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Radiative Transfer Theory at Optical wavelengths applied to vegetation canopies: part 1

Radiative Transfer Theory at Optical wavelengths applied to vegetation canopies: part 1. UoL MSc Remote Sensing Dr Lewis plewis@geog.ucl.ac.uk. Aim of this section. Introduce RT approach as basis to understanding optical and microwave vegetation response enable use of models

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Radiative Transfer Theory at Optical wavelengths applied to vegetation canopies: part 1

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  1. Radiative Transfer Theory at Optical wavelengths applied to vegetation canopies: part 1 UoL MSc Remote Sensing Dr Lewis plewis@geog.ucl.ac.uk

  2. Aim of this section • Introduce RT approach as basis to understanding optical and microwave vegetation response • enable use of models • enable access to literature

  3. Scope of this section • Introduction to background theory • RT theory • Wave propagation and polarisation • Useful tools for developing RT • Building blocks of a canopy scattering model • canopy architecture • scattering properties of leaves • soil properties

  4. Associated practical and reading • Reading • Course notes for this lecture • Reading list

  5. Why build models? • Assist data interpretation • calculate RS signal as fn. of biophysical variables • Study sensitivity • to biophysical variables or system parameters • Interpolation or Extrapolation • fill the gaps / extend observations • Inversion • estimate biophysical parameters from RS • aid experimental design • plan experiments

  6. Radiative Transfer Theory • Applicability • heuristic treatment • consider energy balance across elemental volume • assume: • no correlation between fields • addition of power not fields • no diffraction/interference in RT • can be in scattering • develop common (simple) case here

  7. Radiative Transfer Theory • Case considered: • horizontally infinite but vertically finite plane parallel medium (air) embedded with infinitessimal oriented scattering objects at low density • canopy lies over soil surface (lower boundary) • assume horizontal homogeneity • applicable to many cases of vegetation

  8. Building blocks for a canopy model • Require descriptions of: • canopy architecture • leaf scattering • soil scattering

  9. Canopy Architecture • 1-D: Functions of depth from the top of the canopy (z).

  10. Canopy Architecture • 1-D: Functions of depth from the top of the canopy (z). 1. Vertical leaf area density(m2/m3) • the leaf normal orientation distribution function (dimensionless). 3. leaf size distribution (m)

  11. LAI Canopy Architecture • Leaf area / number density • (one-sided) m2 leaf per m3

  12. Canopy Architecture • Leaf Angle Distribution

  13. Leaf Angle Distribution • Archetype Distributions: · planophile  · erectophile  · spherical  · plagiophile  · extremophile 

  14. Leaf Angle Distribution • Archetype Distributions:

  15. Leaf Dimension • RT theory: infinitessimal scatterers • without modifications (dealt with later) • In optical, leaf size affects canopy scattering in retroreflection direction • ‘roughness’ term: ratio of leaf linear dimension to canopy height also, leaf thickness effects on reflectance /transmittance

  16. Canopy element and soil spectral properties • Scattering properties of leaves • scattering affected by: • Leaf surface properties and internal structure; • leaf biochemistry; • leaf size (essentially thickness, for a given LAI).

  17. Specular from surface Scattering properties of leaves • Leaf surface properties and internal structure optical Smooth (waxy) surface - strong peak hairs, spines - more diffused

  18. Diffused from scattering at internal air-cell wall interfaces Scattering properties of leaves • Leaf surface properties and internal structure optical Depends on refractive index: varies: 1.5@400 nm 1.3@2500nm Depends on total area of cell wall interfaces

  19. Scattering properties of leaves • Leaf surface properties and internal structure optical More complex structure (or thickness): - more scattering - lower transmittance - more diffuse

  20. Scattering properties of leaves • Leaf biochemstry

  21. Scattering properties of leaves • Leaf biochemstry

  22. Scattering properties of leaves • Leaf biochemstry

  23. Scattering properties of leaves • Leaf biochemstry

  24. Scattering properties of leaves • Leaf water

  25. Scattering properties of leaves • Leaf biochemstry • pigments: chlorophyll a and b, a-carotene, and xanthophyll • absorb in blue (& red for chlorophyll) • absorbed radiation converted into: • heat energy, flourescence or carbohydrates through photosynthesis

  26. Scattering properties of leaves • Leaf biochemstry • Leaf water is major consituent of leaf fresh weight, • around 66% averaged over a large number of leaf types • other constituents ‘dry matter’ • cellulose, lignin, protein, starch and minerals • Absorptance constituents increases with concentration • reducing leaf reflectance and transmittance at these wavelengths.

  27. Scattering properties of leaves • Optical Models • flowering plants: PROSPECT

  28. Scattering properties of leaves • Optical Models • flowering plants: PROSPECT

  29. Scattering properties of leaves • leaf dimensions • optical • increase leaf area for constant number of leaves - increase LAI • increase leaf thickness - decrease transmittance (increase reflectance)

  30. Scattering properties of soils • Optical and microwave affected by: • soil moisture content • soil type/texture • soil surface roughness.

  31. soil moisture content • Optical • effect essentially proportional across all wavelengths • enhanced in water absorption bands

  32. soil texture/type • Optical • relatively little variation in spectral properties • Price (1985): • PCA on large soil database • 99.6% of variation in 4 PCs • Stoner & Baumgardner (1982) defined 5 main soil types: • organic dominated • minimally altered • iron affected • organic dominated • iron dominated

  33. Soil roughness effects • Simple models: • as only a boundary condition, can sometimes use simple models • e.g. Lambertian • e.g. trigonometric (Walthall et al., 1985)

  34. Soil roughness effects • Rough roughness: • optical surface scattering • clods, rough ploughing • use Geometric Optics model (Cierniewski) • projections/shadowing from protrusions

  35. Soil roughness effects • Rough roughness: • optical surface scattering • Note backscatter reflectance peak (‘hotspot’) • minimal shadowing • backscatter peak width increases with increasing roughness

  36. Soil roughness effects • Rough roughness: • volumetric scattering • consider scattering from ‘body’ of soil • particulate medium • use RT theory (Hapke - optical) • modified for surface effects (at different scales of roughness)

  37. Summary • Introduction • Examined rationale for modelling • discussion of RT theory • Scattering from leaves • Canopy model building blocks • canopy architecture: area/number, angle, size • leaf scattering: spectral & structural • soil scattering: roughness, type, water

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