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Produced by the NASA Center for Aerospace Information (CASI) 


NASA Technical Memorandum 81222 


..aWe und« NASVf^'l> 

in the interest oleaily and 
semination of Earth 

Program inlorraation am) without iatritim 
tor any use made thereof.’ 

Infrared-Temperature Variability 
in a Large Agricultural Field 

John P. Millard, Robert C. Goettelman, 
and Mary J. LeRoy 

(E80-10331) INFRARED-TEMPEfiATURE N80-3wi822 

(NASA) 26 p HC A03/J1F A0 1 CSCL 02C 

Unci as 

G3/43 00331 


August 1980 


National Aeronautics and 
Space Administration 



NASA Technical Memorandum 81222 


Infrared-Temperature Variability 
in a Large Agricultural Field 

John P. Millard, Ames Research Center, Moffett Field, California 
Robert C. Goettelman, LFE Corporation, Richmond, California 
Mary J. LeRoy, DCA Corporation, Palo Alto, California 


National Aeronautics and 
Space Administration 

Ames Research Center 

Moffett Field California 94035 


Inf rated* temperature variability in a large agricultural field 


Ames Research Center, NASA, Moffett Field, California 94035, U.S.A. 


LFE Corporation, Richmond, California 94804, U.S.A. 

and MARY J. LeROY, DCA Corporation, Palo Alto, California 94303, U.S.A. 

Abstract. Airborne thermal imagery of a large varying*terraln comaer- 
cial barley field was acquired over a full growing season. The data were 
analyzed to determine temperature variability within the field and the 
percentage of area within various size Instantaneous fields of view 
(ifov's) that would be within 1®, 2®, 3®, and 5® C of the mean. There 
appears to be no great advantage in utilizing a small ifov instead of 
a large one for remote sensing of crop temperatures. 

! f «;urch?sed from; 

Send correspondence to: . i L2i- Oj"'*'.'!' 

John P. Millard 'i OuX Falls, SD S'? 

Ames Research Center, NASA, M/S 240-6 
Moffett Field, California 94035 


1 . Introduction 

The measurement of crop canopy temperature Is increasingly being 
suggested as a tool to be used in agricultural crop management and assess- 
ment; for example, Jackson et al. (1977), Idso et £l. (1977), and 
Hatfield (1979). However, the accuracy with which crop tenq>era*-iires 
must be measured and the acceptable temperature variations within a given 
field have not been determined. Knowledge of the required accuracy and 
variability would make it possible to determine appropriate Instantaneous 
fields of view (ifov's) for remote sensors. 

The Dunnigan Agro-Meteorological Experiment (DAME) airborne thermal 
scanner results provide Insight into the temperature variability question. 
DAME was a combined airborne and ground field measurement program which 
was conducted over an entire barley growing season in support of a Heat 
Capacity Mapping Mission spacecraft experiment. It was performed by Ames 
Research Center, USDA/SEA, and the University of California at Davis. 
Measurements of crop temperature, soil moisture, and meteorological param- 
eters were acquired over the growing season. 

This paper is concerned with using the airborne thermal scanner 
results of the experiment and the analysis of data to define (1) the 
temperature variability (coefficient of variation) that may occur within 
various instantaneous fields of view, and (2) the percentage of the area 
within various size ov*s that would be within I®, 2®, 3®, and 5® C 
of the mean. Because of the extreme variability in slope of the DAME 
site, the results may represent a worse-case condition and thus a very 
conservative estimate on which to base future calculations. 


2. DAME site airborne experiment 

The DAME site was located on 1 section of land (I x 1 mile) (1.6 x 1.6 km) 
located near Dunnigan, California « about AO km (25 miles) NW of Sacramento. 

The terrain of this site varied from flat to slopes of about 30 percent; 
thu8» almost any bar ley- growing terrain in the world was duplicated. 

Barley, variety Briggs, was planted in 1 eceober 1977 and harvested in late 
May 1978. The field was not irrigated, but 65 cm (26 in.) of rainfall, 
almost twice normal, was received. 

Figure I is a topographic map of the site, on which Is superimposed 
(1) 16-ha (40-acre) cells; (2) mean slope, m, in percent; (3) standard 
deviation, o, of the slope within each cell; and (A) coefficient of varia- 
tion, Vg, of the slope in each cell. The NE cells are the most rugged, 
and the southern cells are the flattest. 

Airborne thermal Imagery of the site was acquired throughout the 
growing season, from planting to harvest, except in April when the air- 
craft was down for maintenance. Data were acquired both prior to sunup 
and about 1 hour after solar noon; these represent minimum and maximum 
surface temperatures, respectively. Thermal Imagery was acquired with a 
Texas Instruments Model RS-25 infrared scanner operating in the 10.5- to 
12.5-ym bandpass region. This instrument has an Ifov of 2 m (6.6 ft) at 
the flight altitude of 1.2 km (AOOO ft) and a temperature accuracy of 
about 0.2° C. It contains two blackbody calibration sources with platln\im 
resistance thermometers for continuous inflight calibration. All thermal 
data were digitally processed on an HP 3000 computer. In addition to 
thermal data, natural and color-IR photography were acquired on alternate 


flight days with a 70-mm Haaaelblad camera. At the completion of each 
flight, atmospheric tesq>erature and humidity were measured at various levels 
down to near ground level. These were used to correct the thermal->IR data 
for water vapor absorption. 

3. Results 

Airborne photographs acquired throughout the growing season (fig- 
ures 2a-2e) showed that a truly uniform-appearing field never existed. 

This nonuniformity was caused by variable slope, soil color, gullies, and 
drainage-induced crop growth patterns. Figure 2a Is a natural-color photo- 
graph obtained in August prior to planting and when the soil was dry. 

The nonuniformity of the soil is the result of past leaching and the 
presence of alluvial soil In the gullies. Figure 2b is a natural color 
photograph obtained on the 49th day after planting (DAP), after the plants 
had emerged. Much soil background is still apparent. Nonuniformities in 
appearance were caused by farm equipment tracks, varying growth patterns, 
and double-seeded areas. Figure 2c is a color-IR photograph obtained on 
DAP 94. Except for gullies, the scene was rather uniform In appearance. 
Figure 2d is a natural-color photograph obtained on DAP 98. Although only 
4 days after DAP 94, the nonuniform scene appearance is quite striking. 

Much bare soil and many gullies are apparent. The reason for this sudden 
change between DAPS 94 and 98 is unknown, although It may be wind-induced. 
Finally, figure 2e pertains to DAP 154, very close to harvest. This 
shows the effect of crop maturity differences, caused by differing soil 
moisture-holding capacities. The crops on the upper slopes and the tops 
of the ridges matured earlier than those in the gully areas. Thus, there 
are many causes of scene nonuniformities throughout the season. 


The coeff lcient-o£-vari .tlon of afternoon temperatures, pixel by pixel, 
within the DAME site la shown In figure 3. Maximum values of about 0.22 
were obtained near planting tine for bare soil and no winds. Throughout 
the remainder of the growing season values were less than 0.11, and minimum 
values of about 0.02 were reached under wet soil conditions. As an aid to 
interpreting these effects, soil moisture, wind conditions, and agronomic 
values are presented in figure 3. 

Figures 4-8 demonstrate the percentage of area, A^^, within various 
size fields of view that would be within 1“, 2", 3®, and 5® C of the mean. 
These were computed from the airborne scanner data, which consisted of 
equivalent blackbody temperatures for every 2 m (6.6 ft) of the DAME site. 
Figure 4 pertains to 4 ha (10 acre) ifov's. Since there are 64 such 
4-ha (10-acre) cells in the DAME site, we decided to present only values 
for very rugged terrains (the four 4-ha (10-acre) cells in the upper NE 
cell of figure 1] and for gently rolling terrains [the four 4-ha (10-acre) 
cells in the SE cell of figure 2]. These are adequate to bracket the 

Figure 4 shows a large difference in A^^ (percentage of area within 
1®, 2®, 3®, and 5® C of the mean) values between level and rough terrain, 
and even between adjacent 4-ha (10-acre) cells. The explanation for these 
differences is that many 4-ha (10-acre) cells vary in terms of slope, 
gullies, and areas of nonuniform appearing vegetation. Vlhere such condi- 
tions occur, a wide range of temperatures may exist, resulting in low Aj^ 
values. Where the scene is uniform and homogeneous, the spread in tempera- 
ture is small and high A^^ values result. Figure 4 shows chat most 
temperature InViomogeneity occurs for bare soil conditions. As the canopy 


cover Increases, so does temperature homogeneity, especially for rough 
terrain, thus Indicating a smoothing effect of the canopy. For near-level 
terrain, about 95 percent of the data points are within 3* C of the mean 
cell temperature during full-canopy conditions; for very rough terrain, 
about 80 percent are within 3* C. 

The percentage of 16-ha (40-acre) cells within various temperatures 
of the mean Is plotted in figures 5 and 6. Two rather distinct families of 
covers resulted: one for level and intermediate-slope terrains (figure 5) 
and one for hlgh-slope terrain (figure 6). A small amount of crossover, 
or Inconsistent data did exist; the reason for this Is not known. In 
general, however, the temperature uniformity of the level and Intermediate 
terrain cells increased rapidly with crop growth and then remained level 
or dropped slightly throughout the remainder of the season. The cells 
with high slope (figure 6) showed values that Increased steadily 

with crop growth and reached maximum values later In the season. Over 
80 percent of the data points are within 3° C of the mean cell temperature 
over most of the growing season. Comparing the 4-ha (10-acre) cell size 
results of figure 4 with the 16-ha (40-acre) results of figures 5 and 6, 
we find identical trends and nearly the same values of A^, but note that 
individual A^ values for 4-ha (10-acre) cells can be much more variable, 
thus reflecting the uniformity or nonuniformity of the scene. 

Finally, figures 7 and 8 pertain to cell sizes of 65 ha (160 acres) 
and 259 ha (640 acres), respectively. Basically, the same magnitudes 
and trends noted for the previous cell sizes are observed. Approximately 
80 percent of the data points are within S'* C of the mean over most of 
the growing season; for level terrain, the value is 90 percent. 


4. Conclu8ion&' 

The reasons for cen^erature variability within an agricultural field 
are nany. Variability caused not only by varying topography* but also 
by water~carved gullies* varying soil color* nu>isture state of the soil 
and crop* nonuniform ph".nology* and bare spots i Althou^ these various 
effects were nut separated* the cc.abined effect was measured for commer- 
cially grown harlv.y planted on varying terrain. For all but the most rugged 
terrain, over 80 percAr.t of the area within 4-* 16-* 65-* and 259-ha cells 
(10-, 40- 160- , and C40-acre cells) was at temperatures within 3* C of 

the mean cell temperature The result of using relatively small* 4-ha 
(lO-acrc) ifov's for remote sensing applications is that either the worst 
or the best of conditions is often observed. For example* the observed 
temperature uniformity of a homogeneous field containing a stream will 
vary considerably depending on whether the ifov contains the stream. If 
only the homogeneous field is observed* great temperature uniformity might 
be observed, but if the stream is within the ifov* then great nonuniformity 
may be observed. 

There appears to be no great advantage in utilizing a small ifov 
[e.g.* 4 ha (10 acres)] instead of a large one [e.g.* 65 ha (160 acres) 
or 259 ha (640 acres)] for remote sensing of crop canopy temperatures. 

The percentage of the area within any of these Ifov’s that contributes 
temperatures that are within 1“, 2*, 3** and 5* C of the mean is nominally 
the same. The two alternatives for design purposes are then either (1) a 
very high spatial resolution, of the order of a meter or so, where the 
field is very accurately temperature mapped, or (2) a low resolution, 
where the actual size seems to make little difference. 



The authors v->uld like to tlumk Dr. Robert Regiiuito» USDA/SEA* and 
Dr. Jerry Hatfield, U. of California at Davie, for their constructive 
criticism of this manuscript and for providing the agroncmic values ahornn 
in figure 3. 



HATFIELD, J. L. , 1979, Aeron. J . 71, 889-892. 

IDSO, S. B., JACKSON, R. D., end REGINATO, R. J., 1977, Science IH, 19-25. 
JACKSON, R. D., REGINATO, R. J., end IDSO, S. B. , 1977, Water Res . U, 


Figure Cttptions 

Figure 1. Topographic map of DAME aite Including mean alope M» atandard 
deviation a, and coefficient of variance (c/M), Vg. 

Figure 2, Aerial photograph of DAME site: (a) Dec. 9, 1977 (DAP 3)t 

natural color film; (b) Jan. 24, 1978 (DAP 49), natural color film; 

(c) Mar. 10, 1978 (DAP 94), color-IR film; (d) Mar. 14. 1978 (DAP 98), 
natural color film; and (e) May 9, 1978 (DAP 154), color-lR film. 

Figure 3. Coefficient of variation of temperatures within the DAME site. 

Figure 4. Percentage of 4 -ha (lU-acre) cells (1/64 DAME site) that is 

within designated temperature limits of the mean — for the four 4-ha 
(10-acre) cells in the upper NE cell of figure 2, and the four 4-ha 
(10-acre) cells in the SE cell of figure 1; (a) within 1" C of the 

mean; (b) within 2° C of the mean; (c) within 3“ C of the mean; and 

(d) within 5” C of the mean. 

Figure 5. Percentage of l6-ha (40-acre) cells (1/16 DAME site) that is 
within designated temperature limits of the mean - for level and 
intermediate slope terrains; those cells in the central and lower 
part of figure 1: (a) within 1* C of the mean; (b) within 2* C of 

the mean; (c) within 3* C of the mean; and (d) within 5* C of the 


Figure 6. Percentage of 16«ha (40-acre) cella (1/16 DAMP, site) that la 
within deolgnated teoperature Halts of the mean — t%)r hlgh**slope 
terralna; those cella in the upper-right comer of figure 1: 

(a) within 1* C of the mean; (b) within 2* C of the aean; (c) within 
3* C of the mean; and (d) within 5* C of the aean. 

Figure 7. Percentage of 65-ha (160-acre) cells (1/4 DAME site) that Is 
within designated temperature limits of the mean: (a) within 1* C 

of the mean; (b) within 2** C of the mean; (c) within 3* C of the 
mean; and (d) within 5* C of the mean. 

Figure 8. Percentage of 259-ha (640-acre) cells (full DAME site) that 
is within 1*, 2®, 3®, and S® C of the mean temperature. 


Fi«. 1 


Fig. 2b 

'^KIGIXAL page is 



Fig. 2e 



I 1 1 I 1 I I 


0.5 1.1 2.5 3.3 1.2 



1 1 1 1 1 


5 7 11 8 4 



1 1 l 1 1 L 1 


17 21 43 77 98 



Fig. 3 


AREA, percent AREA, percent AREA, percent AREA, percent 



0 I 1 1 1 I I I I I I I I I 

-60 -40 -20 0 20 40 60 80 100 120 140 160 180 

FIr. 4 




AREA, percent AREA, percent AREA,percent AREA, percent 


5 20 

0 - - - . ____ __ . , ^ 

-60 -40 -20 0 20 40 60 80 100 120 140 160180 


Fig. 8