Transcript Slide 1
Radiative Transfer Theory at Optical wavelengths applied to vegetation canopies: part 1 UoL MSc Remote Sensing Dr Lewis [email protected] 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 ul z (m2/m3) 2. the leaf normal orientation distribution function (dimensionless). 3. leaf size distribution (m) Canopy Architecture • Leaf area / number density – 2 3 ul z (one-sided) m leaf per m zH LAI L u z dz l z 0 Canopy Architecture • Leaf Angle Distribution z Inclination to vertical 2 g l Wl d Wl 1 ql Wl Leaf normal vector y fl azimuth x Leaf Angle Distribution • Archetype Distributions: planophile gl l 3 cos2 l erectophile spherical 3 2 g l l sin l 2 plagiophile extremophile gl l 1 15 2 g l l sin 2l 8 15 2 gl l cos 2l 7 Leaf Angle Distribution • Archetype Distributions: 3.0 2.5 g_l(theta_l) 2.0 1.5 1.0 0.5 0.0 0 10 20 30 40 50 60 70 leaf zenith angle / degrees spherical plagiophile planophile extremophile erectophile 80 90 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 Dicotyledon leaf structure 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 Dicotyledon leaf structure Depends on refractive index: varies: 1.5@400 nm 1.3@2500nm Scattering properties of leaves • Leaf surface properties and internal structure optical More complex structure (or thickness): - more scattering - lower transmittance - more diffuse Dicotyledon leaf structure 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 wavelengths. reflectance and transmittance at these 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