Notes adapted from Prof. P. Lewis plewis@geog.ucl.ac.uk

1. 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 plewis@geog.ucl.ac.uk Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel: 7679 0592 Email: mdisney@ucl.geog.ac.uk http://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/GEOGG141.html http://www2.geog.ucl.ac.uk/~mdisney/teaching/3051/GEOG3051.html

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. 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.

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

7. 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

8. 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…..?

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

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

11. 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)

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

13. Canopy Architecture • Leaf Angle Distribution

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

15. Leaf Angle Distribution • Archetype Distributions:

16. 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

17. 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

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

19. 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

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

21. Scattering properties of leaves • Leaf biochemstry

22. 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.

23. 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.

24. 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.

25. 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.

26. 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

27. 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.

28. 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.

29. Scattering properties of leaves • Optical Models • flowering plants: PROSPECT – extension of plate model to N layers http://teledetection.ipgp.jussieu.fr/opticleaf/models.htm

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

31. 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)

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

33. 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.

34. 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).

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

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

37. 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)

38. 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