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

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

- Introduce RT approach as basis to understanding optical and microwave vegetation response
- enable use of models
- enable access to literature

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

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?

- 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

- 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 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

- Require descriptions of:
- canopy architecture
- leaf scattering
- soil scattering

Canopy Architecture

- 1-D: Functions of depth from the top of the canopy (z).

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)

- Leaf Angle Distribution

- Archetype Distributions:
- · planophile
- · erectophile
- · spherical
- · plagiophile
- · extremophile

- Archetype Distributions:

- 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

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

from surface

Scattering properties of leaves

- Leaf surface properties and internal structure

optical

Smooth (waxy) surface

- strong peak

hairs, spines

- more 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

Depends on total area

of cell wall interfaces

Scattering properties of leaves

- Leaf surface properties and internal structure

optical

More complex structure (or thickness):

- more scattering

- lower transmittance

- more diffuse

Scattering properties of leaves

- Leaf biochemstry

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.

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.

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.

Scattering properties of leaves

- Leaf water

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

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.

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.

Scattering properties of leaves

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

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

Scattering properties of leaves

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

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)

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

- 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.

- 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).

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

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

- 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)

- 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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