Remote sensing of water quality is dicussed. Remote sensing penetration depth is a function both of water type and wavelength. Results of three tests to help demonstrate the magnitude of this dependence are presented. The water depth to which the remote-sensor data was valid was always less than that of the Secchi disk depth, although not always the same fraction of that depth. The penetration depths were wavelength dependent and showed the greatest variation for the water type with largest Secchi depth. The presence of a reflective plate, simulating a reflective subsurface, increased the apparent depth of light penetration from that calculated for water of infinite depth

Morris, W. D.

Whitlock, C. H.

Witte, W. G.

English

NASA Technical Reports Server

3 1176 00162 8172
NASA Technical Memorandum 81843
NASA-TM-8 ]843 19800025419
TURBIDWATERMEASUREMENTSOF REMOTE
SENSINGPENETRATIONDEPTHAT VISIBLE
AND NEAR-INFRAREDWAVELENGTH
W.D. Morris, W.G. Witte, and C.H. Whitlock
AUGUST1980 [#OR ._FEP,_NOE
O_f9 1980
I,/,.NGLEY t_ESr:ARCH CENT Kf{
UBRARY,NASA
N/ A
NationalAeronautics and
Space Administration
LangleyFlesearchCenter
Hampton,Virginia 23665
https://ntrs.nasa.gov/search.jsp?R=19800025419 2020-03-21T15:40:47+00:00Z

TURBIDWATERMEASUREMENTSOF REMOTESENSINGPENETRATIONDEPTH
AT VlSlBLE ANDNEAR-INFRAREDWAVELENGTHS
by W. D. Morris, W. G. Witte, Jr., and C. H. Whitlock
Langley Research Center
INTRODUCTION
The quality of impounded water is subject to major changes dUE to
the influx of sediments and dissolved substances from the surrounding
land area. The Use of remote sensing offers a unique potential for
monitoring these individual water bodies on a regional basis and in a
more timely manner than is possible with in situ techniques. Both
multispectral scanners and photographic systems are presently being
used for remote sensing of water quality. They record the reflected
solar radiance from the water at visible and near-infrared wavelengths.
The basis for their use is that changes in water quality are detectible
by the changes in the solar radiance distribution of sunlight whichhas
penetrated the water surface and is then scattered back toward the
remote sensors. These signals are altered from that of clear water by
their interaction with particulate and dissolved matter in the water.
What is not well known, however, is the depth to which the remote
sensors are able to detect these changes in water quality. It is often
assumed that the depth of detection corresponds to the depth at which
a Secchi disk will disappear from sight when lowered into the water,
but previous investigation has indicated that this assumption is not
always correct and may lead to erroneous interpretation of the remotely
sensed data. The penetration depth of light into water as seen by a
remote sensor is frequently less than that of the Secchi disk depth and
can be highly wavelength dependent. An improved knowledge of the
effect of water quality on the remote-sensing penetration depth will
lead to more accurate interpretation of the remotely sensed data and to
better correlation of those data with in situ measurements.
Over the past 2 years, the Marine Environments Branch at NASA-
Langley has been examining the relationship between the reflected solar
signal and water quality in both field test and laboratory experiments.
An integral part of this work has been the determination of the depth
to which the light received by a remote sensor penetrates the water and
how this varies both with the wavelength and with water type. Results
of three field experimentsare presented here.
METHOD
Field measurements were made as illustrated in figure I. Tests
were made for two different types of light penetration. First, water
samples were taken to obtain optical properties used in the determina-
tion of the remote-sensing penetration depth (RSPD) for water which is
not depth limited (i.e., water in which light does not reach the bottom
surface or the bottom surface has a very low reflectance level and does
not affect the signal that is returned to the sensor). These water
Jb -jj s2 7
samples were analyzed in the laboratory for both water quality and
optical parameters. The optical coefficients from these tests were
then used to compute the RSPDby the method described in Gordon and
McCluney (1975) from
1
Z90 - K
where Zgo is the depth above which 90 percent of the diffusely reflected .
radiance originates and K is the diffuse attenuation coefficient.
The diffuse attenuation coefficient is defined by Gordon, Brown, and
Jacobs (1975) as
K - a+b*Cos8o
using the inherent water properties of absorption coefficient (a) and
backscattering coefficient (b*) and the underwater solar zenith angle(Oo).
The second type of measurement was designed to determine the
apparent remote-sensing penetration depth (ARSPD). This is the depth
to which the light will appear to penetrate when a highly reflective
bottom material is present and the light reflecting back from this
surface adds to the signal received by the remote sensor. This effect
would bias the received signal. Determination of the ARSPDwould
define a minimum depth for receiving an unbiased signal in the presence
of a highly reflective bottom. To simulate this condition, a rapid
scanning spectrometer was used to record the reflected signal from a
white plate as it was lowered into the water. Spectral signals were
recorded for discrete depths from near the surface to a depth that was
20 to 40 cm beyond the depth at which the plate could no longer be seen
by the eye. The recorded signals were then plotted versus depth for
selected wavelengths and this figure was compared with the water spec-
trum for those same wavelengths. The depth at which the signal from
the plate matched that of the water was then considered the ARSPD.
The third method was also used to measure penetration depth. A
Secchi disk depth was recorded for each site for comparison with the
RSPDand the ARSPD.
RESULTSANDDISCUSSION
Field tests have been conducted at a number of different sites in
order to obtain data from various characteristic water types. The
results of three of those tests are presented here. Their locations
are shown in figure 2. Visually, each of these rivers had a distinctive
water color. The Appomattox River in Hopewell, Virginia was the
most turbid of the three rivers. The tests were conducted several days
after heavy rains upstream of the test site and the river had a
yellowish-brown, cloudy appearance. The test area on the Back River in
Hampton, Virginia was at the mouth of the river, just before entering
2
Chesapeake Bay. This water had a blue-green appearance and was the
clearest water tested. The Ogeechee River, near Savannah, Georgia,
provides drainage for the swamps in that region and has a characteris-
tic black but clear appearance. The optical and water quality param-
eters for each of these rivers are shown in the tables of figure 3.
As anticipated from the general appearance of the rivers, the minimum
Secchi depth, 51 cm, was in the Appomattox and the maximumwas in the
Back River at 137 cm. The Appomattox is characterized by a large atten-
uation coefficient which is almost 85 percent scattering. This agrees.
with the relatively large amount of total suspended solids which would
be associated with high scattering coefficients. The Ogeechee River
has the highest _mount of dissolved organic carbon and a relatively low
attenuation coefficient. Thirty-six percent of the attenuation in this
river was due to absorption. The Back River had the highest level of
chlorophyll a and total suspended solids and the lowest attenuation
coefficient.- The high level of suspended solids does not correlate
well with the low attenuation coefficient.
The computed remote sensing penetration depths (Zq_) for light
entering water which is not depth limited (i.e., an inffnite water
depth) are presented in figure 4 for each of the three tests. Each
curve is characterized by a maximumpenetration in the 600-to 700-nm
range and an absorption peak near 750 nm. For the Appomattox River,
the remote-sensing penetration depth is approximately half of the
Secchi depth. For the Ogeechee, it is less than a third the depth.
But for the Back River it is almost two-thirds the Secchi depth. In
all cases, the penetration depths are less than that of the Secchi disk
depth. The depths of penetration also vary with thewavelength and the
Back River water appears to be the most highly wavelength dependent of
the three test sites.
The ARSPD's (depth limited) are shown in figure 5 in comparison
with the computed RSPD's (infinite depth) and the Secchi depths. In
each case, the use of the white plate to simulate the effect of a highly
reflective bottom material has extended the effective penetration depth
of the light from that which was calculated (Zon) for water which was
not depth limited (i.e., assumes an infinite b_tom depth). McCluney
(1974) indicates that material with a reflectivity as low as 20 percent
will greatly extend the apparent penetration depth. Each ARSPDcurve
also shows a maximumpenetration in the 600-to 700-nm wavelength range.
For the Back River, this extends from 500 to 700 nm.
These results imply that in the case of Landsat imagery, the results
of each band might be valid for different depths. For example, the
results of band 5 for the Appomattox River (with the same water quality
as was tested) are representative of only the top 25 cm and not to the
50 cm depth read with the Secchi disk. Similarly, in situ samples taken
to correspond with band 5 imagery would have to be taken from that top
25 cm layer to give valid results. This is only true if the band 5
imagery is used in regions where the water depth exceeds 80 cm. If it
is less than this, the results will probably be influenced by bottom
reflectance. The ARSPDcurves essentially define a minimum bottom depth
necessary for remotely sensed data, in order to be assured that the signal
is unbiased by bottom reflectance.
3
CONCLUDINGREMARKS
Remote sensing of water quality can be a valuable aid in monitor-
ing and managing impounded water areas. The remotely sensed signal
represents an integrated result over the water column that isviewed
and to use this information accurately, the water depth through which
the sunlight penetrates and returns to the sensor must be known. This
remote sensing penetration depth is a function both of water type and
wavelength. Results of three tests tohelp demonstrate the magnitude
of this dependence have been presented.
The water depth_to which the remote sensor data was valid was
always less than that of the Secchi disk depth, although not always the
same fraction of that depth. The penetration depths were wavelength
dependent and showed the greatest variation for the water type with the
largest Secchi depth. The presence of a reflective plate, simulating
a reflective subsurface, increased the apparent depth of light penetra-
tion from that calculated for water of infinite depth.
Inferences from remote-sensing data should be limited to depths
less than that of the Secchi disk depth and in situ data taken to
correlate with remote sensing data should also be taken from water
above this depth. Where the water depth is sufficiently shallow, the
presence of a highly reflective bottom may bias the data. Where the
depth is sufficient, a theoretical value of penetration depth may be
used.
A knowledge of the penetration depth is required to best utilize
remotely sensed data. The limited number of tests presented in this
paper are insufficient to draw general conclusions between penetration
depths and water type. Additional testing will be required.
REFERENCES
Gordon, H. R.; Brown, O. B.; and Jacobs, M. M.: "Computed Relation-
ships Between the Inherent and Apparent Optical Properties of a
: Flat HomogenousOcean." Applied Optics, vol. 14, no. 2, 417,
February 1975.
Gordon, H. R. and McCluney, W.R.: "Estimation of the Depth of Sun-
light Penetration in the Sea for Remote Sensing." Applied Optics,
vol. 14, no. 2, 413, February i975.
McCluney, W.R.: "Estimation of Sunlight Penetration in the Sea for
Remote Sensing." Goddard Space Flight Center, NASATMX-70643,
April 1974.
Figure 1.-Il1ustration of field measurements.
5
VIRGINIA ..
:K RIVER
;POMATTOXRI
GEORGIA
OGEECHEERI
I ATLANTIC
-_ OCEAN
Figure 2.- Location of test sites for remote sensing penetration
depth studies.
6
Appomattox Ogeechee Back
River River River
Secchidepth (cm) 51 95 137
AttenuationCoefficient(_)
at 550 nm (m-1) 19 l0 8.9
ScatteringCoefficient(s)
at 550 nm (m-1) 16.3 6.3 8.3
AbsorptionCoefficient(a)
at 550 nm (m-1) 2.7 3.7 .6
Total SuspendedSolids
(mg/l) 18.6 ii 22.8
InorganicSuspended
Solids (mg/l) 14.7 i0 13.9
Volatile Suspended
Solids (mg/l) 3.9 1 8.9
DissolvedOrganic
Carbon (mg/l) <I0 13 <i0
Chlorophylla (_g/l) i0 4 13
Figure 3. Table of water quality and opticalparameters
WAVELENGTH(;k), nm,
400 500 600 700 800 900
0 I I l i
20-
40 - "1::( '
A.R. ./ -
<_ "/////////////////'_60- /E _
o \ /N _
"" 80- _
a. - _O.R.
,= _////////_,
,oo_- ,i!20 -
- B.R. _/
140- '7//////////////////_APPOMATT0X RIVER 0
_- OGEECHEERIVER Q
L_!60 BACK RIVER
Figure 4. - Computed remote-sensing penetration depth
as a function of wavelength for three
test sites. Secchi depths are shown
cross hatched for each river.
Figure 5. - Comparison of the apparent remote-sensing
penetration depth (depth limited) with the
remote-sensing penetration depth (infinite
depth) and the Secchi de.pth (shown cross hatched).

1. Report No, 2. GovernmentAccessionNo, 3. Recipient'sCatalogNo.
NASATM-81843
=4. Title and Subtitle 5. Report Date
Turbid Water Measurements of RemoteSensing Penetration August 1980
Depth at Visi bl e and Near-Infrared Wavelength 6. PerformingOrganizationO_de
7. Author(s) 8. Performing Organ;zation Report No.
W. D. Morris,W. G. Witte, and C. H. Whitlock
10. Work Unit No.
9. Performing Organization Name and Address 691-09-02-01
NASALangley Research Center 11.Contractor GrantNo.
Hampton, Virginia 23665
13. Type of Report and Period Covered
12. Sponsoring Agency Name and Address
Technical Memorandum
NationalAeronauticsand Space Administration 14.SponsoringAgencyCode
Washington,D.C. 20546 -
15. SupplementaryNotes
This paper was presented at the Symposiumon Surface-Water Impoundments,
Minneapolis, Minnesota, June 2-5, 1980.
16. Abstract
Remote sensing of water quality can be a valuable aid in monitor-
ing and managing impounded water areas. The remotely sensed signal
represents an integrated result over the water column that is viewed
and to use this information accurately, the water depth through which
the sunlight penetrates and returns to the sensor must be known. This
remote sensing penetration depth is a function both of water type and
wavelength: Results of three tests to help demonstrate the magnitude
of this dependence have been presented.
The water depth to which the remote-sensor data was valid was
always less than that of the Secchi disk depth, although not always the
same fraction of that depth. The penetration depths were wavelength
dependent and showed the greatest variation for the water typewith the
largest Secchi depth. The presence of a reflective plate, simulating
a reflective subsurface, increased the apparent depth of light penetra-
tion from that calculated for water of infinite depth.
17. Key Words (Sugg_ted by Author(s)) 18. Distribution Statement
Remote Sensing Unclassified - Unlimited
•" Water QualityMeasurement
Data Analysis SubjectCategory 45
19. S_curity Cla_if. (of this report} 20. Security Classif. (of this page) 21. No. of Pages 22. Price"
Unclassified Unclassified 9 A02 ..
:_-_,_ ForsalebytheNationalTechnicalInformationService,Springfield,.Virginia22161





Turbid water measurements of remote sensing penetration depth at visible and near-infrared wavelength

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