Theoretical modeling as well as laboratory and field measurement were coupled with analysis of aircraft data obtained from controlled sites in an effort to enhance understanding of the microwave response due to soil moisture so as to specify sensor parameters and develop inversion algorithms. Models to predict the complex dielectric constant were produced which led to the interpretation of the results in terms of a matrix potential rather than simply moisture content. Similar advances were made in the development of coherent and incoherent radiative transfer models and rough surface scattering models

Waite, W. P.

English

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AgRISTARS
Made awadalNe under WA sponsolU*
•n th4 interest of early and wide dis-
serniution of Eerth Resources Survey
F rogram information and without liability
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Soil Moistu^
SM-A2-04322
A Joint Program for
Agriculture and
Resources Inventory
Surveys Through
Aerospace
Remote Sensing
May 1982
MEASUREMENT OF SOIL MOISTURE USING REMOTE
SENSING MULTISENSOR RADIATION TECHNIOUES
W. P. Wa;te^
s
(L83- IC 12)
	
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•	 Department of Electrical Engineering
University of Arkansas
Fayettevile, Arkansas 72701
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Lyndon B. Johnson Space Center
Houston. Texas 77058
UW" nee! a r/wd111111
1. ReptaK No. 2. Govw meet Accession No. 3. Recipient's Catalog No.
SM-A2-04322
4. Title and Subtitle S. Report Date
Measurement of Soil Moisture Using Remote Sensing May 1982
6. Performing Organization CodeMultisensor Radiation Techniques
7. Author(s) B. Performing Organization Report No.
W. P. Waite
10. Work Unit No.
9. Performing Organization Name and Address
Department of Electrical Engineering
University of Arkansas
Fayetteville, Arkansas 	 72701
11. Contract or Grant No.
NAS9-14251
13. Type of Report and Period Covered
Technical
12. Sponsoring Agency Name and Address
AgRISTARS - Soil Moisture Project
NASA - Johnson Space Center 14. Sponsoring Agency code
Houston, Texas	 77058
15. Supplementary Notes
16. Abstract
The purpose of this study was to investigate means for the microwave remote sensing
of soil moisture.. 	 The effort was to be d4rected toward the specification of sensor
parameters and the development of inversion algorithms.
The conduct of the investigation was a mixture of theoretical modeling, laboratory
and field measurement, coupled with analysis of aircraft data obtained from controlled
sites.
17. met Words iSuggested by Author(s)) 18. Distribution Statement
I
1
i
19. Security C4=if. (of this repo") 20.	 Securit . • Classif. (of this Pagel 21. No. of Pages 22.	 Price'
Unclassified Unclassified I
'Fur sale by the National Technical Information Service, WngfiNd, Virginia 22161
JSC Farm 1424 (Rev Nov 75)
	 NASA — JSC
tFINAL REPORT
NASA CONTRACT NAS 9-14251
MEASUREMENT OF SOIL MOISTURE USING REMOTE
SENSING MULTISENSOR RADIATION TECHNIQUES
By
W.P. Waite
Y
Department of Electrical Engineering
University of Arkansas
Fayetteville, Arkansas 72701
May 1982
Measurement of Soil Moisture Using Remote
Sensing Multisensor Radiation Techniques
INTRODUCTION
The original purpose of this study was to investigate means for
the micrcwave remote sensing of soil moisture. This was both to
demonstrate feasibility and to ascertain the effects of perturbing
factors suc li as surface roughness. The effort was to be directed
toward the specification of sensor parameters and the development of
inversion algorithms. This original goal was subsequently more
sharply focussed to concentrate on the development of algorithms specs
fically useful for agricultural applications, primarily crop yield
forecasting.
The conduct of the investigation was a mixture of theoretical
modeling, laboratory and field measurement, coupled with analysis of
aircraft data obtained from controlled sites. The laboratory and
field measurement program was conducted with a bistatic reflectometer.
This instrument, while active in nature, was used to measure the spe.
cularly reflected component which is more similar to the passive indi-
metric measurement than the backscattered component measured by a nor-
mal radar. This type operation has the advantage of providing swept
(continuous) frequency measurement of the basic radiometric behavior
which can only be accomplished using multiple direct frequencies with
radiometers.
HISTORICAL SUMMARY
The initial thrust of the program was to evaluate both active
(radar) and passive (.zdiometric) techniques. These were viewed at
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ORIGINAL PAGE 13
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the time not so much as complimentary sensors as competitors. The
argument favoring radar centered pria<arily on its ability to.provide.
fine resolution even at spacecraft altitudes. The principle disadvan-
tage was the known extreme sensitivity to surface roughness. The case
for the radiometer was precisely the opposite. It had been observed
that the roughness sens:tivity was substantially less than that of
radar, however, resolution could only be provided by the real antenna
beam and would be quite large at spacecraft altitudes.
The initial investigations at the Universl,ty of Arkansas con-
centrated on the effects of surface roughness as a function of fre-
quency and the analysis of passive data available from the first
aircraft missions. The investigation of roughness concentrated on
defining the variation of the specularly reflected (coherent) com-
ponent as a function of frequency (Hancock, 1977). The hope was to
use multiple (continuous) frequency measurement to define the rate of
decay with freluency of the specularly reflected component. Once this
rate was defined, it would be possible to invert to an equivalent
smooth surface reflectivity from which the determination of dielectric
constant and then soil moisture would be a relativeLy
 simple mutter.
It should again be noted that although the measurement conducted
utilized active swept frequency techniques the implementation method
would be multiple frequency passive since it is desired to monitor the
•	 coherent component of the scattered signal. This is possible, of
course, only over the range where the incoherent (diffuse) component
may be neglected in comparison to the coherent component. This stud
and continuing investigations of rough surface effects generated the
following conclusions:
s^
M
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ORIGINAL PAGE 13
OF POOR QUALITY
1) Correction for roughness can be accomplished., however, even
for frequencies as low as 1GH, the effective valve :of the rms
height that can be accomodated is significantly less than 1/em.
2) The effective roughness of real surfaces with discontinuities
is substantially greater than their measured rms height.
3) The measurements exhibited a marked sensitivity to changes in
the correlation distance of the surface.
These observations could be further sumim rized to say that multi-
parameter measurement of the exponential decrease in the coherent
scattered component does not appear feasible for realistic agri-
cultural surfaces unless frequencies substantially below 1GHz are
used.
These conclusions still have significance today in that they
imply that the observed decreased sensitivity due to roughness of the
passive versus active sensors is due to the fact that the passive
system is responding to the diffuse component of the scattered signal.
This in turn implies that the exponential correction factor applied to
explain the sensitivity compression due to roughness is incorrect.
The correction should be a reduction from the plane surface value to
the diffuse value since for the measured data the diffuse component is
significantly greater than the specular component.
r
The analysis of airborne passive iYadiometric data presented an
entirely different problem. The difficulty here was not in showing a
pronounced sensitivity due to soil moisture, but in interpreting the
significance of the results. The only means of evaluating the data
is by linear correlation analysis. The results consistently
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ORIGINAL PAGE 93
OF POOR QUALITY
demonstrated a slightly higher correlation with the upper levels of
the moisture profile. both at X - and L. band.. •Howeveri."the • internal '.
correlation of the profile also leads to correlation of brightness
temperature with moisture content at depths vtll below any reasonable
expectation of penetration.
At approximately this time the emphasis of the program was
sharply focussed on the application to crop yield estimation. This
immediately led to consideration of means to estimate the total plant
water availability which in turn leads to requiring a knowledge of the
profile to depths determined by root density which may range to six
feet or more. Thus, while the measurement technique demonstrated
increasingly good correlation with approximately the upper one inch of
the surface the application suddenly required estimation to six feet.
A s.. J of soil moisture budget models was conducted to assess
the feasibility of periodic surface measurements to update a moisture
budget model driven by meteorologic data. This, in essence, periodi-
cally corrects the budget model by enforcing a measured boundary con-
dition.
Considerations such as this led to the need for maximizing the
actual measurement depth. While there is no hope of performing
measurements to the depth required by the application, the greater
the depth that can be sampled, the greater will be the accuracy andi
the longer can be the period between measurements.
At approximately this time it could be concluded that microwave
remote measurement of the soil moisture of bare surfaces was certainly
feasible. In fact, it appeared this could be done quite accurately
ORIGINAL PAGE IS
OF POOR QUALITY
even with the perturbations introduced by surface roughness. The
problem lie in determining the depth to which the moisture might be
sensed and defining at least a moisture gradient for the upper sur-
face. In line with this a program of laboratory measurements of arti-
ficially constructed and layered moisture profiles was conducted.
This program definitly has established the ability to detect discon-
tinuities in moisture to significant depths (greater than six inches).
The problem with such artifical profiles was that although the layer
interference pattern could be used to accurately measure depth and
infer moisture content, it likewise required continuous frequency
measurement.
The next step in the process was to conduct a series of field
measurements over controlled moisture plots to monitor the effects of 	 •.
natural gradients under controlled conditions. This program offered
as well the first opportunity to compare the microwave measurements
with soil matric potential rather than with soil moisture. As had
been predicted this offered improved correlation.
The field measurement program demonstrated that naturally
occurring moisture gradients in the upper surface could in fact yield
the effect of a subsurface layer. While for each individual frequency
over the range the reflectivity tracks the decrease in moisture with
evaporation, the continuous frequency trace may exhibit an inter-
Terence pattern. The coherent component transmitted into the surface
and leading to the interference effect is apparently decreased by the
surface roughness. Thus, even natural surfaces, unless sufficiently
rough, will produce pronounced coherent effects introducing a scatter
into the measurement.
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ORIGINAL PACE 19
OF POOR QUALITY
Also near the end of this program data became available Prom the
Colby test site. The initial analysis of these data confirmed th,-
measurements of SCZ showing row orientatio,. to have a pronounced
effect upon the active backscatter return even at K Land. This was in
distinct contradiction to the conclusions reached from the measure-
meats of the University of Kansas MAS system from which it was
concluded that row effects would be insignificant at C band.
CONCLUSIONS
The preceding has been a brief synopsis of the studies conducted
at the University of Arkansas. These were, of course, simply part of
an overall program including primarily NASA/JSC, NASA Goddard, the
University of Kansas, and Texas A & M University as well. This
program has undoubtedly aided substantially in the understanding of
the microwave response due to soil moisture. As one example of this
the development of models to predict the complex dielectric constant
have been produced and have lead to interpretetion of the results in
terms of a metric potential rather than simply moisture content. This
appears to have both a be%.ter theoretical basis and likewise to be
the parameter of more concern to the user as it is directly related
to the water availab-e to the plant. Similar advances have been made
in the development of coherent and incoherent radiative transfer
models and rough surface scattering models.
While these advances in understanding have been substantial they
have not as yet answered the question of whether satellite remote
sensing of soil moisture is feasible or practical for agricultural and
Lydrologic uses. The greatest single problem is not lack of
demonstrated sensit. 3ity but definition as to what moisture the
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ORIGINAL PAGE 19
OF POOR QUALITY
instrument is responding. With the internal correlation of virtually
any moisture.proflle, &.limited data.sample me demonstrate excellent.
correlation at even substantial depths. It still remains to determine
the best inversion algorithm for moisture input to budget models over
a wide range of profile shapes.
The degree of the row orientation effect is still to be comple-
tely specified and may well force active systems to adopt a cross
polarized measurement. The effect of screening vegetation remains
somewhat controversial with a wide range of results from differing
measurement programs. There is still some disagreement as to the
severity of coherent effects for differing profile shapes particularly
in the near surface. Efforts in this area are hampered by a lack of
data for near surface profiles particularly with the depth resolution
needed.
In final conclusion it would appear that the advances in the
understanding of microwave soil moisture measurement have been
substantial. These have been for the most part accomplished with
relatively isolated data sets from both ground measurement and
aircraft programs. It would appear that perhaps the most pressing
need is for measurement programs relatively small in area but pro-
viding continuous measurement over an extended period of time. This
should not be surprising as agriculturists have used this approach for
years due to the great variability encountered. Since the soil is the
object studied here the same method would seem to apply.
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ORIGINAL. PA % s
OF POOR QUALITY
REPORTS AND PUBLICATIONS
The basic reporting mechanism during the conduct of this program
was presentation of results at quarterly review sessions held at JSC.
Approximately 20 of these presentations were made with the delivery o
all presentation material and descriptive synopsis. These formed the
basis for directing the program and most importantly for rapid
interaction between all members of the investige.tive team.
Publications and in depth reports on specific aspects are listed below:
Waite, W.P., "Frequency Dependence of the Microwave Brightness
Temperature of Rough Dielectric Surfaces,"
Proc. Am. Soc. of Photogrammetry, Little Rock, AR, pp. 377-392.
ct. 1977•
Waite, W.P., et al., "Long Wavelength Radar for Geological Analysis in
Vegetated Terrain," Proc. Am. Soc. of Photogrammetry,
Albuquerque, NM, pp. 386-39A, Oct. 1978.
Waite, et al. "Dual Polarized Long Wavele%th Radar for Discrimination
of Agricultural Land Use," Proc. Ar— Soc. of Photogrammetry,
Albuquerque, NM, pp. 595-607.
Waite, W. ."Aircraft Experiments Required for Development of Microwave
Soil Moisture Measurement," NASA hdgtrs. Joint Soil Moisture
Committee Meeting.Washington, DC, Jan. 1979.
Waite, W.P. et al. "Data Documentation for the Bare Soil Ex;eriment at
the University of Arkansas." p. 127, Jan. 1980.
Waite, W.P. et al., "Use of Satellite Radar Imagery for the Detectior.
of Standing Water Beneath Forest Vegetation,"
Proc. Amer. Soc. Photogramm., Niagara Falls, NY, Oct. 1980.
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ORIGINAL 1 AUi A^ a^
OF POOR QUALITY
Waite, W.P. et al., "Look Direction Dependence of Radar Backscattering
Cross Section for Agricu-tural Fields,"
Proe. Amer. Soc. Photoframm., Niagara Falls, NY, Oct. 1950.
Stiles, W.	 "The Ef:euL of Rurface Roughness on the Microwave
Measurement of Soil Moisture Content," MS Thesis, University of
Arkansas, 19T4.
Turner., C.H., "The Effect of Low Level Vegetation on the Microwave
Measurement of Soil Moisture Content," MS Thesis, University of
Arkansas, 1974.
Moss, R.A., "Analysis of Airborne "assive Microwave Data for the
Remote Measurement of Soil Moisture," MS Thesis, University of
Arkansas, 1977•
McCall, E. /M., "Analysis of Radiative Transfer Model for the 	
t
Prediction of Microwave Emission and Soil Moisture," MS Thesis
University of Arkansas, 1977•
Hancock, G.D., "Broad Spectrum Microwave Measurement of Rough
Discontinuous Surfaces for the Determination of Moisture
Content," MS Thesis, University of Arkansas, 1977•
Corball, B.H., "The Effects of Row Orientation and Soil Moisture on
Radar Backscattering from Agricultural Fields," MS Thesis,
University of Arkansas, 1980.
Reid, C.S., "The Utility of Microwave Remote Sensing Techniques for
Soil Moisture Profile Determination," MS Thesis, University of
Arkansas, 1977•
Hancock, G.D., "Inversion Algorithms for the Microwave Remote Sensing
of Soil Moisture, Ph.D. Dissertation, University of Arkansas, 1982.
—9—


Measurement of soil moisture using remote sensing multisensor radiation techniques

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