NAS9-15466
A Joint Program for Agriculture and Resources Inventory Surveys Through Aerospace Remote Sensing March 1981
Technical Report
Canopy Reflectance as Influenced by Solar Illumination Angle by J.C. Kollenkark, V.C. Vanderbilt, C.S.T. Daughtry, and M. E. Bauer
Purdue University Laboratory for Applications of Remote Sensing West Lafayette, Indiana 47907
NI\S/\
SR-Pl-04039 NAS9-l5466 LARS 021681
Canopy Reflectance as Influenced by Solar Illumination Angle
J.C. Kollenkark, V.C. Vanderbilt, C.S.T. Daughtry, M.E. Bauer
Purdue University Laboratory for Applications of Remote Sensing West Lafayette, Indiana 47907
March 1981
Star Information Form 1.
Report No. SR-Pl-04038
4.
3.
Recipient's Catalog No.
Title and Subtille
5.
Canopy Reflectance as Influenced by Solar Illumination Angle
Report Date February, 1981
6.
Performing Organization Code
2.
Government Accession No.
7.Author(s)J.C. Kollenkark, V.C. Vanderbilt, C.S.T. Daughtry and M.E. Bauer Performing Organization Name and Address Purdue University Laboratory for Applications of Remote Sensing 1220 Potter Drive West Lafayette, IN 47906 1-------.:..--...:.------------------------1 12. Sponsoring Agency Name ana Address NASA Johnson Space Center Remote Sensing Research Division Houston, TX 77058 15. Supplementary Notes F.G. Hall, Technical Monitor M.E. Bauer, Principal Investigator 9.
Performing Organization Report No. 021681 10. Work Unit No. 8.
Contract or Grant No. NAS9-ls466 13. Type of Report and Period Covered Technical 11.
14.
Sponsoring Agency Code
Abstract
16.
An experiment was conducted at West Lafayette, Indiana in 1979 to quantitatively describe the interaction of the solar illumination angle and row azimuth angle on the measured reflectance factor (RF) of soybean canopies consisting of 11 plots. Nine of the plots were planted in 71 cm wide rows; the other plots were of bare soil and soybeans with 100 percent soil cover. Reflectance factor data in four spectral bands, 0.5-0.6, 0.6-0.7, 0.7-0.8, and 0.8-1.1 ~m, were taken at 15 minute intervals during three clear days, August 12 and 31 and September 19 over nine plots of differing azimuthal direction with a Landsat-band radiometer (Exotech Model 100) at 5.2 meters above the soil. Diurnal changes of nearly 140 percent were observed in the red wavelength region when canopies covered 64 percent of the soil. The amount of shadow observed was a function of the plant geometry and row width. As soil cover approached 100 percent, the diurnal changes diminished. A function that describes the solar illumination angle with respect to the row azimuth explained most of the diurnal variation in the measured RF. Variation in near infrared response was much less and did not appear to be as strongly related to sun-row angle interactions. The ratio, near infrared/red, was highly sensitive to sun angle-row direction interactions, whereas the greenness function, utilizing all four spectral bands, was not. 17. Key Words (Suggested by Author(s)) 18. Distribution Statement Remote sensing, crop canopy, sun angle, Glycine max 1..
19.
Security Classi! (ofth,s report)
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'For sale by the National Technical Information Service. Springfield. Virginia
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No. of Pages
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1
Introduction Understanding illumination necessary models
and
the
crop canopy
to utilize
have
been
reflectance factor
effect
of
the
geometry
on
reflectance factor proposed
to
of plant canopies as
between
the spectral
and
predict
a function of
solar
response
data effectively.
explain
sun angle, and view angle (Suits, 1972; et al., 1975).
interactions
is
Numerous
the
measured
plant geometry,
Smith et al., 1975;
Richardson
The models by Suits and Smith deal with a canopy with no
horizontal spatial variations. Richardson et al. with distinct soil,
modeled the reflectance of
horizontal spatial variations,
and shadow
various shaped
(1975)
components.
objects,
percent of the variance in the
vegetation
viewed are
particular canopy
surface covered with to explain 80
to 85
(1979) assumes an incomplete canopy
The fractions of sunlit and shaded soil and
calculated as
condition,
was able
of plant,
reflectance measurements due to shadows.
A model suggested by Jackson et al. of rectangular-shaped rows.
as a function
By illuminating a
Egbert (1977)
a row crop,
a function
described
ratio, row spacing and direction,
of view
by plant
time of day,
cover,
angle for
a
height/width
day of year,
latitude,
and size of the radiometer resolution element. Studies of the effect of sun have supported that
the predictions of
the reflectance
factor
zenith angle on reflectance generally the Suit's canopy
should increase
as
reflectance model
the solar
elevation
2
increases (Colwell, 1974; attributes this Field data response
Chance and LeMaster,
to changes in the
have shown
sun elevations
LeMaster, 1977; Jackson et al., 1979). and
non-symmetric
components
Colwell (1974)
amount of shadow within
minor to significant
with decreasing
1977).
increases in
(Duggin,
the canopy. the infrared
1977;
Chance
and
Crecelius (1978) noted symmetric
about solar
noon
that
influenced
observed variation
in reflectance throughout
component,
solar
angle,
variation.
Other effects, such as drying of the soil surface and plant
wilting
will be
explained
asymmetric about
the
solar
the day.
the
majority
noon and
The symmetric of
the
may be
observed
significant
factors to consider. Further investigation of reflectance factor data taken in 1978 over incomplete soybean canopies revealed possible time of day effects in the Landsat band regions as illustrated in the red, 0.6-0.7 pm, and the near infrared, 0.8-1.1 pm, in Figure 1. row direction.
Plots were planted in a north-south
Both bands were plotted with and without a 1.5 hour time
restriction about solar noon.
Low responses were noted over those plots
that were measured more than 1.5 in RF resulted from
hours from solar noon.
shadows between the rows and a
These changes
lower response from
the soil component. The objective soybean canopy
of this research was
as a function of
azimuth and zenith angles.
to model the reflectance
row width,
row direction,
of a
and solar
The hypothesis was that by varying only the
14
0.6-0.7J,1m
70
...... ~ 12~" ":"',• , ,,' I'" ,',.:. .. 10 't It ,1," ' t o .... ••. I,' ' • (J
1'0
u.
g
....1'0 ~
;:
.9
: ",,' '
6
."
:,'::-:. :,' u' • , "
"""
'.
c
't
..,
• '"
,
4
I,'
..
,. "
'
., ..:.. . . ....:.
'"
":
o
,"
2 O'~--------~------~
________ ________
-L________
~
'I
c::
'i'/ .'.' " ,~I
.. .. .... ,
!3 o
." • '/ 'J'-' ••,I .1.': !:jl!liI.,·1 .' 1 . *,.: !' l,. ' I
G)
'f.'
i a:
,:1/
.. ...,; I! ..
.' '. .:"
G)
•I·,'..•• r,..,'I'',f"
"
50
~
,
II)
a:
60
o
,, , "
"
,
~
,t' ••
a
G)
.-.. ......
a
___
o,a-1,1J,1m
,.,!,," ,tl
I, ,-" " • II,'
':!.
I" ,
. •'
,I
• ' :. 1
•
", ,t
e' ',"
'"•••• ,-
•
,I'
"..
"
'
oLI____~____~____~__~____~
~
70
-......
b
ot:'!' • . . .. I •• ! ....
..
~
... ....oo
"
-,
o 50 ....
.', 'I.,
o
~ 40
G)
G)
o
c::
•"
....1'0o
','
"
(1)
g ....1'0
..
, II:
;:
.',
2
.. 'I;, '/ ""'!I
I .. , .;,
• ',I
'It •.••• 1
.
"
," ,:, "·1' . .•s :.t: ..... ..,...•
;;::: 20
20
40
60
Soil Cover ('Yo)
ao
100
I.: ,
..
,
I
.
"
,
:.
•••
I
I I ' ... , -
,"
'
I,
:.
I'
,
"
.
.... ,.
• ,"
'I,:
'
(1)
a:
10
0'
0
.
30
(1)
.. '
(1)
0
.• ",:1
~ ......
1'0 U.
a:
d
...... 60
o
!
20
!
40
I
!
I
60
ao
100
Soil Cover (%)
Figure 1. Time of day effect on the RF seen in the 1978 data for the red (0.6-0.7 ~) and the near infrared (0.8-1.1 ~) wavelength regions, (a,c) All 1978 data minus dates which include wet soil and/or senescing vegetation. (b,d) Same as a and c above, but only includes data in the three hour time period centered about solar noon.
w
4
row direction, by
changes
the variation in reflectance would be explained entirely
in
sun
zenith
and azimuth
angle
with
respect
to
row
direction. Materials and Methods Experimental Conditions The experiment was
conducted in 1979 on the
Soybeans (Glycine max (L.) Merr.
Purdue Agronomy Farm.
"Amsoy 71") were planted on a Chalmers
silty clay loam (typic Argiaquoll) on June 25, 1979. because they have dense foliage with of the season.
Soybeans were used
distinct row patterns through much
This is in contrast
to many of the
other major crops
such as wheat and corn that have a much more complex canopy geometry and shadow pattern. season,
Because of extended periods of cloudy days early in the
spectral data
were collected only on
development stages after
full bloom. The experiment consisted
of 11 randomly arranged
3.5 m wide and 5.2 m long (Figure 2).
plots which were
Nine plots were planted in 71-cm
rows with the following azimuthal directions:
0-180,
90-270, 105-285,
and 165-345 degrees from
north.
120-300,
135-315,
150-330,
30-210,
60-240,
Another plot was planted in 25-cm wide east-west and north-south
rows to obtain a canopy with negligible was included soybean plots.
to monitor the sunlit Row
row effects.
A bare soil plot
soil background reflectance
directions were selected to
of the
favor data collection
during the morning hours when cloud-free conditions were more likely.
5
Figure 2. Illustration of field spectral data acquisition over the row direction plots in 1979.
6
Three development stages with 65, 78, the 71-cm rows were represented with canopy with 78 percent
and 94 percent soil cover on
the three measurement dates.
soil cover was obtained by trimming
canopy just prior to the start of senescence.
The
a near full
The cross sectional shape
of the canopy was determined by placing a large piece of poster board in the canopy,
perpendicular to the row azimuth,
at several locations and
drawing the perimeter of the canopy on the board.
The canopy shapes for
each date are illustrated in Figure 3. Spectral Measurements Radiance measurements, were taken
used to
over all the plots
determine reflectance factor (RF),
with a Landsat band
radiometer (Exotech
Model 100) at 15-minute intervals throughout the day on three clear days (August 12, August 31, and September 19). Robinson and obtaining
Biehl (1979)
the
descibe
reflectance
bidirectional
reflectance
radiometer with
pm.
Data
which
factor.
The
field of
following wavelength regions: 0.8-1.1
the conditions and
factor,
a 15-degree
Nicodemus et al.
Exotech
view that
0.5-0.6,
were taken
closely
only under
(1977) and
proceedures for approximates
100
a
4-band
acquires data
in the
0.6-0.7,
is
the
0.7-0.8,
near cloud-free
and
conditions
(especially in the vicinity of the sun). A mobile
truck-mounted radiometer
efficient data collection in the field. the
truck permitted
the radiometer
placed 5.2 m above the crop canopy data were
collected over two
and
system was
used for
quick and
A boom mounted on the back of a motor-driven
camera to
and 3.5 m from the truck.
locations in each
plot on August
be
Spectral 12 and
7
80 60
o
'--_---l..LL._ _..l.-_----:U-L_ _....I
August 12
~_
August 31
-
E
o ........ +'"
.c
0> .(1)
J: _J_.J.......L_ _""___---I.~'"_ _ _ _
80
oL-_..l..-...1....:=L--_.l.-._
o
_J_-'--...I..-_.....I
35
0
Se ptember 19
35
Distance (em) Figure 3. Soybean canopy shapes and dimensions for three dates of data collection.
8
over four locations instruments were
in each plot on
carefully leveled
nadir look angle. averaged to
to obtain
all spectral
Several measurements were taken over
insure a representative sampling
biased values for on-row or bands were
August 31 and September
During data collection,
The
data at
a
each plot and
of the plot and
off-row measurements.
taken concurrently and recorded
19.
to avoid
Measurements in all
by a printing
data logger.
photographs were taken periodically
over each
plot for soil cover determination and shadow assessment. Agronomic Measurements Agronomic
measurements included
plant height,
maturity stage (Fehr and Caviness, 1977), fresh and
dry biomass,
Percent soil
and
stem,
pod,
cover was determined by
leaf area
surface soil moisture,
index, total
and green leaf
dry biomass.
placing a grid over
the vertical
photograph and counting the intersections occupied by green vegetation. Data Analysis The reflectance
factor data
reflectance data were and Thomas
for Landsat MSS
(1976)
=
BBand 3 * 0.17289)
*
0.612498 •
+
(Band4
Band1 to
bands.
um)/(0.6-0.7 pm)]
The
as band
transformed into greenness as
data (Malila and Gleason, 1977).
Landsat
were analyzed
*
means.
The
described by Kauth
data and modified
for spectrometer
The data transformation was: Greenness 0.59538)] - 8Band1
band4 refer
to the
near infrared/red
was also considered
variance and Newman-Keuls tests were
*
0.48935)
RF measured
reflectance
in the
ratio
in the analysis.
+
(Band2 four
[(0.8-1.1 Analysis of
performed to determine significant
effects of row-solar angle interaction and RF.
9
Results and Discussion The maximum
response of RF to
the sun azimuth
angle was equal to
Diurnal changes
in RF of
occurred when
the row azimuth angle
(Figure 4a).
nearly 140 percent
0.6-0.7 pm,
wavelength region,
changes in sun angle
were observed in
on August 12.
The highest reflectance
values were obtained when the soil was
sunlit and the lowest,
soil was
in the
shaded.
Diurnal variations
wavelength region,
0.8-1.1 pm,
were lower
the red
RF in the
when the
near infrared
(relative to the minimum RF
value observed) than that noted in the visible region and not as clearly related to sun-row interactions (Figure 4b). in RF are about the same. not
be as
pigment
dark as
The
shadows of the near infrared region may
those observed
absorption and
multiple
Note the absolute changes
in the
visible region
scattering
in the
due to
canopy
low
(Colwell,
1974). The effect of
sun-row azimuth interactions are shown
The reflectance was directions.
The
plotted over time for three plots peak response
three plots was not row azimuth. the
rows,
in the red
in Figure 5.
of different row
wavelength region
only at different times,
but also
for the
in order of the
Again the peak response was when the sun was shining down lighting
the
soil surface,
and
thus
giving
a
higher
wavelength region for two
of the
reflectance reading. The diurnal
response in the red
key canopy components, 6.
sunlit soil and vegetation,
are shown in Figure
Very little change in reflectance factor was observed as a function
of zenith
angle for either
the plot containing
bare soil or
the plot
10
10
0.6-0.7Jjm
-
~ ....... a..
0
.....
•
0
cu
u.
6
CD 0
c .....cu
4
0 CD
..... CD a:
•• 2
0 70
-
60
a..
50
0.8 -1.1 JJm
~ .......
0
..... 0
cu u. 40
••
CD 0
c 30 .....cu 0 CD
20
..... CD a: 10 0
-80
-40
80
Solar Azimuth - Row Azimuth Figure 4. Changes in the BRF in (a) the red wavelength band (0.6-0.7 and (b) the near infrared wavelength band (0.8-1.1 pm) plotted against the difference between solar and row azimuth. Row azimuth=180o.
pm)
11
10 s-
o
0.6 - 0.7 jJm
8
........ (,)
co
u..
"
......... ..... .. --'-..-, '" ... .... " ......
6
(]) (,)
c co
~
.....
"'
4
-+-' (,) Q)
-
,
~
...... ~ ........
.......
Row Direction 2
........ 2400 ---180°
.......
.... ..
....
--...--
1650 o~----~--~----~----~--~~--~----~
11
12
1
2
3
4
Time Figure 5. RF in the red wavelength region (0.6-0.7 ~) for three row directions over time on August 12, 1979.
5
12
10
--... .... 0~
0.6- 0.7jJm • •
•
•
•
•
•
•
• •
• Bare Soil
8
0 0
co
u.
6
a>
0
c::
....0co
4
• •• •
•
• •
•
•
• •
•
•
•
•
• Full Canopy
QJ 'to-
QJ
0:
2
30
40
50
60
Solar Zenith Angle Figure 6. Reflectance factor in the red wavelength (0.6-0.7 ~m) for the bare soil and full vegetative canopy plots against changes in zenith angle on August 12, 1979.
13
with 100
percent soil cover.
Note
the large differences
sunlit soil and sunlit vegetation. size,
and
soil width with
between the
Interaction of the canopy shape and
the sun
angle produces varying
shadow cast on both the soil and the vegetation. the diurnal variations in RF observed in
Thus,
amounts of
it may be that
Figures 4 and 5 were caused by
changes in the amount of shadow in the field of view of the instrument. Equations to
predict the shadow
cast by rectangular
rows as the sun
zenith and azimuth change throughout the
defined by many
investigators (Idso and Baker,
1979;
Verhoff and Bunnik, 1978).
to express the solar zenith (9) a plane
1972;
or spherical day have been
Jackson et al.,
For this study, an equation was used (~)
and azimuth angle
projected
ray onto
perpendicular
function,
called the projected solar angle,
to the 8sp
in relation to a
row azimuth.
This
= tan-1(tan8sin~),
is
illustrated in Figure 7. The
response
reflectance ratio, August 12 are the
of
the
red,
near
infrared,
and greenness transformation
presented in Figure 8.
greenness transformation
are
The near
often related
biomass, soil cover, and/or leaf area index. expressed as
a percent
value observed that day,
increase in
near
infrared/red
to changes in
8sp on
infrared/red ratio and to
changes in
plant
If the diurnal changes are
response relative
to the
minimum
the red and near infrared/red ratio were quite
sensitive to changes in esp, whereas the greenness and the near infrared region were not.
The near infrared wavelength region does show similar
absolute changes in response during the day, clearly related to the changes in
but the pattern is not as
sun-row angle and the variation about
the mean for any given 8sp is much higher for the near infrared region.
14
.
.~
,",\
\ /
/
Aug 21
,,--
~O-
Solar Angle Date
Time
Zenith
Azimuth
asp
Aug
15
12:00
26.3
180.0
26.3
Aug
21
2:09
40.1
234.2
26.3
4:49
73.7
261.7
26.3
Sept
10
Illustration of the solar projected angle e as observed Figure 7. when looking to the west. Three dates and the correspon~~ng observation !limes result in the same shadow pattern and e ~260. sp
10r
0.6-0.7 JIm
25
o
~ a:
-*
8
~ 20
a
C ~
~
~
o
ti
~ ~
c !!
IJ
...
~15
6..
~
4
a:~
_.,
~
a:
....
~
'
10
~ ~
IJ
i
c.
IJ
~.
--------
..... .
~
2
~
5~
«i
~
z 0' 70 r
0 0.8-1.1J1m
50 t-'
\J1
_ ~
~
r.
~.; ~
~
b
35
IJ
•
Purdue e-Pubs LARS Technical Reports
Laboratory for Applications of Remote Sensing
1-1-1981
Canopy Reflectance as Influenced by Solar Illumination Angle J. C. Kollenkark V. C. Vanderbilt C. S. T. Daughtry M. E. Bauer
Follow this and additional works at: http://docs.lib.purdue.edu/larstech Kollenkark, J. C.; Vanderbilt, V. C.; Daughtry, C. S. T.; and Bauer, M. E., "Canopy Reflectance as Influenced by Solar Illumination Angle" (1981). LARS Technical Reports. Paper 14. http://docs.lib.purdue.edu/larstech/14
This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information.
AgRISTARS Supporting Research
SR-PI-04039 NAS9-15466
A Joint Program for Agriculture and Resources Inventory Surveys Through Aerospace Remote Sensing March 1981
Technical Report
Canopy Reflectance as Influenced by Solar Illumination Angle by J.C. Kollenkark, V.C. Vanderbilt, C.S.T. Daughtry, and M. E. Bauer
Purdue University Laboratory for Applications of Remote Sensing West Lafayette, Indiana 47907
NI\S/\
SR-Pl-04039 NAS9-l5466 LARS 021681
Canopy Reflectance as Influenced by Solar Illumination Angle
J.C. Kollenkark, V.C. Vanderbilt, C.S.T. Daughtry, M.E. Bauer
Purdue University Laboratory for Applications of Remote Sensing West Lafayette, Indiana 47907
March 1981
Star Information Form 1.
Report No. SR-Pl-04038
4.
3.
Recipient's Catalog No.
Title and Subtille
5.
Canopy Reflectance as Influenced by Solar Illumination Angle
Report Date February, 1981
6.
Performing Organization Code
2.
Government Accession No.
7.Author(s)J.C. Kollenkark, V.C. Vanderbilt, C.S.T. Daughtry and M.E. Bauer Performing Organization Name and Address Purdue University Laboratory for Applications of Remote Sensing 1220 Potter Drive West Lafayette, IN 47906 1-------.:..--...:.------------------------1 12. Sponsoring Agency Name ana Address NASA Johnson Space Center Remote Sensing Research Division Houston, TX 77058 15. Supplementary Notes F.G. Hall, Technical Monitor M.E. Bauer, Principal Investigator 9.
Performing Organization Report No. 021681 10. Work Unit No. 8.
Contract or Grant No. NAS9-ls466 13. Type of Report and Period Covered Technical 11.
14.
Sponsoring Agency Code
Abstract
16.
An experiment was conducted at West Lafayette, Indiana in 1979 to quantitatively describe the interaction of the solar illumination angle and row azimuth angle on the measured reflectance factor (RF) of soybean canopies consisting of 11 plots. Nine of the plots were planted in 71 cm wide rows; the other plots were of bare soil and soybeans with 100 percent soil cover. Reflectance factor data in four spectral bands, 0.5-0.6, 0.6-0.7, 0.7-0.8, and 0.8-1.1 ~m, were taken at 15 minute intervals during three clear days, August 12 and 31 and September 19 over nine plots of differing azimuthal direction with a Landsat-band radiometer (Exotech Model 100) at 5.2 meters above the soil. Diurnal changes of nearly 140 percent were observed in the red wavelength region when canopies covered 64 percent of the soil. The amount of shadow observed was a function of the plant geometry and row width. As soil cover approached 100 percent, the diurnal changes diminished. A function that describes the solar illumination angle with respect to the row azimuth explained most of the diurnal variation in the measured RF. Variation in near infrared response was much less and did not appear to be as strongly related to sun-row angle interactions. The ratio, near infrared/red, was highly sensitive to sun angle-row direction interactions, whereas the greenness function, utilizing all four spectral bands, was not. 17. Key Words (Suggested by Author(s)) 18. Distribution Statement Remote sensing, crop canopy, sun angle, Glycine max 1..
19.
Security Classi! (ofth,s report)
20.
Security Classi!. (of this page)
'For sale by the National Technical Information Service. Springfield. Virginia
22161
21.
No. of Pages
22.
Price' NASA - JSC
1
Introduction Understanding illumination necessary models
and
the
crop canopy
to utilize
have
been
reflectance factor
effect
of
the
geometry
on
reflectance factor proposed
to
of plant canopies as
between
the spectral
and
predict
a function of
solar
response
data effectively.
explain
sun angle, and view angle (Suits, 1972; et al., 1975).
interactions
is
Numerous
the
measured
plant geometry,
Smith et al., 1975;
Richardson
The models by Suits and Smith deal with a canopy with no
horizontal spatial variations. Richardson et al. with distinct soil,
modeled the reflectance of
horizontal spatial variations,
and shadow
various shaped
(1975)
components.
objects,
percent of the variance in the
vegetation
viewed are
particular canopy
surface covered with to explain 80
to 85
(1979) assumes an incomplete canopy
The fractions of sunlit and shaded soil and
calculated as
condition,
was able
of plant,
reflectance measurements due to shadows.
A model suggested by Jackson et al. of rectangular-shaped rows.
as a function
By illuminating a
Egbert (1977)
a row crop,
a function
described
ratio, row spacing and direction,
of view
by plant
time of day,
cover,
angle for
a
height/width
day of year,
latitude,
and size of the radiometer resolution element. Studies of the effect of sun have supported that
the predictions of
the reflectance
factor
zenith angle on reflectance generally the Suit's canopy
should increase
as
reflectance model
the solar
elevation
2
increases (Colwell, 1974; attributes this Field data response
Chance and LeMaster,
to changes in the
have shown
sun elevations
LeMaster, 1977; Jackson et al., 1979). and
non-symmetric
components
Colwell (1974)
amount of shadow within
minor to significant
with decreasing
1977).
increases in
(Duggin,
the canopy. the infrared
1977;
Chance
and
Crecelius (1978) noted symmetric
about solar
noon
that
influenced
observed variation
in reflectance throughout
component,
solar
angle,
variation.
Other effects, such as drying of the soil surface and plant
wilting
will be
explained
asymmetric about
the
solar
the day.
the
majority
noon and
The symmetric of
the
may be
observed
significant
factors to consider. Further investigation of reflectance factor data taken in 1978 over incomplete soybean canopies revealed possible time of day effects in the Landsat band regions as illustrated in the red, 0.6-0.7 pm, and the near infrared, 0.8-1.1 pm, in Figure 1. row direction.
Plots were planted in a north-south
Both bands were plotted with and without a 1.5 hour time
restriction about solar noon.
Low responses were noted over those plots
that were measured more than 1.5 in RF resulted from
hours from solar noon.
shadows between the rows and a
These changes
lower response from
the soil component. The objective soybean canopy
of this research was
as a function of
azimuth and zenith angles.
to model the reflectance
row width,
row direction,
of a
and solar
The hypothesis was that by varying only the
14
0.6-0.7J,1m
70
...... ~ 12~" ":"',• , ,,' I'" ,',.:. .. 10 't It ,1," ' t o .... ••. I,' ' • (J
1'0
u.
g
....1'0 ~
;:
.9
: ",,' '
6
."
:,'::-:. :,' u' • , "
"""
'.
c
't
..,
• '"
,
4
I,'
..
,. "
'
., ..:.. . . ....:.
'"
":
o
,"
2 O'~--------~------~
________ ________
-L________
~
'I
c::
'i'/ .'.' " ,~I
.. .. .... ,
!3 o
." • '/ 'J'-' ••,I .1.': !:jl!liI.,·1 .' 1 . *,.: !' l,. ' I
G)
'f.'
i a:
,:1/
.. ...,; I! ..
.' '. .:"
G)
•I·,'..•• r,..,'I'',f"
"
50
~
,
II)
a:
60
o
,, , "
"
,
~
,t' ••
a
G)
.-.. ......
a
___
o,a-1,1J,1m
,.,!,," ,tl
I, ,-" " • II,'
':!.
I" ,
. •'
,I
• ' :. 1
•
", ,t
e' ',"
'"•••• ,-
•
,I'
"..
"
'
oLI____~____~____~__~____~
~
70
-......
b
ot:'!' • . . .. I •• ! ....
..
~
... ....oo
"
-,
o 50 ....
.', 'I.,
o
~ 40
G)
G)
o
c::
•"
....1'0o
','
"
(1)
g ....1'0
..
, II:
;:
.',
2
.. 'I;, '/ ""'!I
I .. , .;,
• ',I
'It •.••• 1
.
"
," ,:, "·1' . .•s :.t: ..... ..,...•
;;::: 20
20
40
60
Soil Cover ('Yo)
ao
100
I.: ,
..
,
I
.
"
,
:.
•••
I
I I ' ... , -
,"
'
I,
:.
I'
,
"
.
.... ,.
• ,"
'I,:
'
(1)
a:
10
0'
0
.
30
(1)
.. '
(1)
0
.• ",:1
~ ......
1'0 U.
a:
d
...... 60
o
!
20
!
40
I
!
I
60
ao
100
Soil Cover (%)
Figure 1. Time of day effect on the RF seen in the 1978 data for the red (0.6-0.7 ~) and the near infrared (0.8-1.1 ~) wavelength regions, (a,c) All 1978 data minus dates which include wet soil and/or senescing vegetation. (b,d) Same as a and c above, but only includes data in the three hour time period centered about solar noon.
w
4
row direction, by
changes
the variation in reflectance would be explained entirely
in
sun
zenith
and azimuth
angle
with
respect
to
row
direction. Materials and Methods Experimental Conditions The experiment was
conducted in 1979 on the
Soybeans (Glycine max (L.) Merr.
Purdue Agronomy Farm.
"Amsoy 71") were planted on a Chalmers
silty clay loam (typic Argiaquoll) on June 25, 1979. because they have dense foliage with of the season.
Soybeans were used
distinct row patterns through much
This is in contrast
to many of the
other major crops
such as wheat and corn that have a much more complex canopy geometry and shadow pattern. season,
Because of extended periods of cloudy days early in the
spectral data
were collected only on
development stages after
full bloom. The experiment consisted
of 11 randomly arranged
3.5 m wide and 5.2 m long (Figure 2).
plots which were
Nine plots were planted in 71-cm
rows with the following azimuthal directions:
0-180,
90-270, 105-285,
and 165-345 degrees from
north.
120-300,
135-315,
150-330,
30-210,
60-240,
Another plot was planted in 25-cm wide east-west and north-south
rows to obtain a canopy with negligible was included soybean plots.
to monitor the sunlit Row
row effects.
A bare soil plot
soil background reflectance
directions were selected to
of the
favor data collection
during the morning hours when cloud-free conditions were more likely.
5
Figure 2. Illustration of field spectral data acquisition over the row direction plots in 1979.
6
Three development stages with 65, 78, the 71-cm rows were represented with canopy with 78 percent
and 94 percent soil cover on
the three measurement dates.
soil cover was obtained by trimming
canopy just prior to the start of senescence.
The
a near full
The cross sectional shape
of the canopy was determined by placing a large piece of poster board in the canopy,
perpendicular to the row azimuth,
at several locations and
drawing the perimeter of the canopy on the board.
The canopy shapes for
each date are illustrated in Figure 3. Spectral Measurements Radiance measurements, were taken
used to
over all the plots
determine reflectance factor (RF),
with a Landsat band
radiometer (Exotech
Model 100) at 15-minute intervals throughout the day on three clear days (August 12, August 31, and September 19). Robinson and obtaining
Biehl (1979)
the
descibe
reflectance
bidirectional
reflectance
radiometer with
pm.
Data
which
factor.
The
field of
following wavelength regions: 0.8-1.1
the conditions and
factor,
a 15-degree
Nicodemus et al.
Exotech
view that
0.5-0.6,
were taken
closely
only under
(1977) and
proceedures for approximates
100
a
4-band
acquires data
in the
0.6-0.7,
is
the
0.7-0.8,
near cloud-free
and
conditions
(especially in the vicinity of the sun). A mobile
truck-mounted radiometer
efficient data collection in the field. the
truck permitted
the radiometer
placed 5.2 m above the crop canopy data were
collected over two
and
system was
used for
quick and
A boom mounted on the back of a motor-driven
camera to
and 3.5 m from the truck.
locations in each
plot on August
be
Spectral 12 and
7
80 60
o
'--_---l..LL._ _..l.-_----:U-L_ _....I
August 12
~_
August 31
-
E
o ........ +'"
.c
0> .(1)
J: _J_.J.......L_ _""___---I.~'"_ _ _ _
80
oL-_..l..-...1....:=L--_.l.-._
o
_J_-'--...I..-_.....I
35
0
Se ptember 19
35
Distance (em) Figure 3. Soybean canopy shapes and dimensions for three dates of data collection.
8
over four locations instruments were
in each plot on
carefully leveled
nadir look angle. averaged to
to obtain
all spectral
Several measurements were taken over
insure a representative sampling
biased values for on-row or bands were
August 31 and September
During data collection,
The
data at
a
each plot and
of the plot and
off-row measurements.
taken concurrently and recorded
19.
to avoid
Measurements in all
by a printing
data logger.
photographs were taken periodically
over each
plot for soil cover determination and shadow assessment. Agronomic Measurements Agronomic
measurements included
plant height,
maturity stage (Fehr and Caviness, 1977), fresh and
dry biomass,
Percent soil
and
stem,
pod,
cover was determined by
leaf area
surface soil moisture,
index, total
and green leaf
dry biomass.
placing a grid over
the vertical
photograph and counting the intersections occupied by green vegetation. Data Analysis The reflectance
factor data
reflectance data were and Thomas
for Landsat MSS
(1976)
=
BBand 3 * 0.17289)
*
0.612498 •
+
(Band4
Band1 to
bands.
um)/(0.6-0.7 pm)]
The
as band
transformed into greenness as
data (Malila and Gleason, 1977).
Landsat
were analyzed
*
means.
The
described by Kauth
data and modified
for spectrometer
The data transformation was: Greenness 0.59538)] - 8Band1
band4 refer
to the
near infrared/red
was also considered
variance and Newman-Keuls tests were
*
0.48935)
RF measured
reflectance
in the
ratio
in the analysis.
+
(Band2 four
[(0.8-1.1 Analysis of
performed to determine significant
effects of row-solar angle interaction and RF.
9
Results and Discussion The maximum
response of RF to
the sun azimuth
angle was equal to
Diurnal changes
in RF of
occurred when
the row azimuth angle
(Figure 4a).
nearly 140 percent
0.6-0.7 pm,
wavelength region,
changes in sun angle
were observed in
on August 12.
The highest reflectance
values were obtained when the soil was
sunlit and the lowest,
soil was
in the
shaded.
Diurnal variations
wavelength region,
0.8-1.1 pm,
were lower
the red
RF in the
when the
near infrared
(relative to the minimum RF
value observed) than that noted in the visible region and not as clearly related to sun-row interactions (Figure 4b). in RF are about the same. not
be as
pigment
dark as
The
shadows of the near infrared region may
those observed
absorption and
multiple
Note the absolute changes
in the
visible region
scattering
in the
due to
canopy
low
(Colwell,
1974). The effect of
sun-row azimuth interactions are shown
The reflectance was directions.
The
plotted over time for three plots peak response
three plots was not row azimuth. the
rows,
in the red
in Figure 5.
of different row
wavelength region
only at different times,
but also
for the
in order of the
Again the peak response was when the sun was shining down lighting
the
soil surface,
and
thus
giving
a
higher
wavelength region for two
of the
reflectance reading. The diurnal
response in the red
key canopy components, 6.
sunlit soil and vegetation,
are shown in Figure
Very little change in reflectance factor was observed as a function
of zenith
angle for either
the plot containing
bare soil or
the plot
10
10
0.6-0.7Jjm
-
~ ....... a..
0
.....
•
0
cu
u.
6
CD 0
c .....cu
4
0 CD
..... CD a:
•• 2
0 70
-
60
a..
50
0.8 -1.1 JJm
~ .......
0
..... 0
cu u. 40
••
CD 0
c 30 .....cu 0 CD
20
..... CD a: 10 0
-80
-40
80
Solar Azimuth - Row Azimuth Figure 4. Changes in the BRF in (a) the red wavelength band (0.6-0.7 and (b) the near infrared wavelength band (0.8-1.1 pm) plotted against the difference between solar and row azimuth. Row azimuth=180o.
pm)
11
10 s-
o
0.6 - 0.7 jJm
8
........ (,)
co
u..
"
......... ..... .. --'-..-, '" ... .... " ......
6
(]) (,)
c co
~
.....
"'
4
-+-' (,) Q)
-
,
~
...... ~ ........
.......
Row Direction 2
........ 2400 ---180°
.......
.... ..
....
--...--
1650 o~----~--~----~----~--~~--~----~
11
12
1
2
3
4
Time Figure 5. RF in the red wavelength region (0.6-0.7 ~) for three row directions over time on August 12, 1979.
5
12
10
--... .... 0~
0.6- 0.7jJm • •
•
•
•
•
•
•
• •
• Bare Soil
8
0 0
co
u.
6
a>
0
c::
....0co
4
• •• •
•
• •
•
•
• •
•
•
•
•
• Full Canopy
QJ 'to-
QJ
0:
2
30
40
50
60
Solar Zenith Angle Figure 6. Reflectance factor in the red wavelength (0.6-0.7 ~m) for the bare soil and full vegetative canopy plots against changes in zenith angle on August 12, 1979.
13
with 100
percent soil cover.
Note
the large differences
sunlit soil and sunlit vegetation. size,
and
soil width with
between the
Interaction of the canopy shape and
the sun
angle produces varying
shadow cast on both the soil and the vegetation. the diurnal variations in RF observed in
Thus,
amounts of
it may be that
Figures 4 and 5 were caused by
changes in the amount of shadow in the field of view of the instrument. Equations to
predict the shadow
cast by rectangular
rows as the sun
zenith and azimuth change throughout the
defined by many
investigators (Idso and Baker,
1979;
Verhoff and Bunnik, 1978).
to express the solar zenith (9) a plane
1972;
or spherical day have been
Jackson et al.,
For this study, an equation was used (~)
and azimuth angle
projected
ray onto
perpendicular
function,
called the projected solar angle,
to the 8sp
in relation to a
row azimuth.
This
= tan-1(tan8sin~),
is
illustrated in Figure 7. The
response
reflectance ratio, August 12 are the
of
the
red,
near
infrared,
and greenness transformation
presented in Figure 8.
greenness transformation
are
The near
often related
biomass, soil cover, and/or leaf area index. expressed as
a percent
value observed that day,
increase in
near
infrared/red
to changes in
8sp on
infrared/red ratio and to
changes in
plant
If the diurnal changes are
response relative
to the
minimum
the red and near infrared/red ratio were quite
sensitive to changes in esp, whereas the greenness and the near infrared region were not.
The near infrared wavelength region does show similar
absolute changes in response during the day, clearly related to the changes in
but the pattern is not as
sun-row angle and the variation about
the mean for any given 8sp is much higher for the near infrared region.
14
.
.~
,",\
\ /
/
Aug 21
,,--
~O-
Solar Angle Date
Time
Zenith
Azimuth
asp
Aug
15
12:00
26.3
180.0
26.3
Aug
21
2:09
40.1
234.2
26.3
4:49
73.7
261.7
26.3
Sept
10
Illustration of the solar projected angle e as observed Figure 7. when looking to the west. Three dates and the correspon~~ng observation !limes result in the same shadow pattern and e ~260. sp
10r
0.6-0.7 JIm
25
o
~ a:
-*
8
~ 20
a
C ~
~
~
o
ti
~ ~
c !!
IJ
...
~15
6..
~
4
a:~
_.,
~
a:
....
~
'
10
~ ~
IJ
i
c.
IJ
~.
--------
..... .
~
2
~
5~
«i
~
z 0' 70 r
0 0.8-1.1J1m
50 t-'
\J1
_ ~
~
r.
~.; ~
~
b
35
IJ
•
