Remote Sensing of Water

Transcription

1 Remote Sensing of Water Carolina Distinguished Professor Department of of Geography University of of South Carolina Columbia, South Carolina sc.edu

2 Earth: The Water Planet 74% of of the the Earth s s surface is is water 97% of of the the Earth s s volume of of water is is in in the the saline oceans 2.2% in in the the permanent icecap Only 0.02% is is in in freshwater streams, river, lakes, reservoirs Remaining water is is in: in: -- underground aquifers (0.6%), -- the the atmosphere in in the the form of of water vapor (0.001%) Jensen, Jensen,

3 Water Surface, Subsurface Volumetric, and Bottom Radiance The The total total radiance, (L( (L t ) t ) recorded by by a remote sensing system over over a waterbody is is a function of of the the electromagnetic energy from from four four sources: L t t = L p p + L s s + L v v + L bb L p is p is the the the the radiance recorded by by a sensor resulting from from the t the downwelling solar solar (E ((E sun ) sun ) and and sky sky (E( (E sky ) sky ) radiation. This This is is unwanted path path radiance that that never reaches the the water. L s is s is the the radiance that that reaches the the air-water interface (free (free-surface layer layer or or boundary layer) ) but but only only penetrates it it a millimeter or or so so and and is is then then reflected d from from the the water surface. This This reflected energy contains spectral information about the the near-surface characteristics of of the the water. L v is v is the the radiance that that penetrates the the air-water interface, interacts with with the the organic/inorganic constituents in in the the water, and and then then exits exits the the water column without encountering the the bottom. It It is is called subsurface volumetric radiance and and provides information about the the internal bulk bulk characteristics of of the the water column L b is b is the the radiance that that reaches the the bottom of of the the waterbody,, is is reflected from from it it and and propagates back back through the the water column, and and then then exits exits the the water column. This This radiance is is of of value if if we we want want information about the the bottom (e.g., depth, color).

4 Total radiance, (L( (L t ) t ) recorded by by a remote sensing system over water is is a function of of the the electromagnetic energy received from: L p L = p atmospheric path radiance L s = s free-surface layer reflectance L v = v subsurface volumetric reflectance L = b bottom reflectance Jensen, Jensen,

5 Water Surface, Subsurface Volumetric, and Bottom Radiance The The total total radiance, (L( (L t ) t ) recorded by by a remote sensing system over over a waterbody is is a function of of the the electromagnetic energy from from four four sources: L t t = L p p + L s s + L v v + L bb L p is p is the the the the radiance recorded by by a sensor resulting from from the t the downwelling solar solar (E ((E sun ) sun ) and and sky sky (E( (E sky ) sky ) radiation. This This is is unwanted path path radiance that that never reaches the the water. L s is s is the the radiance that that reaches the the air-water interface (free (free-surface layer layer or or boundary layer) ) but but only only penetrates it it a millimeter or or so so and and is is then then reflected d from from the the water surface. This This reflected energy contains spectral information about the the near-surface characteristics of of the the water. L v is v is the the radiance that that penetrates the the air-water interface, interacts with with the the organic/inorganic constituents in in the the water, and and then then exits exits the the water column without encountering the the bottom. It It is is called subsurface volumetric radiance and and provides information about the the internal bulk bulk characteristics of of the the water column L b is b is the the radiance that that reaches the the bottom of of the the waterbody,, is is reflected from from it it and and propagates back back through the the water column, and and then then exits exits the the water column. This This radiance is is of of value if if we we want want information about the the bottom (e.g., depth, color).

6 Examples of of Absorption of of Near-Infrared Radiant Flux by by Water and Sunglint Black Black and and white white infrared infrared photograph photograph of of water water bodies bodies in in Florida Florida Black Black and and white white infrared infrared photograph photograph with with sunglint sunglint

7 Absorption and Scattering Attenuation in in Pure Water Molecular water absorption dominates in in the the ultraviolet (<400 nm) nm) ) and and in in the the yellow through the the near-infrared portion of of the the spectrum (>580 nm). Almost all all of of the the incident near-infrared and and middle-infrared infrared (740 ( nm) nm) ) radiant flux flux entering a pure pure water body body is is absorbed with with negligible scattering taking place.

8 Absorption and Scattering Attenuation in in Pure Water Scattering in in the the water column is is important in in the the violet, dark dark blue, blue, and and light light blue blue portions of of the the spectrum (400 ( nm). This This is is the the reason water appears blue blue to to our our eyes. eyes. The The graph truncates the the absorption data data in in the the ultraviolet and and in in the the yellow through near-infrared regions because the the attenuation is is so so great.

9 Monitoring the Surface Extent of Water Bodies The best wavelength region for for discriminating land from pure water is is in in the the near-infrared and middle-infrared infrared from ,500 nm. In In the the near- and middle-infrared infrared regions, water bodies appear very dark, even black, because they absorb almost all all of of the the incident radiant flux, especially when the the water is is deep and pure and contains little suspended sediment or or organic matter. Jensen, Jensen,

10 Water Penetration Cozumel Island Palancar Reef Caribbean Sea SPOT Band SPOT 1 Band ( ( μm) green μm) green SPOT Band SPOT 2 Band ( ( μm) red μm) red SPOT SPOT Band Band 3 3 (0.79 ( μm) μm) NIR NIR

11 Spectral Response of of Water as as a Function of 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 we are are usually most most interested in in measuring the the subsurface volumetric radiance, L v exiting v the the water column toward the the sensor. The The characteristics of of this this radiant ant energy are are a function of of the the concentration of of pure pure water (w),( (w), inorganic suspended minerals (SM (SM), organic chlorophyll a (Chl (Chl), dissolved organic material ((DOM), and and the the total total amount of of absorption and and scattering attenuation that that takes takes place in in the the water column due due to to each each of of these these constituents, c( c( λ ): λ ): L v v = f f [w [w c(λ) c(λ),,sm c(λ) c(λ),,chl c(λ) c(λ),,w w c(λ) c(λ) ]. ]. It It is is useful to to look look at at the the effect that that each each of of these these constituents ts has has on on the the spectral reflectance characteristics of of a water column.

12 Spectral Response of of Water as as a Function of of Inorganic and Organic Constituents Minerals such as as silicon, aluminum, and iron oxides are are found in in suspension in in most natural water bodies. The particles range from fine clay particles ( ( μm m in in diameter), to to silt silt (5 ( μm), to to fine-grain sand ( μm), and coarse grain sand ( μm). Sediment comes from a variety of of sources including agriculture erosion, weathering of of mountainous terrain, shore erosion caused by by waves or or boat traffic, and volcanic eruptions (ash). Most suspended mineral sediment is is concentrated in in the the inland and nearshore water bodies. Clear, deep ocean (Case 1 water) far far from shore rarely contains suspended minerals greater than 1 μm m in in diameter.

13 Space Shuttle Photograph of of the the Suspended Sediment Plume at at the the Mouth of of the the Mississippi River near New Orleans, Louisiana STS STS #51 #51

14 Secchi Disk Used to to measure suspended sediment in in water bodies

15 In In situ Spectroradiometer Measurement of of Water with Various Suspended Sediment and Chlorophyll a Concentrations Spectroradiometer 165 cm 15Þ 35 cm diameter water surface 90 cm diameter = 3.66 m Lodhi Lodhi et et al., al., 1997; 1997; Jensen, ; 2000;

16 clay clay Percent Reflectance Clayey soil clear water ,000 mg/l 300 In In situ Spectroradiometer Measurement of of Clear Water with Various Levels of of Clayey and Silty Soil Suspended Sediment Concentrations 0.5 a Wavelength (nm) silt silt Percent Reflectance Silty soil ,000 mg/l 600 Reflectance peak peak shifts toward longer wavelengths as as more more suspended sediment is is added 2 clear water b Wavelength (nm) Lodhi Lodhi et et al., al., 1997; 1997; Jensen, 2000

17 Spectral Response of of Water as as a Function of of Organic Constituents - Plankton Plankton is is the the generic term term used used to to describe all all the the living organisms (plant( and and animal) present in in a waterbody that that cannot resist the the current (unlike fish). fish). Plankton may may be be subdivided further into into algal algal plant plant organisms (phytoplankton), animal organisms ((zoolankton), bacteria ((bacterio- plankton), and and lower platn platn forms such such as as algal algal fungi. Phytoplankton are are small single-celled plants smaller than than the the size size of of a pinhead. Phytoplankton, like like plants on on land, land, are are composed of of substances that that contain carbon. Phytoplankton sink sink to to the the ocean or or water-body floor floor when they they die. die. All All phytoplankton in in water bodies contain the the photosynthetically active pigment chlorpohyll a. a.. There are are two two other other phytoplankton photosynthesizing agents: carotenoids and and phycobilins. Bukata et et al al (1995) suggest, however, that that chlorphyll a is is a reasonable surrogate for for the the organic component of of optically complex natural waters.

18 Micrograph of of A Photosynthesizing Diatom Micrograph of of Blue Reflected Light from a Green Algae Cell (Micrasterias sp.). chloroplast material cell wall

19 Percent Reflectance a clear water algae-laden water Wavelength (nm) Percent reflectance of of clear and algae-laden laden water based on on in in situ spectroradiometer measurement. Note the the strong chlorophyll a absorption of of blue light between 400 and 500 nm and strong chlorophyll a absorption of of red light at at approximately 675 nm Percent Percent Reflectance Reflectance Algae-Laden Water with Various Suspended Sediment Concentrations 500 mg/l Percent reflectance of of algae-laden laden water with various concentrations of of suspended sediment ranging from mg/l b. 0 0 mg/l Wavelength (nm) Han, Han, 1997; 1997; Jensen, Jensen,

20 Chlorophyll in Ocean Water A remote estimate of of near-surface chlorophyll concentration generally constitutes an an estimate of of near-surface biomass (or (or primary productivity) for for deep deep ocean (Case 1) 1) water where there there is is little little danger of of suspended mineral sediment contamination. Numerous studies have have documented a relationship between selected spectral bands and and ocean chlorophyll (Chl( (Chl) ) concentration using the the equation: Chl Chl = x [L([ [L(λ 1 )/L(λ 1 )/L(λ 2 )] 2 )] y y Where L(λ L(λ 1 ) 1 ) and and L(λ L(λ 2 ) 2 ) are are the the upwelling radiances at at selected wavelengths recorded by by the t the remote sensing system and and x and and y are are empirically derived constants. The The most most important SeaWiFS algorithms involve the the use use of of band band ratios of of 443/355 nm nm and and 490/555 nm. nm.

21 Global Chlorophyll a (g/m 33 ) ) Derived from SeaWiFS Imagery Obtained from September 3, 3, 1997 through December 31, 31, 1997

22 True-color True-color SeaWiFS SeaWiFS image image of of the the Eastern Eastern U.S. U.S. on on September September 30, 30, Chlorophyll Chlorophyll a a distribution distribution on on September September 30, 30, derived derived from from SeaWiFS SeaWiFS data data

23 Spectral Response of of Water as as a Function of of Dissolved Organic Constituents Sunlight penetrates into into the the water column a certain photic depth (the (the vertical distance from from the the water surface to to the the 1 percent subsurface irradiance level). Phytoplankton within the the photic depth of of the the water column consume nutrients and and convert them them into into organic matter via via photosynthesis. This This is is called primary production. Zooplankton eat eat the the phytoplankton and and create organic matter. Bacterioplankton decompose this this organic matter. All All this this conversion introduces dissolved organic matter (DOM) into into oceanic, nearshore, and and inland water bodies. In In certain instances, there there may may be be sufficient dissolved organic matter in in the the water to to reduce the the penetration of of light light in in the the water column (Bukata et et al., al., 1995). The The decomposition of of phytoplankton cells cells yields carbon dioxide, inorganic nitrogen, sulfur, and and phosphorous compounds.

24 Spectral Response of of Water as as a Function of of Dissolved Organic Constituents The The more more productive the the phytoplankton, the the greater the the release of of dissolved organic matter. In In addition, humic substances may may be be produced. These often often have have a yellow appearance and and represent an an important colorant agent in in the the water column, which may may need need to to be be taken into into consideration. These dissolved humic substances are are called yellow substance or or Gelbstoffe and and can can 1) 1) impact the the absorption and and scattering of of light light in in the the water column, and and 2) 2) change the the color color of of the the water. There are are sources of of dissolved organic matter other other than than phytoplankton. For For example, the the brownish-yellow color color of of the the water in in many rivers in in the the northern United States is is due due to to the the high high concentrations of of tannin from from the the eastern hemlock (Tsuga canadensis) and and various other other species of of trees trees and and plants grown in in bogs bogs in in these these areas areas (Hoffer, 1978). These tannins can can create problems when remote sensing inland water bodies.

25 GOES-East GOES-East East Visible Visible GOES-East GOES-East East Thermal Thermal Infrared Infrared GOES-East East Images of of the the United States and Portions of of Central America on on April 17, 17, 1998 GOES-East GOES-East East Water Water Vapor Vapor

26 Cold middle level clouds or semi-transparent high clouds tops of large convective clouds middle level convective clouds Cloud Type Determination Based on on Multispectral Measurements in in the the Visible and Thermal Infrared Regions of of the the Spectrum Thermal Infrared Thermal Infrared low cumuliform clouds Warm Dark sea Visible land Bright

27 Reflectance of of Clouds and Snow in in the Wavelength Interval μm

28 Sea-surface Temperature (SST) Maps Derived from A Three-day Composite of of NOAA AVHRR Infrared Data Centered on on March 4, 4, 1999 Adjusted Adjusted to to highlight highlight nearshore nearshore temperature temperature differences differences Adjusted Adjusted to to highlight highlight Gulf Gulf Stream Stream temperature temperature differences differences

29 Composite Sea-surface Temperature (SST) Map of of the the Southeastern Bight Derived from AVHRR Data C 60 F

30 Worldwide Sea-surface Temperature (SST) Map Derived From NOAA-14 AVHRR Data Three-day composite of of thermal thermal infrared infrared data data centered on on March March 4, 4, Each Each pixel pixel was was allocated the the highest highest surface surface temperature that that occurred during during the the three three days. days.

31 Reynolds Monthly Sea-surface Temperature ( C)( ( C) Maps Derived from from In In situ situ Buoy and and Remotely Sensed Data Data La La Nina Nina December, December, Normal Normal December, December, El El Nino Nino December, December,

32 Tropical Rainfall Measurement Mission (TRMM) Microwave Imager (TMI) Data Obtained on on March 9, 9, 1998 A passive passive microwave microwave sensor sensor that that measures measures in in five five frequencies: frequencies: (45 (45 km km spatial spatial resolution), resolution), 19.4, 19.4, 21.3, 21.3, 37, 37, and and GHz GHz (5 (5 km km spatial spatial resolution). resolution). It It has has dual dual polarization polarization at at four four of of the the frequencies. frequencies. Swath Swath width width is is miles miles (780 (780 km). km). The The GHz GHz frequency frequency provides provides a a a linear linear response response to to rainfall. rainfall.

33 A B TRMM Precipitation Radar (PR) (PR) data data obtained on on March 9, 9, Height (km) 5 z(dbz dbz) A B Distance (km) Along-track cross-section section of of TRMM TRMM Precipitation Radar Radar data data obtained on on March March 9, 9,

34 Nonpoint Source Pollution Modeling Based on on the the Agricultural NonPoint Source (AGNPS) Pollution Water Quality Model Applied to to Two Two Subbasins in in the the Withers Swash Watershed in in Myrtle beach, SC SC