Radiative Transfer Theory at Optical wavelengths applied to

Radiative Transfer Theory at Optical wavelengths applied to vegetation canopies: part 1 Uo. L 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 • 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

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

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

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 (m 2/m 3) 2. the leaf normal orientation distribution function (dimensionless). 3. leaf size distribution (m)

Canopy Architecture • Leaf area / number density – LAI (one-sided) m 2 leaf per m 3

Canopy Architecture • Leaf Angle Distribution

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

Leaf Angle Distribution • Archetype Distributions:

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

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).

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

Scattering properties of leaves • Leaf surface properties and internal structure optical Diffused from scattering at internal air-cell wall interfaces Depends on total area of cell wall interfaces Depends on refractive index: varies: 1. 5@400 nm 1. 3@2500 nm

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

Scattering properties of leaves • Leaf biochemstry

Scattering properties of leaves • Leaf biochemstry

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

Scattering properties of leaves • Optical Models – flowering plants: PROSPECT

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 – soil type/texture – soil surface roughness.

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

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

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)

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

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

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)

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