74% of the Earth s surface is water

CHAPTER 12: Remote Sensing of Water REFERENCE: Remote Sensing of the Environment John R. Jensen (2007) Second Edition Pearson Prentice Hall Why y we ...
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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?

1

THE BLUE PLANET 74% of the Earth’s surface is water

WATER RESERVOIRS

2

PROCESSES AFECTING THE REMOTE SIGNAL

3

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

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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 ƒ Minerals such as silicon, aluminum, and iron oxides are found in suspension in most natural water bodies. ƒ 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). ƒ 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) ƒ Most suspended mineral sediment is concentrated in the inland and nearshore water bodies. ƒ 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)

14

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

14

Añasco River Plume-IKONOS

Culebrinas River Plume-IKONOS

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Spectral Response of Water as a Function of Organic Constituents - Plankton ƒ 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). ƒ Plankton may be subdivided further into algal plant organisms (phytoplankton), animal organisms (zoolankton), bacteria (bacterio-plankton), and lower plant forms such as algal fungi. ƒ Phytoplankton are small single-celled plants smaller than the size of a pinhead. ƒ Phytoplankton, like plants on land, are composed of substances that contain carbon. ƒ 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. ƒ There are two other phytoplankton photosynthesizing agents: carotenoids and phycobilins. ƒ 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)

17

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

5

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)

23

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

26

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

29

True-color SeaWiFS image of the Eastern U.S. on September 30, 1997

Chlorophyll a distribution on September 30, 1997 derived from SeaWiFS data

30

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)

31

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)

33

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

40

SeaWiFS ALGORITHMS

41

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

43

Launched on December 18, 1999

Launched on May 4, 2002

44

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

45

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

46

Standard MODIS Chlorophyll

Sea Surface Temperature (Celsius Degree)

Phytoplankton Chlorophyll-a (mg m^3)

47

Weekly Ocean Net Primary Productivity

PHYTOPLANKTON ROLE IN THE CARBON CYCLE?

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