CHAPTER 12: Remote Sensing of Water REFERENCE: Remote Sensing of the Environment John R. Jensen (2007) Second Edition Pearson Prentice Hall
Why y we study y the water with remote sensing?
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THE BLUE PLANET 74% of the Earth’s surface is water
WATER RESERVOIRS
2
PROCESSES AFECTING THE REMOTE SIGNAL
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DIFFERENT LAYERS
Total radiance, (Lt) recorded by a remote sensing system over water is a function of the electromagnetic energy received from:
Lp = atmospheric path radiance Ls = free-surface layer reflectance Lv = subsurface volumetric reflectance Lb = bottom reflectance
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Water Surface, Subsurface Volumetric, and Bottom Radiance The total radiance, (Lt) recorded by a remote sensing system over a waterbody is a function of the electromagnetic energy from four sources:
Lt = Lp + Ls + Lv + Lb • Lp is i the h the h radiance di recorded d d by b a sensor resulting l i from f the h downwelling d lli solar l (E ( sun) andd sky (Esky) radiation. This is unwanted path radiance that never reaches the water. • Ls is the radiance that reaches the air-water interface (free-surface layer or boundary layer) but only penetrates it a millimeter or so and is then reflected from the water surface. This reflected energy contains spectral information about the near-surface characteristics of the water. • Lv is i the th radiance di that th t penetrates t t the th air-water i t interface, i t f interacts i t t with ith the th organic/inorganic i /i i constituents in the water, and then exits the water column without encountering the bottom. It is called subsurface volumetric radiance and provides information about the internal bulk characteristics of the water column. • Lb is the radiance that reaches the bottom of the waterbody, is reflected from it and propagates back through the water column, and then exits the water column. This radiance is of value if we want information about the bottom (e.g., depth, color).
BRIGHTNESS IS CHANGED TO Lt AND Rrs
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WATER SURFACE CONDITIONS THAT AFFECT LS
Examples of Absorption of Near-Infrared Radiant Flux by Water and Sunglint
Black and white infrared photograph of water bodies in Florida
Black and white infrared photograph with sunglint
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CHANGES IN DEPTH AFFECT LS AND LV
BATHYMETRY
7
Monitoring the Surface Extent of Water Bodies The best Th b t wavelength l th region i f for di i i ti discriminating land from pure water is in the near-infrared and middle-infrared from 740 - 2,500 nm. In the near- and middle-infrared regions, water bodies appear very dark, dark even black, black because they absorb almost all of the incident radiant flux, especially when the water is deep and pure and contains little suspended sediment or organic matter.
Water Penetration
Cozumel Island
Palancar Reef SPOT Band 1 (0.5 - 0.59 mm) green
Caribbean Sea SPOT Band 2 (0.61 - 0.68 mm) red
SPOT Band 3 (0.79 - 0.89 mm) NIR
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SEPARATING THE REMOTE SIGNAL What we Measure
Water Column Reflected Radiance
Reflected Bottom Radiance
• Inherent Optical Properties • Bottom Reflectance (Albedo)
FromNEMO Overview Nemo.nrl.navy.gov
PROPERTIES
AFFECTING THE WATER LEAVING RADIANCE (LW)
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Spectral Response of Water as a Function of Organic and Inorganic Constituents - Monitoring Suspended Minerals (Turbidity), Chlorophyll, and Dissolved Organic Matter When conducting water-quality studies using remotely sensed data, data we are usually most interested in measuring the subsurface volumetric radiance, Lv exiting the water column toward the sensor. The characteristics of this radiant energy are a function of the concentration of pure water (w), inorganic suspended minerals (SM), organic chlorophyll a (Chl), dissolved organic material (DOM), and the total amount of absorption and scattering attenuation that takes place in the water column due to each of these constituents, c(λ):
Lv = f [Wc(λ), SMc(λ), Chlc(λ), DOMc(λ) ] It is useful to look at the effect that each of these constituents has on the spectral reflectance characteristics of a water column.
Absorption in Pure Water Molecular water absorption dominates in the ultraviolet (580 nm). Almost all of the incident near-infrared and middle-infrared (740 2500 nm) radiant flux entering a pure water body is absorbed with negligible scattering taking place.
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Scattering in Pure Water SScattering i in i the h water column is important in the violet, dark blue, and light blue portions of the spectrum (400 - 500 nm). This is the reason water appears blue to our eyes. The graph truncates the absorption data in the ultraviolet and in the yellow through near-infrared regions because the attenuation is so great.
Spectral Response of Water as a Function of Inorganic Constituents Minerals such as silicon, aluminum, and iron oxides are found in suspension in most natural water bodies. The particles range from fine clay particles ( 3 - 4 μm in diameter), to silt (5 - 40 μm), to fine-grain sand (41 - 130 μm), and coarse grain sand (131 - 1250 μm). Sediment comes from a variety of sources including agriculture erosion, weathering of mountainous terrain, shore erosion caused by waves or boat traffic, traffic and volcanic eruptions (ash). (ash) Most suspended mineral sediment is concentrated in the inland and nearshore water bodies. Clear, deep ocean far from shore rarely contains suspended minerals greater than 1 μm in diameter.
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AMOUNT OF TURBIDITY
5
clay
4.5
300 250
3.5 Percent Reflectance
In situ Spectroradiometer Measurement of Clear Water with Various Levels of Clayey and Silty Soil Suspended Sediment Concentrations
1,000 mg/l Clayey soil
4 200 150
3
100
2.5
50
2 clear water
15 1.5 1 0.5
a.
0 400
450
500
55 0
600 65 0 700 Wavelength (nm)
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silt
12
550 500 450
soil
Percent Reflecctance
85 0
900
400
Reflectance peak shifts toward longer wavelengths as more suspended sediment is added
600
300 250 200
10
150
8
100
6
50
4 clear water
2 0 400
800
1,000 mg/l Silty
350
b.
75 0
450
500
55 0
600 65 0 700 Wavelength (nm)
75 0
800
85 0
900
Lodhi et al., 1997; Jensen, 2000
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Space Shuttle Photograph of the Suspended Sediment Plume at the M th off the Mouth th Mississippi River near New Orleans, Louisiana
Mississippi River Plume-TM
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Mayaguez Bay-AOCI
Añasco River Plume-ATLAS
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Añasco River Plume-IKONOS
Culebrinas River Plume-IKONOS
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Spectral Response of Water as a Function of Organic Constituents - Plankton Plankton is the generic term used to describe all the living organisms (plant and animal) present in a waterbody that cannot resist the current (unlike fish). Plankton may be subdivided further into algal plant organisms (phytoplankton), animal organisms (zoolankton), bacteria (bacterio-plankton), and lower plant forms such as algal fungi. Phytoplankton are small single-celled plants smaller than the size of a pinhead. Phytoplankton, like plants on land, are composed of substances that contain carbon. Phytoplankton sink to the ocean or water-body floor when they die. All phytoplankton h l k i water bodies in b di contain i the h photosynthetically h h i ll active i pigment i chlorpohyll a. There are two other phytoplankton photosynthesizing agents: carotenoids and phycobilins. Bukata et al (1995) suggest, however, that chlorphyll a is a reasonable surrogate for the organic component of optically complex natural waters.
PHYTOPLANKTON Photosynthesis
Ocean Color
chloroplast material
cell wall
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INHERENT OPTICAL PROPERTIES
bw, Morel (1974) aw, Pope and Fry (1997)
bw 10-2 m-1
Phytoplankton bw 10-2 m-1
Pure Seawater
bchl,Loisel and Morel (1998) achl, Sathyendranath et al. (2001)
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Different pigments absorb at different wavelengths
4
Percent Reflectance
3 .5 3
clear water
2.5
algae-laden water
2 1.5 1 0.5
a.
0 400
25
500
600 7 00 Wavelength (nm)
800
900
Percent reflectance of clear and algae-laden water based on in situ spectroradiometer measurement. Note the strong chlorophyll a absorption of blue light between 400 and 500 nm and strong chlorophyll a absorption of red light at approximately 675 nm
A lgae-Laden Water with Various Suspended Sediment Concentrations
Percent Reflectan nce
20 500 mg/l
Percent e ce t reflectance e ecta ce of o algaea gae laden water with various concentrations of suspended sediment ranging from 0 - 500 mg/l
15 10 5
b.
0 400
0 mg/l 5 00
600 700 Wavelength (nm)
800
900
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Spectral Response of Water as a Function of Dissolved Organic Constituents • Sunlight penetrates into the water column a certain photic depth (the vertical distance from the water surface to the 1 percent subsurface irradiance level). level) • Phytoplankton within the photic depth of the water column consume nutrients and convert them into organic matter via photosynthesis. This is called primary production. • Zooplankton eat the phytoplankton and create organic matter. • Bacterioplankton decompose this organic matter. • All this conversion introduces dissolved organic matter (DOM) into oceanic, nearshore, and inland water bodies. • In certain instances, there may be sufficient dissolved organic matter in the water to reduce the penetration of light in the water column (Bukata et al., 1995). • The decomposition of phytoplankton cells yields carbon dioxide, inorganic nitrogen, sulfur, and phosphorous compounds.
Spectral Response of Water as a Function of Dissolved Organic Constituents • • •
• •
•
The more productive the phytoplankton, the greater the release of dissolved organic matter. matter In addition, addition humic substances may be produced. produced These often have a yellow appearance and represent an important colorant agent in the water column, which may need to be taken into consideration. These dissolved humic substances are called yellow substance or Gelbstoffe and can 1) impact the absorption and scattering of light in the water column, and 2) change the color of the water. There are sources of dissolved organic matter other than phytoplankton. For example, the brownish-yellow color of the water in many rivers in the northern United States is due to the high concentrations of tannin from the eastern hemlock (Tsuga canadensis) and various other species of trees and plants grown in bogs in these areas (Hoffer, 1978). These tannins can create problems when remote sensing inland water bodies.
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Absorption Coefficient of CDOM at Different Stations in the Mayagüez Bay 1 .0 St at ion St at ion St at ion St at ion St at ion St at ion St at ion St at ion St at ion St at ion St at ion St at ion
0 .9 0 .8 8 0 .7
ag ( m-1 )
0 .6 0 .5
1 4 5 7 9 11 13 15 17 19 21 23
0 .4 0 .3 0 .2 0 .1 0 .0 350
400
450
500
550
600
650
700
Wavelengt h ( nm)
Salinity vs. CDOM Absorption Coefficient (Ag 300 nm-1) Correlation During the Wet Season
Absorp ption Coefficient at 300 nm
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r = 0.72 n = 32
4
3
2
1
0 33.0
33.5
34.0
34.5
35.0
35.5
36.0
Salinity (ppt)
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Salinity vs. CDOM Absorption Coefficient (Ag 300 nm-1) Correlation During the Dry Season r = 0.21 n = 30
Absorp ption Coefficient at 300 nm
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 34.6
34.8
35.0
35.2
35.4
35.6
35.8
36.0
Salinity (ppt)
MAIN COMPONENTS ABSORVING LIGHT IN THE WATER COLUMN
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TYPES OF WATERS
BASED ON OPTICAL PROPERTIES
Oceanic Waters
Coastal Waters
MEASURING THE WATER QUALITY WITH REMOTE SENSORS
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Secchi Disk
Used to measure the clarity (related with suspended particles) in water bodies
BIO--OPTICAL PACKAGE BIO Data Logger Pump p OCR-200 OCR(Ed)
CTD
HS--6 HS
AC--9 AC
Battery Pack
Fluorometer
OCROCR-200 (Lu)
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WATER COLUMN VARIABILITY
SURFACE SPATIAL VARIABILITY
Salinity
Fluorescence
Absorption @412
Backscattering @589
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OCEANOGRAPHIC BUOYS
ONDULATING UNDERWATER VEHICLES
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AUTONOMOUS UNDERWATER VEHICLES
REMOTE SENSING REFLECTANCE SUN GER Ed
Absorption Scattering
Lo
Lo L sky L water
Absorption Scattering
Therefore,
R = rs
Ls
L − fL E 0
s
d
Where, f=Fresnel Number (Percent of radiation reflected back into the atmosphere). At 45o angle is 0.028.
Ed
GER--1500 GER
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Three typical spectral shapes of remote sensing reflectance curves found in Mayagüez Bay.
Remote Sensing Reflectance (Rrs) during the Dry and Rainy Seasons Low Sediment input by Rivers
High Sediment input by Rivers
Remote Sensing Reflectance (Rrs) for April 06
Remote Sensing Reflectance (Rrs) During August 05
0.018
High Chl-a Signal
0.07
0.016
High Sediment load
A1
0.06
A1
0.014
A2
AAA2
0.008 Y1
0.006
AAA1
0.04
AAA2 Y1
0.03 Y2
Y2
0.02
0.004
G1
G2
0.002 0 400
0.05 -1
-1
Rrs (sr )
AAA1
0.01
Rrs (sr )
A2
0.012
0.01
500
600 Waveleght (nm)
700
0 400
G2
500
600
700
Waveleght (nm)
27
OCEAN COLOR WITH REMOTE SENSING
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Chlorophyll in Ocean Water A remote estimate of near-surface chlorophyll concentration generally constitutes an estimate of near-surface biomass (or primary productivity) for deepp ocean ((Case 1)) water where there is little danger g of CDOM and suspended sediment contamination. Numerous studies have documented a relationship between selected spectral bands and ocean chlorophyll (Chl) concentration using the equation: Chl = x [L(λ1)/L(λ2)]y Wh Where L(λ1) andd L(λ2) are the th upwelling lli radiances di att selected l t d wavelengths l th recorded by the remote sensing system and x and y are empirically derived constants. For example, the most important SeaWiFS algorithms involve the use of band ratios of 443/355 nm and 490/555 nm.
Global Chlorophyll a (g/m3) Derived from SeaWiFS Imagery Obtained from September 3, 1997 through December 31, 1997
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True-color SeaWiFS image of the Eastern U.S. on September 30, 1997
Chlorophyll a distribution on September 30, 1997 derived from SeaWiFS data
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SPACEBORNE OCEAN COLOR INSTRUMENTS 1. Coastal Zone Color Scanner (CZCS) 2. Modular Optoelectronic Scanner (MOS) 3. Ocean Color and Temperature Scanner (OCTS) 4. Sea-viewing Wide Field-of-view Sensor (SeaWiFS) 5. Ocean Color Imager (OCI) 6 Moderate Resolution Imaging Spectroradiometer 6. (MODIS) 7. Global Imager (GLI) 8. Medium Resolution Imaging Spectrometer (MERIS)
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OCEAN COLOR INSTRUMENTS Instrument
Satellite
Dates of Operation
Spatial Resolution
Swath Width
CZCS
Nimbus-7
10/24/78-6/22/86
825 m
1556 km
MOS
IRS P3
3/21/96-Present
520 m
200 km
MOS
Priroda
4/23/96-Present
650 m
85 km
OCTS
ADEOS
8/17/96-7/1/97
700 m
1400 km
SeaWiFS
Orbview-2
8/1/97-Present
1100 m
2800 km
OCI
ROCSAT-1
1/99-Present
800 m
690 km
MODIS
Terra/Aqua
12/18/99-Present
1000 m
2330 km
GLI
ADEOS-2
scheduled
1000 m
1600 km
MERIS
ENVISAT-1
scheduled
1200 m
1450 km
Comparison of Wavelength & Bandwidth for Spaceborne Ocean Color Instruments
32
COASTAL ZONE COLOR SCANNER (CZCS)
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SCANNING GEOMETRY OF THE CZCS
34
CZCS BANDS
35
36
37
PROCESSING ALGORITHMS Based on Gordon et al. (1980) and Gordon et al. (1983) The algorithm used for estimating the pigments content of the ocean from CZCS measurements involves the use of radiance ratios. The general form of the equation is
log(C) = a + b*log[Lw(1)/Lw(2)] Where C is the pigment concentration (mg/m^3) a,b are regression coefficients Lw(1),Lw(2) are the atmospherically corrected radiances for a pair of CZCS channels h l
For CZCS pigments processing, these channel pairs are (443, 550 nm), for C < 1.5 mg/m^3 (520, 550 nm), for C > 1.5 mg/m^3
Monthly Composite of CZCS During September 1979
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39
Sea--viewing Wide Field Sea Field--of of--view Sensor
(SeaWiFS) Band 1 2 3 4 5 6 7 8
Wavelength (nm) 412 443 490 510 555 670 765 865
CZCS BANDS
Phytoplankton ChlChl-a
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SeaWiFS ALGORITHMS
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GLOBAL ESTIMATION OF PHYTOPLANKTON CHLOROPHYLL--A USING SEAWIFS DATA CHLOROPHYLL
RECEIVING CAPABILITIES OF SeaWiFS AT UPRM
Orbview 2
L-BAND ANTENNA
42
COASTAL UPWELLING IN THE CARIBBEAN SEA
AVHRR Sea Surface Temperature
SeaWiFS Chlorophyll--a Chlorophyll
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Launched on December 18, 1999
Launched on May 4, 2002
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MODIS Technical Specifications Orbit:
705 km, 10:30 a.m. descending node (Terra) or 1:30 p.m. ascending node (Aqua), sun-synchronous, near-polar, circular
Scan Rate:
20.3 rpm, cross track
Swath Dimensions:
2330 km (cross track) by 10 km (along track at nadir)
Telescope:
17.78 cm diam. off-axis, afocal (collimated), with intermediate field stop
Size:
1.0 x 1.6 x 1.0 m
Weight:
228.7 kg
Power:
162.5 W (single orbit average)
Data Rate:
10.6 Mbps (peak daytime); 6.1 Mbps (orbital average)
Quantization:
12 bits
Spatial Resolution:
250 m (bands 1-2) 500 m (bands 3-7) 1000 m (bands 8-36)
Design Life:
6 years
MODIS BANDS Primary Use
Band
Bandwidth1
Land/Cloud/Aerosols Boundaries
1
620 - 670
Spectral Radiance2 21.8
Required SNR3 128
2
841 - 876
24.7
201
Land/Cloud/Aerosols Properties
3
459 - 479
35.3
243
4
545 - 565
29.0
228
5
1230 - 1250
5.4
74
6
1628 - 1652
7.3
275
7
2105 - 2155
1.0
110
8
405 - 420
44.9
880
9
438 - 448
41.9
838
10
483 - 493
32.1
802
11
526 - 536
27.9
754
12
546 - 556
21.0
750
13
662 - 672
9.5
910
14
673 - 683
8.7
1087
15
743 - 753
10.2
586
16
862 - 877
6.2
516
17
890 - 920
10.0
167
18
931 - 941
3.6
57
19
915 - 965
15.0
250
Ocean Color/ Phytoplankton/ Biogeochemistry
Atmospheric Water Vapor
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MODIS BANDS Primary Use
Band
Bandwidth1
Surface/Cloud Temperature
20
3.660 - 3.840
Spectral Radiance2 0.45(300K)
Required NE[delta]T(K)4 0.05
21
3.929 - 3.989
2.38(335K)
2.00
22
3.929 - 3.989
0.67(300K) ( )
0.07
23
4.020 - 4.080
0.79(300K)
0.07
Atmospheric Temperature
24
4.433 - 4.498
0.17(250K)
0.25
25
4.482 - 4.549
0.59(275K)
0.25
Cirrus Clouds Water Vapor
26
1.360 - 1.390
6.00
150(SNR)
27
6.535 - 6.895
1.16(240K)
0.25
28
7.175 - 7.475
2.18(250K)
0.25
Cloud Properties
29
8.400 - 8.700
9.58(300K)
0.05
Ozone
30
9.580 - 9.880
3.69(250K)
0.25
Surface/Cloud Temperature
31
10.780 - 11.280
9.55(300K)
0.05
32
11.770 - 12.270
8.94(300K)
0.05
Cloud Top Altitude
33
13.185 - 13.485
4.52(260K)
0.25
34
13.485 - 13.785
3.76(250K)
0.25
35
13.785 - 14.085
3.11(240K)
0.25
36
14.085 - 14.385
2.08(220K)
0.35
Standard MODIS Algorithm OC3M MODIS ChlorChlor-a C = 10
(0.2830 − 2.753 R3 M + 1.457 R32M + 0.659 R33M − 1.403 R34M )
where R 3M = log 10 (R
443 490 >R ) 550 550
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Standard MODIS Chlorophyll
Sea Surface Temperature (Celsius Degree)
Phytoplankton Chlorophyll-a (mg m^3)
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Weekly Ocean Net Primary Productivity
PHYTOPLANKTON ROLE IN THE CARBON CYCLE?
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