Transcript Sensor Readings in Corn as Affected by Time of Day

Sensor Readings in
Corn as Affected by
Time of Day
Peter Scharf
University of Missouri
With thanks to Larry Mueller,
Joao Medeiros, Bettina Coggeshall,
and Amanda Coggeshall
Why consider time of day?
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Most of our readings were collected within
one hour in our calibration research
Our method requires comparison with a
reference area
If spectral properties change during the day,
the time between measuring the reference
area and fertilizing may introduce error
There is some evidence in the literature that
spectral properties change during the day
Literature: color vs. time
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Hoel & Solhaug, 1998:
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Wheat SPAD highest at dusk & dawn
SPAD dropped when plants were transferred from
low to high intensity light in growth chambers
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5 units in 30 minutes in shade plant
6% in one hour in wheat
SPAD goes back up when light intensity is dropped
Brugnoli & Bjorkman, 1992:
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Sunflower transmittance higher at higher light intensity
Due to chloroplast movement (arrangement &
orientation)
Methods
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Experiments conducted
in 2004 near Columbia
Sensors were mounted
in place over individual
plants
Sensor measurements
were taken hourly from
6am to 9pm (approx.
weekly)
Experiment 1: June 16
(V8) to June 29 (V12)
Experiment 2: July 7
(V6) to July 28 (V14)
NDVI vs. time: green is most
sensitive
6/29/04
1
0.8
NDVI
yellow NDVI
green NDVI
0.6
0.4
0.2
0
6
10
14
time
18
22
Green NDVI consistently drops
a lot in the middle of the day
GNDVI (with Greenseeker)
0.9
16-Jun
0.7
17-Jun
22-Jun
0.5
29-Jun
0.3
6
10
14
time
18
22
Green NDVI consistently drops
a lot in the middle of the day
GNDVI (with Greenseeker)
0.9
7-Jul
13-Jul
20-Jul
0.7
0.5
28-Jul
0.3
6
10
14
time
18
22
Regular NDVI also consistently
drops in the middle of the day
NDVI (with Greenseeker)
1
0.9
7-Jul
0.8
13-Jul
20-Jul
28-Jul
0.7
0.6
0.5
6
10
14
time
18
22
Yellow NDVI also drops in mid-day,
but perhaps the least of the three
YNDVI (with Crop Circle)
1
0.9
0.7
7-Jul
13-Jul
20-Jul
0.6
28-Jul
0.8
0.5
6
10
14
time
18
22
What’s going on?--NIR
NIR (with Crop Circle)
8
7-Jul
13-Jul
6
4
20-Jul
28-Jul
2
0
6
10
14
time
18
22
What’s going on?--VIS
yellow (with Crop Circle)
2
7-Jul
1.5
1
13-Jul
20-Jul
0.5
28-Jul
0
6
10
14
time
18
22
What’s going on?--SPAD
SPAD chlorophyll meter, ave. 8 dates
56
55
54
53
52
6
10
14
time
18
22
Why were NDVI values lower in
the middle of the day?
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Ideas:
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SPAD suggests that these
may change late in day
Change in canopy architecture
Change in internal leaf structure
Change in leaf pigment concentration or
arrangement
Influence of shadows (readings were in northsouth rows: shadows in the background change
dramatically during the day)
Why were NDVI values lower in
the middle of the day?
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Ideas:
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Change in canopy architecture
Change in internal leaf structure
Change in leaf pigment concentration or
arrangement
Influence of shadows (readings were in northsouth rows: shadows in the background change
dramatically during the day)
We were curious about
this and decided to test it
Influence of shadows
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Greenseeker green & red, Crop Circle,
stationary over soybean canopy
Take reading in sun, then move shade over
sensor & canopy, take reading, repeat
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Ten pairs afternoon 8/9/2005
Ten pairs morning 8/10/2005
Photo & measurements: Larry Mueller
Influence of shadows
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Greenseeker green NDVI was consistently
much higher when measured in shadow
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Mean 0.684 in sun, 0.761 in shade
Different with p = 0.0001
Greenseeker red NDVI was consistently
higher when measured in shadow
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Mean 0.742 in sun, 0.769 in shade
Different with p = 0.04
Influence of shadows
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Crop Circle yellow NDVI was not different
measured in sun or shade
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Mean 0.707 in sun, 0.700 in shade
p = 0.27
What about visible/NIR ratio?
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Visible/NIR ratio can be calculated exactly
from NDVI—the exact same information is
contained in both parameters
So I won’t show vis/NIR vs. time of day
But proportionally, differences are larger in
vis/NIR than in NDVI analogs
Summary
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We thought that with active light sensors, we
would not have problems with reflectance
measurements changing as light conditions
changed (time of day, clouds)
With all three sensors, NDVI values were
lower in mid-day and higher near dawn and
dusk
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This pattern was much more pronounced with the
Greenseeker green sensor than the other two
Summary
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Some of the diurnal change in NDVI may be
due to actual changes in the crop
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SPAD is higher in evening
Canopy architecture may also change
Change in value for NDVI (or vis/NIR) during
the course of the day is an obstacle to
making good N rate decisions
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Need to solve this to be a useful on-farm tool
Frequent reference reading is one possibility (N
applied across rows with airplane?)
Another possibility is to use a correction function
Summary
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We also observed differences in sensor readings in
sun vs. shade
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Biggest in green Greenseeker
Small in red Greenseeker
No problem in Crop Circle
Change in sensor readings when a cloud comes
over is also an obstacle to making good N rate
decisions
We also have data from passive sensors in these
experiments—not yet analyzed, but we will
eventually compare them with active sensors