A Semiannual Status Report

on

CHARACTERIZATION OF VEGETATION BY MICROWAVE AND OPTICAL

REMOTE SENSING

C.S.T. Daughtry (P.I.)/ K.J. Ranson, and L.L. .Biehl

Laboratory for Applications of Remote Sensing

Purdue University

West Lafayette, IN 47907

NASA Research Grant NAGW-799

August 1985 - January 1986

r- (E86-10012 NASA-Cfi^176«05) CHABACTEBIZATION N86-J18766

OF VEGETATION. BI 8ICB08AVE AHD OPTICAL

BJEMOTE SENSING Seoianuual Status fieport,

Aug. .,1985: -

 ;Jan.,1986> (Purdue Univ.) 15 p Unclas

HC A02/MF A01, CSCL 02C G3/43 00012
INTRODUCTION

For more than two decades, scientists have studied

separately the properties of vegetation in the optical and

microwave regions of the electromagnetic spectrum. Each spectral

region appears to contain unique information about vegetation.

Measurements by optical sensors depend on the differential

scattering and absorption caused by chlorophyll, leaf structure,

green leaf area, leaf water content, and soil background (Bunnik,

1978; Knipling, 1970). Microwave sensors respond to larger scale

structure of the canopy, plant water content, plant part size

distributions, and soil surface conditions (Attema and Ulaby,

1978; Brumfeldt and Ulaby, 1984a).

However, most microwave research related to vegetation has

focused only on species identification (Bush and Ulaby, 1978) or

the effects of vegetation on the determination of soil moisture

(Dobson et al., 1984;Dobson and Ulaby, 1986). Strong evidence

suggests that the backscatter returns of active microwave

systems are related to vegetation canopy density and structure

(Allen and Ulaby, 1984; Brumfeldt and Ulaby, 1984b).

It is significant to note that the parameters affecting

optical and microwave radiation appear to be complementary in

that they represent different, but interrelated, aspects of the

scene (Goel, 1985) . Because the physical parameters affecting

both optical and microwave radiation are related to the quantity

and quality of phytomass, the opportunity exists to utilize

remote sensors to obtain data that are needed to model the growth

and net productivity of vegetation. These are key concerns for

modeling global carbon cycles, as well as assessing the ability

of ecosystems to support human life.

Relatively little remote sensing research has been devoted

to assessing the quantity or vigor of vegetation. The key

research issue is how to determine the important biophysical

characteristics of diverse vegetative canopies using

multispectral (optical and microwave) remote sensing.

The overall objective of this multi-year research project

is to measure and analyze microwave and optical data, singly and

in combination, for characterizing diverse vegetative canopies.

The specific objectives of this research were:

1. Determine the relationships between backscatter and

the biophysical characteristics of a broad range of

canopy types.

2. Determine the feasibility of using simulated tree

canopies for evaluating backscattering from forest

scenes.

3. Implement and exercise backscattering models using

data acquired by the C-Band microwave scatterometer.
Such information will be important to scientists in

understanding the relationships between remotely sensed

(microwave and/or optical) data and the biophysical

characteristics of vegetation. The goals of the Space Station

Earth Observing System make it imperative that these

relationships be understood. This research will help establish a

sound physical and biological basis for interpreting the

radiation-absorbing and -scattering properties of vegetation.

EXPERIMENT DESIGN

Two series of carefully controlled experiments were

conducted in 1984 and 1985. First, plots of important crops

(corn, soybeans, and sorghum), prairie grasses (big bluestem,

switchgrass, tall fescue, orchardgrass, bromegrass') , and forage

legumes (alfalfa, red clover, and crown vetch) were manipulated

to produce wide ranges of phytomass, leaf area index, and canopy

architecture. Second, coniferous forest canopies .were simulated

using small (1.3 m) balsam fir trees grown in large pots of soil

and arranged systematically on a large (5 m) platform. Rotating

the platform produced many 'new1 canopies for frequency and

spatial averaging of the backscatter signal. In both series of

experiments, backscatter of 5-.0 GHz (C-Band) was measured as a

function of view angle (0 to 60 degrees in 10-degree increments)

and polarization. Biophysical measurements included leaf area

index, fresh and dry phytomass, water content of canopy elements,

canopy height, and soil roughness and moisture content. For a

subset of the above plots, additional measurements were acquired

to exercise microwave backscatter models (Attema and Ulaby,

1978; Eom and Fung, 1984). These measurements included size and

shape of leaves, stems, and fruit and the probability density

function of leaf and stem angles. The dielectric properties of

canopy components were measured by the Radiation Laboratory at

the University of Michigan.

•Optical data of these experiments were also acquired using

the Barnes 12-1000 (TM bands) and Exotech 100 (MSS bands)

radiometers. In future analyses, optical and microwave data

will be combined. The relationships of the backscattering

coefficients and the biophysical properties of the canopies

were evaluated using statistical correlations, analysis of

variance, and regression analysis. Results from the corn density

and balsam fir experiments are discussed and analyses of data

from the other experiments are summarized.

Corn Density Experiment

[2.5, 5.0, 10.0, and 15.

(Zea mavs L. 'Pioneer 3732') were planted in 0.76 m rows on 14

Four densities (2.5, 5.0, 10.0, and 15.0 plants/m2) "of corn
May and 4 June 1984 at the Purdue University Agronomy Farm. The

second experiment was planted on 23 May 1985 and consisted of

three densities (2.5, 5.0 and 10.0 plants/m ) of corn (Pioneer

3732) in 0.76 m rows and three densities (5.0, 10.0, and 20.0

plants/m2) of corn in 0.38 m rows. High levels of soil

fertility were maintained by applying 250, 44, and 130 kg/ha of

N, P, and K, respectively, as indicated by soil tests. The four

densities of corn produced four distinct levels of fresh and dry

phytomass and leaf area index throughout most of the growing

season (Fig. 1). Plant water content (not shown) also differed

significantly among the plant densities and was highly correlated

to fresh phytomass. Plant height and stage of development were

not significantly affected by plant density. As competition

among the plants increased, the phytomass and leaf area per plant

decreased and the number of barren plants (i.e., plants with no

grain on their ears) increased.

The C-Band microwave scatterometer (MSC) was mounted on a

mobile aerial tower (Hi-Ranger truck) and backscattering data

were acquired as a function of polarization (HH, HV, W) and view

zenith angle (0 to 60 degrees in 10-degree increments). The

backscatter coefficients measured parallel to the row direction

decreased rapidly as the view zenith angle increased from 0

degrees (vertical) to 30 degrees (Fig. 2). Changes in

backscatter were much smaller at view zenith angles greater than

30 degrees. Both like- (HH, W) and cross- (HV) polarizations

responded similarly to changes in view angle although the

cross-polarization backscatter was approximately 5 dB lower than

the like-polarization backscatter.

Selected relationships between the biophysical

characteristics of corn and the backscatter coefficients at 30

degrees for HH polarization are shown in Figure 3. In general,

as phytomass and leaf area index increased, the backscatter

coefficients (in real units) also increased linearly. A portion

of the scatter about the regression lines (Fig. 3) is undoubtedly

associated with experimental error; however, each backscatter

coefficient is the mean of at least 25 independent observations

and each biophysical measurement is the- mean of at least 10

plants per plot. Other sources of variations in the backscatter

at 5.0 GHz are related to soil moisture content, soil texture

(particle size distribution), and soil roughness (Dobson et al.,

1984). In this experiment, soil texture was a silt loam and

was uniform throughout all plots. Soil moisture and .surface

roughness changed during the season but were not significantly

different among the plots on each measurement date.

Additional research is needed to carefully evaluate the

effects of soil moisture and surface roughness on microwave

backscatter coefficients of vegetation. It may be possible and

profitable to use multiple view angles and polarizations to

minimize the variation in microwave backscatter due to the soil
10

1 6

"- Z,

ISO 170 190 210 230 . 250

130 ISO 170 ISO 210 230 250

x 4'

I -

0 -

130 ISO 170 190 210 230

omrs

250

Figure 1. Changes in phytomass and leaf, area index

of four plant densities of corn during 1984.
5

0

-5

T3 -10

il

U.

U

CC 5

Ul

tS o

10

-10

-15

-20

-1

+ HJune 1984 p|Qnl

D\ Density

^ >y ' plonts/m

\\

 +

 — + " \ \ D D 10.0

S """v

3 August 1984 Plon,

Density

D.

_ S plonts/m

+ , \ D D 10.0

1 1 1 1

0 0 10 20 30

Phytomoss

LAI Fresh Dry

* m*/mj g/m1 g/m2

0.5 463 56

1.8 1510 185

1 1

Phytomoss

LAI Fresh Dry

1

 m'/m* g/m1 g/m*

1.3 3639 706

3.7 6568 1281

~~

D— • --. _

1 1

40 50

|

|

60

VIEW ZENITH ANGLE, degrees

Figure 2. Changes in backscattering coefficients of

two corn canopies on two dates in 1984 as a function of view zenith angle with HH polarization.

Data are means of 25 observations per plot and

were acquired at a view azimuth parallel to the

row direction.
to

a

6

1

Y •-1.980 + 68.910 X

R

?

- 0.67

0.00 0.02 0.04 0.08 0.08 0.10 O.U O.U O.IS

2.5 H

2.0 -

•5(3

£ 1.5 -

CO

*

g uo -

i o.s -

0.0 -

Y • -O.W8 + It.178 X

R* - 0.71

0.00 0.02 0.01 0.06 0.08 0.10 0.12 O.U 0.18

o.oo o.oz o.u o.oe o.oa a. 10 o.iz o.u o.ie

BACKSCATTER COEFFICIENT (K2/M2)

Figure 3. Relationships of phytomass and leaf

area index to microwave backscatter coefficients of corn in 1984. Microwave data

were acquired at a view zenith angle of 30°

with HH polarization. Similar results were

also observed for VV polarization.
8

background condition and thus enrich the information content

about vegetation. Knowledge gained from these analyses will be

valuable in interpreting multitemporal microwave data.

Balsam Fir Experiment

The purpose of the balsam' fir C-band experiment was to

examine the radar backscattering from coniferous forest canopies

arranged with different densities. Balsam fir has linear

flattened needles and a well defined pyramidal shaped crown.

Similar trees were used for an optical reflectance experiment in

1984 (Ranson et al., 1986). In 1985, live seven year old balsam

fir (Abies balsamea (L.) Mill.) trees were transplanted from a

nursery to 28 liter pots filled with soil. The trees averaged

1.3 m in height with a mean crown diameter of 0.76 m. The trees

were placed on a 5 m diameter rotatable platform (turntable) in

an equidistant arrangement. The densities used were 0.7, 1.4,

2.0 and 3.1 trees/m2 resulting in canopies with fresh phytomass

of 1.6, 3.3, 4.7 and 7.3 kg/m , respectively. Other canopy

measurements included needle area index, (ratio of one sided leaf

area to horizontally projected ground area), dry phytomass, water

content, and percent ground cover. One representative tree was

sent to the University of Michigan's Radiation Laboratory for

determination of dielectric properties. In addition, needle and

branch angle probability density functions were measured in 1984

using balsam fir trees of similar age and growth habit.

The C-band scatterometer was mounted on the boom of the HiRanger truck and positioned to view the canopies at view zenith

angles ranging from 0° (nadir) to 50° (Figure 4). A series of

backscatter measurements were made at 10° rotation intervals of

the turntable (0-350°) for four polarizations (HH, HV, VH and

W). At each turntable position the average of 8 backscatter

observations (in real units) was calculated. The 36 mean

observations for a complete turntable rotation were averaged to

obtain the the mean radar backscattering coefficient for a canopy

density at each view zenith angle and polarization.

To eliminate radar backscatter from the metal turntable and

pots, 0.10 m of microwave absorbing material (ie. , Echosorb) was

placed beneath the trees directly over the pots. The effectiveness of the absorber material was tested by measuring the

backscatter of (1) the bare metal turntable surface; (2) the

turntable with soil filled pots covered with absorber (no' trees);

and (3) the turntable arranged with trees in pots covered by

absorber. The results of this test for the W polarization are

shown in Figure 5. There was a 28 db reduction in backscatter

coefficient with the absorber material at 0° view zenith angle.

The trees with absorber showed a 23 db decrease in backscatter

from the bare turntable and a 5 db increase over that from the

absorber + pots. Similar reductions were noted from the HH, HV
5m

Turntable

Figure 4. The C-band scatterometer was mounted' on an aerial

boom truck and positioned to view the balsam fir

canopies on a turntable.

J3

TJ

4*

c

01

20

10

t °

01

o

u

u -10

01

u -20

in

.*

u

fl

«,-30 _I " " ......

0 10 20 30 40

VIEW ZENITH ANGLE (DE6.)

Figure 5. C-band backscatter coefficient versus view zenith

angle for 1) bare metal turntable surface, 2) turntable

surface with soil filled pots (no trees) covered with

10 cm of microwave absorber (Echosorb) and 3) balsam

fir trees with Echosorb covering pots and bare

turntable surface.
10-

and VH polarizations as well. We concluded that the absorber

material effectively minimized the radar back scattering from the

background and the radar return from the scene was dominated by

the canopy scattering and attenuation.

The relationships between backscattering coefficient and

illumination (view zenith) angle for the balsam fir canopies is

illustrated in Figure 6. There was a general decrease in radar

return as view angle increased for HH and W polarizations for

all canopy densities. The HV cross polarization backscatter for

the lowest density canopy decreased until 20° view zenith angle

and then increased rapidly as view zenith angle increased. As

the view angle increased (more vegetation in the field of view)

the backscatter coefficient increased and then leveled off. It

appears for this case that the scattering characteristics of the

vegetation are less important for near-nadir viewing angles. The

order of the curves suggests that radar backscatter coefficients

increase with increasing amounts of vegetation. However, there

were small differences in backscatter coefficient between the'

three densest canopies with 3.3, 4.7 and 7.3 kg/nr of fresh

phytomass, respectively.

Figure 7 shows the relationship between backscatter

coefficient and phytomass more clearly for view zenith angles of

0, 20 and 50° and HH polarization. The cause of the lack of

difference in backscatter coefficient as the amount of vegetation

increased may be attributed to attenuation of the radar signal by

the outside foliated branches. Balsam fir has linear flattened

needles which have length and width much smaller than the

wavelength of C-band radar. Needle lengths average about 1.5 cm

and needle widths average about 0.15 cm in comparison with the

average radar wavelength of 6.0 cm. In a recent report sieber

(1985) observed that the outside green branches of spruce trees

were the prime scattering centers for L- and X- Band radar with

higher attenuation for the X-band frequency. A similar effect

may be a contributing factor for our C-band measurements of

balsam fir.

Accomplishments

One of the key milestones achieved in 1984 was learning how

"to obtain reliable data with the C-Band microwave scatterometer

system. In 1984 and 1985 procedures for operating the

scatterometer were refined and tested for field plots where the

instrument was moved to acquire multiple samples per plot' and for

simulated canopies on the turntable where the instrument remained

stationary and the scene moved. More than 4500 independent

observations were acquired with the C-band scatterometer in the

corn, soybeans, sorghum, prairie grasses and simulated forest

canopies. At least three research papers are being prepared for

publication. The results may be summarized by the. following:
11

-10

-IS

-20

-as

-30

-

Dvnmlty .

Trees/in"

0-0-0 0.0

A^4 0.7

o-B-e i. it

*-»-• E.O

«"«"• 3.1

MAI

of /of

0.0

1.2

2.5

3.5

S.<t

Phy to<&«.»« — •©

Met Dry

kg/ae

0.0 0.0

1.6 O.B

3.3 1.7

it. 7 3.«»

7.3 '3.7

10 20 30

VIEW ZENITH ANGLE,

HO

degrees

Figure 6. Relationships of C-band backscatter coefficient

and view zenith angle for five densities of balsam fir

and three polarizations. A) HH, B) HV and C) W

polarization.
12

JO

* -5

4*

c

Ql -1

H*

u

HI

o

U

 -20

u

01

4*

4* -26

IB

U

in

-

30

50'

123456 7

Fresh Phytomass, kg/n>B

Figure 7. Relationships of C-band backscatter coefficient

and fresh balsam fir phytomass at three view zenith

angles.
13

1. As phytomass and leaf area index increased, the

backscatter coefficient also increased. However, variations

in soil moisture from date to date significantly affected

the backscatter from all canopies. In some cases the

relative ranking of the backscatter coefficients with

respect to the phytomass of the canopies changed, depending

on soil moisture. Analysis techniques to extract

information about vegetation in the presence of "noise" due

to the background are needed.

2. Tree canopies for microwave experiments can be simulated

by using 10 cm of microwave absorber to obscure the background of the turntable. Many of the procedures developed

for simulating canopies for our optical experiments are

applicable for microwave experiments. The backscattering

coefficient from the tree canopy appears to approach a

maximum as fresh phytomass exceeds 3.3 kg/m2 * * In these

experiments the contribution from the background surface to

the measured backscatter response was negligible.

Additional experiments with the balsam fir trees and natural

background materials (eg. soil or grass sod) should be

conducted to quantify the contribution from the background.

3. Large changes in the phytomass of the grasses and

legumes produced only small changes in the backscatter

coefficients — presumably because the foliage elements are

much smaller than the wavelength of the C-Band microwaves (5

to 7 cm) and the background surface contributes significant

radar backscatter.

4. The development of grain heads in sorghum significantly

•increased the backscatter coefficient (Fig. 8). These

multitemporal microwave data may be valuable for determining

critical stages in the development of sorghum, wheat,

barley, sunflowers, and other crops.

5. An initial version of a "cloud" backscattering model

•(Attema and Ulaby, 1978) was implemented and exercised. A

new and improved scattering model for vegetation (Eom and

Fung, 1984) will be implemented in. 1986 when it is

available.
14

CD

TJ

UJ

o

UJ

o

o

o:

UJ

O

(/)

X

O

<

CO

— D>^ After Heading

10 20 30 40 50 60 70

VIEW ZENITH ANGLE,degrees

80

Figure 8. Changes in backscattering coefficients of a sorghum canopy

before and after emergence of the panicles (grain heads). Data are

means of 25 observations per plot and were acquired at a view azimuth

parallel to the row direction.
References

Allen, C.T., and F.T. Ulaby, 1984. Modeling the polarization

dependence of the attenuation in vegetation canopies, pp.

119-124, Proc. IGARSS'84 Symp., Strasbourg, France, Aug.

27-30, 1984.

Attema, E.P.W., and F.T. Ulaby. 1978. Vegetation modeled as a

water cloud. Radio Science 13:357-364.

Brumfeldt, D.R., and F.T. Ulaby. 1984. Measured microwave

emission and scattering in vegetation canopies. IEEE Trans.

Geosci. Remote Sens. GE-33:520-524.

Brumfeldt, D.R., and F.T. Ulaby. 1984. The effect of row

direction on the microwave emission from vegetation

canopies, pp. 125-130, Proc. IGARSS'84 Symp. Strasbourg,

France, Aug. 27-30, 1984.

Bunnik, N.J.J. 1978. The multispectral reflectance of shortwave

radiation by agricultural crops in relation with their

morphological and optical properties. Mededelingen

Landbouwhogeschule, Wageningen, The Netherlands. 78-1.

Bush, T.I"./ and F.T. Ulaby. 1978. An evaluation of radar as a

crop classifier. Remote Sens. Environ. 7:15-36.

Dobson, M.C., F. Kouyate, and F.T. Ulaby. 1984. A reexamination

of soil textural effects on microwave emission and

backscattering. IEEE Trans. Geosci. Remote Sens. GE-22:530-

536.

Dobson, M.C. and F.T. Ulaby. 1986. Active microwave soil

moisture research. IEEE Trans. Geosci. and Remote Sens.

GE-24:23-36.

Eom, H.J., and A.K. Fung. 1984. A scatter model for vegetation

up to Ku-Band. Remote Sens. Environ. 15:185-200.

Goel, N.S. 1985. Modeling canopy reflectance and backscattering

coefficient. Remote Sens. Environ. 18:235-253.

Ranson, K.J., C.S.T. Daughtry and L.L. Biehl. 1986. Sun angle,

view angle and background effects on spectral response of

simulated balsam fir canopies. Photogramm. Engin. Remote

Sens. (In press).

Sieber, A.J. 1985. Forest signatures in microwave scatterometer

data. IEEE Geoscience and Remote Sensing Society Newsletter

9(4):4-10. t .