Thermal images from an aircraft-mounted scanner are used to evaluate the effectiveness of crop freeze protection devices. Fuel oil heaters, wind machines and irrigation systems are evaluated from flights at an altitude of 450 m over an experimental citrus grove of 1.5 hectares

Sutherland, R. A.

English

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Paper No. 22
REMOTE SENSING APPLIED TO THE EVALUATION OF CROP FREEZE
PROTECTION DEVICES
R. A. Sutherland, Institute of Food and Agricultural Sciences, University of
Florida, Gainesville, Florida 32611
ABSTRACT
Thermal images from an aircraft-mounted scanner are used
to evaluate the effectiveness of crop freeze protection
devices. Fuel oil heaters, wind machines and irrigation
systems are evaluated from flights at an altitude of 450 m
over an experimental citrus grove of 1.5 hectares.
INTRODUCTION
In the past it has been difficult to determine the effec-
tiveness of various measures that are taken in order to prevent
freeze damage to crops. This difficulty is caused in great part
by the fact that temperature measurements made with conventional
sensors such as thermocouples or thermisters are limited to a
small number of discrete points. Usually, it has been through
trial and error that methods were learned.
In recent years, however, infrared technology has advanced
to the point where remote sensing in the 8-14_ "thermal" radi-
ation region has become a common and reliable research tool.
This provides a new perspective on evaluating freeze protection
methods because measurements can be made _rom a scanning aircraft
giving a spacially continuous overview of surface radiation pat-
terns of a relatively large area. The full impact of the tech-
nology lies in the fact that under most conditions the magnitude
of the emitted radiation is directly related to the surface (or
crop) temperature (Sutherland and Bartholic, 1977). The data,
when properly reduced give a temperature "picture" of an entire
area as opposed to the more conventional ground measurements of
a few discrete points. A major purpose of this paper is to
report progress made by using the remote sensing technique.
Description of apparatus and test site
The test site is an experimental citrus grove located in
Northern Florida near the University of Florida (~29.5°N, 53
m.a.s.l.). An aerial photograph of the grove is shown in Fig. I.
The trees in the northern 2/3 of the grove are approximately
'4 m in height. Those in the southern I/3 are shorter (3 m).
Tree spacings are 3.05 m east-west (rows) and 4.88 m north-south.
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Except for a few missing trees, the grove is homogeneous in the
east-west direction. The grove is located on flat terrain of
Kanapaha Fine Sand.
Figure 1. Aerial photograph of the test site.
A computerized data acquisition facility for ground truth
measurements is located near the southwest corner. The grove is
equipped with two heater types, Spots and Scheu on alternate rows
spaced at every other tree along rows. The undertree irrigation
sprinkler heads are in every row, centered in the row and spaced
at every other tree. The irrigation pump is a 20 Kw electric
motor and pumps well water (21°C) from a depth of approximately
30 m. The pump valve was adjusted to give a deposition rate at
the sprinkler heads of 0.2 cmmin-i. The wind machine is lO m
high with a single propeller 4.7 m in length, is driven by a gaso-
line engine and rotates about a vertical axis once every 5 min.
The overall spacial resolution of the aircraft scanner is
approximately 1.25 mZ which is sufficient to resolve individual
trees. The data which is recorded on magnetic tape as an analog
signal aboard the aircraft was later digitized at a rate which
maintained this resolution. In this paper the term pixel will
be applied to this area. A more detailed description of the air-
craft and scanner can be found elesewhere (Sutherland and
Bartholic, 1975). A linear approximation was used to convert
the scanner signal to surface temperature and emissivities are
approximated as unity. Errors due to these approximations for
the scanner bandpass of 8_4u are small (Sutherland, et. al.,
1978)
Results
Fig. 2 shows a typical thermal image of the test site which
was taken during a time when the grove was in the natural (un-
protected) condition. In Fig. 2, lighter areas are warmer and
128
darker areas the colder. The entire range of the data is -8.9
to +0.56°C. The darker (colder) rows in the grove are the tree
tops and the lighter (warmer.) rows are bare soil.
Figure 2. Typical thermal image of the test site taken
wchen the grove was in the unprotected condition.
The digitized data used to produce the image of Fig. 2 was
further analyzed by sectoring off only the area containing the
grove and calculating the number of pixels (16000 total) found
in the scene between various temperature intervals (i.e. histo-
grams).
For evaluation of the wind machine the entire unprotected
grove was considered the control then the data from a previous
flight when the wind machine was operating is compared. For the
heaters and irrigation the west half of the grove was considered
control while the east half was first irrigated then heated.
All of the flights reported here were made on the night of
22-23 Feb. 1978. This was a calm night with no detectable wind.
The experiments were started near midnight after most cooling had
occured. Flights were then made approximately every two hours
during which time nocturnal cooling was occuring at a nearly
steady rate of O.15°C/hour.
The resultant histograms of the thermal images are shown in
Fig. 3. The mean temperatures of the distributions are marked
along the x axis.
The results of Fig. 3a and 3b show the overall effectiveness
of the wind machine, based upon the mean temperature of the grove
to be 1.06°C. The broader distribution for the wind protected
grove is indicative of the fact that the amount of protection
provided by the wind is a function of distance from the machine
which is located near the southwest corner of the grove. This
was borne out in the accompanying thermal images (not shown).
129
The results of Fig. 3c and 3d show the overall effectiveness
of the undertree irrigation, based upon mean temperatures only,
to be 2.1°C. This mean, however, could be somewhat misleading
since it is influenced by temperatures of the irrigation water
itself, which was pumped in at elevated temperature. This is
apparent from the bimodal distribution of Fig. 3d where the
second peak at a higher temperature is certainly due to this fact.
Perhaps a more accurate assessment of the effectiveness of the
irrigation technique is the low temperature cut-off which by sub-
tracting the lower thresholds gives an effectiveness of 0.78°C.
The results of Fig. 3e and 3f show the effectiveness of the
heaters to be 1.5°C based upon means. This is again somewhat
misleading for the same reason as before since some of the higher
temperatures are the heaters themselves. By subtracting the
lower thresholds, however, the effectiveness of the heaters is
approximately 0.63°C, about the same as the irrigation technique.
This latter observation is somewhat complicated by the fact that
it was necessary to operate the wind machine during the heater
experiment in order to protect the control portion of the grove
from killing freeze damage. Perhaps a better direct comparison
between irrigation and heating is to use the result of Fig. 3c
as control for both. Viewed this way, the effectiveness of each
is about the same.
Conclusions
The technique of remote sensing has been shown to be useful
in evaluating freeze protection methods. Although more data is
desired it can be concluded from thp _,J1+c chow _ that th_ tcch
nique of undertree irrigation competes effectively with that of
heating. The economic significance of this latter conclusion
lies in the fact that the cost of heating is about $20 per acre
per hour while undertree irrigation is $.75 per acre per hour.
References
Sutherland, R. A. and J. F. Bartholic: 1974, "Aircraft-
Mounted Thermal Scanner to Determine Grove Temperatures During
Freeze Conditions," Proceedings of the Florida State Horticul-
tural Society. 87, 65-69.
Sutherland, R. A. and J. F. Bartholic: Significance of
Vegetation in Interpreting Thermal Radiation from a Terrestrial
Surface," Journal of Applied Meteorology. 16, 759-763.
Sutherland, R. A., J. F. Bartholic and J. F. Gerber: 1978,
"Emissivity Correction for Interpreting Thermal Radiation from
a Terrestrial Surface," Journal of Applied Meteorology. In
review.
130
darker areas the colder. The entire range of the data is -8.9
to +0.56°C. The darker (colder) rows in the grove are the tree
tops and the lighter (warmer.) rows are bare soil.
Figure 2. Typical thermal image of the test site taken
wbhen the grove was in the unprotected condition.
The digitized data used to produce the image of Fig. 2 was
further analyzed by sectoring off only the area containing the
grove and calculating the number of pixels (16000 total) found
in the scene between various temperature intervals (i.e. histo-
grams).
For evaluation of the wind machine the entire unprotected
grove was considered the control then the data from a previous
flight when the wind machine was operating is compared. For the
heaters and irrigation the west half of the grove was considered
control while the east half was first irrigated then heated.
All of the flights reported here were made on the night of
22-23 Feb. 1978. This was a calm night with no detectable wind.
The experiments were started near midnight after most cooling had
occured. Flights were then made approximately every two hours
during which time nocturnal cooling was occuring at a nearly
steady rate of O.15°C/hour.
The resultant histograms of the thermal images are shown in
Fig. 3. The mean temperatures of the distributions are marked
along the x axis.
The results of Fig. 3a and 3b show the overall effectiveness
of the wind machine, based upon the mean temperature of the grove
to be 1.06°C. The broader distribution for the wind protected
grove is indicative of the fact that the amount of protection
provided by the wind is a function of distance from the machine
which is located near the southwest corner of the grove. This
was borne out in the accompanying thermal images (not shown).
129


Remote sensing applied to the evaluation of crop freeze protection devices

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