Remote Sensing of Water, Soil, and Urban Areas Lecture 6

1. Remote Sensing of Water, Soil, and Urban AreasLecture 6 Summer Session 28 July 2011

2. Spectral characteristics of water

4. http://www.itek.norut.no/vegetasjon/fenologi/introduction/ndvi.htmlhttp://www.itek.norut.no/vegetasjon/fenologi/introduction/ndvi.html

5. Liquid Water Absorption

6. Important things to remember in using VIS/RIR data to monitor water surfaces • Pure water absorbs EM energy in most of the IR region • Pure, deep (> 50 m) water bodies have low reflectance in the visible and very near IR region of the EM spectrum • However, some EM energy in this region is transmitted into the water column, where it reacts with particles suspended in the water column

7. Reflection off the bottom in shallow water

8. Variations in image color/intensity in the Bahamas region is due to reflectance off the ocean bottom

9. MODIS Image of Phytoplankton – Bering Sea, Alaska

10. Plankton off of the West coast of Mexico

11. Plankton off of Tasmania, Australia

12. Pyrrophytes (dinoflagellates) Oblique aerial photograph of red tide

13. MODIS Satellite Image of Suspended Sediments Mississippi River Delta

14. Simple model of flux off of a water surface p- scattered/reflected from particles suspended in water r- surface reflection i s- scattered from water b – reflected from the bottom

15. What can happen to EM energy reaching a water surface? • Reflected off the surface • Transmitted into the water column • Absorbed by the water • Scattered by the water • Absorbed by materials suspended in the water • Reflected or scattered by matter suspended in the water • Reflected off of the bottom

16. Water Attenuation c() c () =  () + b () where • () = water absorption coefficient b () = water scattering coefficient

18. Absorption and wavelength • Note that wavelengths > 0.9 m have large absorption coefficients • Because of this, sea surface remote sensing systems do not have bands > 0.9 m • Absorption will change if you have dissolved inorganic or organic material in the water, depending upon the compounds that are dissolved

19. ultrviolet

21. Role of suspended sediments Suspended sediments in the water column do two things • Absorb/transmit EM energy • Scatter/reflect EM energy

22. Simple model of flux off of a water surface p- scattered/reflected from particles suspended in water r- surface reflection i s- scattered from water

23. p - total scattering from the water column is dependent on • b () - water scattering coefficient • SM () - Suspended inorganic minerals • DOM() - Dissolved organic material • Chl () - Chlorophyll within the column Wavelength dependent!

24. T - total absorption from the water column is dependent on •  () - water absorption coefficient • SM () - Suspended inorganic minerals • DOM () - Dissolved organic material • Chl () - Chlorophyll within the column Wavelength dependent!

25. Water reflectance as a function of sediment concentration 12-8 in Jensen

26. Chlorophytes Euglenophytes Haptophytes Bacillariophytes Glaucophytes Pyrrophytes (dinoflagellates)

27. Reflectance vs. phytoplankton density

28. Without sediment present Effects of phytoplankton on water surface reflectance @ different sediment concentrations Figure 12.9 Jensen

29. Effects of bottom reflectance • In clear, shallow (< ~30 m) water, the reflectance properties of the bottom influences the total flux off of the water surface • Therefore, in clear shallow water, variations in total flux are related to the composition of the bottom itself

31. Sources of variation in bottom reflectance • Variations in mineral content of soil (sediment), gravel, rocks on bottom • Presence of coral • Presence of sea grass

32. Simple model of flux off of a water surface r- surface reflection i • Surface reflection has three components • Direct specular reflection of sunlight • Specular reflection of indirect, scattered light • Sunglint

33. Sunglint occurs when the sensor and the reflected sunlight having the same angle • Wind results in small waves (capillary waves) on any water surface. • These waves results in facets that result in direct specular reflection of a certain portion of sunlight. • Specular reflection off of a smooth ocean = artificially high signal at sensor. • Specular reflection off of a rough surface = artificially low signal at sensor.

34. Sunglint • “Sunglint is a phenomenon that occurs when the sun reflects off the surface of the ocean at the same angle that a satellite sensor is viewing the surface. • “In the affected area of the image, smooth ocean water becomes a silvery mirror, while rougher surface waters appear dark.” Source: Wikipedia – generally not to be trusted but good for sunglint!

35. Water Summary In summary, total radiance from a water surface, t-wis comprised of 1. Radiance from surface reflection 2. Radiance from water scattering 3. Radiance from reflection/scattering from particles/phytoplankton suspended in the water column 4. Radiance from reflection of EM energy from the bottom of the water body

36. Clouds, Snow, and Ice

37. Clouds • Do not reflect Solar radiation well in all directions • Need multiple observations from different points • Thermal properties are important

38. Albedo • a measure of reflectivity of a surface or a body • a ratio of EM radiation reflected to the amount incident upon it • Reflectance of an object aggregated over a broader segment of the EM spectrum (0.3-2.4 microns) in all directions • Expressed in 0-100% • Clouds: varies from 10-90%, depending on drop sizes, liquid water or ice content, thickness of a cloud, and the solar zenith angle.

39. Clouds Information and imagery from http://earthobservatory.nasa.gov/Library/Clouds

40. High Clouds High clouds increase greenhouse effect and subsequently increase surface temperature Information and imagery from http://earthobservatory.nasa.gov/Library/Clouds

41. Middle and Low Clouds Low clouds decrease greenhouse effect and subsequently decrease surface temperature Information and imagery from http://earthobservatory.nasa.gov/Library/Clouds

42. Deep Convective Clouds Deep convective clouds do not influence greenhouse effect and are neutral to surface temperature Information and imagery from http://earthobservatory.nasa.gov/Library/Clouds

43. Precipitation Hurricane Bonnie precipitation from TRMM data

44. Snow and Ice • Climate change observations • Hazards (avalanches etc.) • Sea ice extent • Fresh water supply

45. Clouds and Ice: spectral response • Very similar in visible and NIR wavelengths • Very different in wavelengths over 1.5µm - snow and ice strongly absorb the energy - clouds strongly reflect the energy Source: http://www.cps-amu.org/sf/notes/m1r-1-8.htm

46. Spectral Characteristics of Soil

47. Soil composition • Several layers contributing to energy flux off of soils: • litter • Organic matter content • minerals • iron compounds • bedrock http://cals.arizona.edu/pubs/garden/mg/soils/images/p3large.gif http://www.physicalgeography.net/fundamentals/images/soil_breakdown.gif

48. Energy-Target Interactions General Rule: I = R+A+T Where R – reflected radiation; A – absorbed radiation; T – transmitted radiation For Soils: I = R+A T is near 0

49. General Spectral Characteristics • Dry soil: increase in reflectance with increase in wavelength in visible and NIR portion of the spectrum • The differences in reflectance of various soils are relatively consistent throughout various wave length regions.

50. Soil properties influencing its spectral characteristics • texture • organic matter • iron oxide