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GEOGG141/ GEOG3051 Principles & Practice of Remote Sensing (PPRS) Radiative Transfer Theory at o ptical wavelengths applied to vegetation canopies: part 1. Notes adapted from Prof. P. Lewis [email protected] Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building

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notes adapted from prof p lewis plewis@geog ucl ac uk

GEOGG141/ GEOG3051Principles & Practice of Remote Sensing (PPRS)Radiative Transfer Theory at optical wavelengths applied to vegetation canopies: part 1

Notes adapted from Prof. P. Lewis [email protected]

Dr. Mathias (Mat) Disney

UCL Geography

Office: 113, Pearson Building

Tel: 7679 0592

Email: [email protected]

http://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/GEOGG141.html

http://www2.geog.ucl.ac.uk/~mdisney/teaching/3051/GEOG3051.html

aim of this section
Aim of this section
  • Introduce RT approach as basis to understanding optical and microwave vegetation response
  • enable use of models
  • enable access to literature
scope of this section
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
reading
Reading

Full notes for these lectures

http://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/rt_theory/rt_notes1.pdf

http://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/rt_theory/rt_notes2.pdf

Books

Jensen, J. (2007) Remote Sensing: an Earth Resources Perspective, 2nd ed., Chapter 11 (355-408), 1sted chapter 10.

Liang, S. (2004) Quantitative Remote Sensing of Land Surfaces, Wiley, Chapter 3 (76-142).

Monteith, J. L. and Unsworth, M. H. (1990) Principles of Environmental Physics, 2nd ed., ch 5 & 6.

Papers

Feret, J-B. et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.

Jacquemoud. S. and Baret, F. (1990) PROSPECT: A model of leaf optical properties spectra, RSE, 34, 75-91.

Nilson, T. and Kuusk, A. (1989) A canopy reflectance model for the homogeneous plant canopy and its inversion, RSE, 27, 157-167.

Price, J. (1990), On the information content of soil reflectance spectra RSE, 33, 113-121

Walthall, C. L. et al. (1985) Simple equation to approximate the bidirectional reflectance from vegetative canopies and bare soil surfaces, Applied Optics, 24(3), 383-387.

why build models
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
radiative transfer theory
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
radiative transfer theory1
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
  • But…..?
building blocks for a canopy model
Building blocks for a canopy model
  • Require descriptions of:
    • canopy architecture
    • leaf scattering
    • soil scattering
canopy architecture
Canopy Architecture
  • 1-D: Functions of depth from the top of the canopy (z).
canopy architecture1
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)

slide12

LAI

Canopy Architecture

  • Leaf area / number density
    • (one-sided) m2 leaf per m3
slide13

Canopy Architecture

  • Leaf Angle Distribution
slide14

Leaf Angle Distribution

  • Archetype Distributions:
      • · planophile 
      • · erectophile 
      • · spherical 
      • · plagiophile 
      • · extremophile 
slide15

Leaf Angle Distribution

  • Archetype Distributions:
slide16

Leaf Dimension

  • RT theory: infinitesimal 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
slide17

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).

Excellent review here:

http://www.photobiology.info/Jacq_Ustin.html

slide18

Specular

from surface

Scattering properties of leaves

  • Leaf surface properties and internal structure

optical

Smooth (waxy) surface

- strong peak

hairs, spines

- more diffused

slide19

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: [email protected] nm

[email protected]

Depends on total area

of cell wall interfaces

slide20

Scattering properties of leaves

  • Leaf surface properties and internal structure

optical

More complex structure (or thickness):

- more scattering

- lower transmittance

- more diffuse

slide22

Scattering properties of leaves

  • Leaf biochemstry

Feret, Jacquemoud et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.

slide23

Scattering properties of leaves

  • Leaf biochemstry

Feret, Jacquemoud et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.

slide24

Scattering properties of leaves

  • Leaf biochemstry

Feret, Jacquemoud et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.

slide25

Scattering properties of leaves

  • Leaf water

Feret, Jacquemoud et al. (2008) PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments, RSE, 112, 3030-3043.

slide26

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
slide27

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.
slide28

Scattering properties of leaves

  • Optical Models
    • flowering plants: PROSPECT – a generalised plate model

Figure from: http://teledetection.ipgp.jussieu.fr/opticleaf/models.htm& see for more detail on various approaches to leaf optical properties modelling

Jacquemoud. S. and Baret, F. (1990) PROSPECT: A model of leaf optical properties spectra, RSE, 34, 75-91.

slide29

Scattering properties of leaves

  • Optical Models
    • flowering plants: PROSPECT – extension of plate model to N layers

http://teledetection.ipgp.jussieu.fr/opticleaf/models.htm

slide30

Scattering properties of leaves

  • leaf dimensions
    • optical
      • increase leaf area for constant number of leaves - increase LAI
      • increase leaf thickness - decrease transmittance (increase reflectance)
slide31

Scattering properties of soils

  • Optical and microwave affected by:
    • soil moisture content
      • Wetter soils are darker (optical); have lower dielectric (microwave)
    • soil type/texture
    • soil surface roughness
      • shadowing (optical)
      • coherent scattering (microwave)
slide32

soil moisture content

  • Optical
    • effect essentially proportional across all wavelengths
      • enhanced in water absorption bands
slide33

soil texture/type

  • Optical
    • relatively little variation in spectral properties
    • Price (1990):
      • 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

Price, J. (1990), On the information content of soil reflectance spectra RSE, 33, 113-121.

slide34

Soil roughness effects

  • Affects directional properties of reflectance (optical particularly)
  • Simple models:
    • as only a boundary condition, can sometimes use simple models
      • e.g. Lambertian
      • e.g. trigonometric (Walthall et al., 1985; Nilson and Kuusk 1990)
      • where θv,i are the view and illumination (sun) zenith angles; ϕ is relative azimuth angle (ϕi - ϕv).
slide35

Soil roughness effects

  • Rough roughness:
    • optical surface scattering
      • clods, rough ploughing
        • use Geometric Optics model (Cierniewski)
        • projections/shadowing from protrusions
slide36

Soil roughness effects

  • Rough roughness:
    • optical surface scattering
      • Note backscatter reflectance peak (‘hotspot’)
      • minimal shadowing
      • backscatter peak width increases with increasing roughness
slide37

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)
slide38

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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