Nadia [email protected]
Richard Fernandes
& Michael Chelle
Does Needle Clumping Affect Shoot Scattering and Canopy BRDF ?
Outlook
Context
Background
Smolander Approach
Objectives
Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF
A simple shoot parameterization
Impact of simple shoot parameterization on canopy BRDF
Conclusion
Outlook
ContextContext Background
Smolander Approach
Objectives
Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF
A simple shoot parameterization
Impact of simple shoot parameterization on canopy BRDF
Conclusion
Conteoxt Monitoring the seasonal development and carbon uptake by vegetation in
relation with global climatic change.
Northern vegetation (Dominant forests) represent significant carbon sink.
VEGETATION
MISR
Spectral & Directional Information
Reflectance or Radiative Transfer Models
Empirical approaches(Vegetation Indices)
Biophysical variables Leaf Area Index
fcoverfAPARalbédo
Functioning Model
PROCESSESPhotosynthesis
Growth
Energy & mass exchanges
Understand & model processes at various scales
LAI EvaluationR
efe
rence
LA
I x1
0
Refe
rence
LA
I x1
0
VGT LAI x10 VGT LAI x10
Watson Lake, Yukon Kejimikujik, Nova Scoatia
/ 1.4
Outlok
Context
Background Background Smolander Approach
Objectives
Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF
A simple shoot parameterization
Impact of simple shoot parameterization on canopy BRDF
Conclusion
Coniferous Forest Modeling Background
Tree scale
Shoot scale
Structure organization at various scales constrains canopy radiative transfer.
Clumping around
branch
Clumping around
Twig
Spatial Distribution
Stand Scale
Empirical approaches : Implicit taking into account within field measurements
Reflectance & Radiative transfer models: Turbid media models all structure captured in single element
Geometrical Optical (GO): opaque crowns are described by geometric shapes (cone, spheroid, cylinder …)
Geometric-Optical & Radiative Transfer GORT: Radiative transfer within crowns are considered
Ray Tracing/ Radiosity: explicit placement of elements – how much complexity?
-
Shoots are typically assumed the basic element
Needle clumping parameterization
Background
Tree scale
Shoot structure variability : (specie, age, canopy depth)
Stand ScalePinus Bancksiana (Jack Pine) Picea-Mariana (Black Spruce) Pinus-sylvestris (Scot Pine)
How needle clumping affect shoot scattering ?
Shoot silhouette to total needle area ratio of the Shoot ‘STAR’: (Oker-Blom 1985) Projected Shoot area divided by total needle area Average over spherical shoot orientation : Variability [0.09 0.22]
Needle to shoot area ratio ‘ ’: (Chen and Cihlar 1995 )
Coefficient factor within GORT models and Optical LAI measurements (TRAC, hemispherical photographs,
LAI2000)
STAR
STAR41
Outlook
Context
Background
Smolander’s ApproachSmolander’s Approach Objectives
Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF
A simple shoot parameterization
Impact of simple shoot parameterization on canopy BRDF
Conclusion
Shoot scattering parameterization
Smolander’s Approach
Tree scale
Impact of STAR variable on Shoot scattering albedo sh: (Smolander & Stenberg 2003)
STARp
p
p
sh
Nsh
shNsh
41
)(.1
1).()(
N: Needle albedopsh: probabilty of more than 1 interaction within
the shoot
Validation over Scot pine using “constrained” ray tracing simulations Shoot scale - Shoot phase function shows a hot spot in illumination direction - Shoot phase function averaged over all directions is bi-lambertian Canopy scale: - Spherical oriented shoots may be as bi-lambertian flat leaves with G()=2STAR. - Good agreement in nadir view
5SCALE simulation on BOREAS Old Black Spruce
Is it sufficient to reproduce BRDF ?
STAR effect at canopy scale
Smolander’s Approach
Is such shoot parameterization sufficient to reproduce Canopy BRDF ?
Outlook
Context
Background
Smolander Approach
ObjectivesObjectives Impact of needle reflectance and transmittance on shoot
scattering and canopy BRDF
A simple shoot parameterization
Impact of simple shoot parameterization on canopy BRDF
Conclusion
Objectives
1- What is the impact of Needle reflectance and
transmittance on shoot scattering and canopy BRDF?
2- Can we design a simple shoot with same effective
optical properties as a detailed shoot ?
3- What is the impact of using simplified shoot on
canopy scale BRDF?
Outlook Context
Background
Smolander Approach
Objectives
Impact of needle reflectance and transmittance Impact of needle reflectance and transmittance
on shoot scattering and canopy BRDF on shoot scattering and canopy BRDF A simple shoot parameterization
Impact of simple shoot parameterization on canopy BRDF
Conclusion
Method: Detailed Shoot (S)Impact of needle reflectance and transmittance …
Twig length 7.7 cm
Twig width 0.3 cm
Needle length 2.85 cm
Needle width 0.092 cm
Needle Number 190
Total Needle area 199.28 cm2
Needle-twig angle 40.5
STAR 0.104
Shoot structure: Scot pine
Main Assumptions
• Needle as rectangular box defined by its width and length• Twig as decahedron with given width and length• Same needle number is fixed on each decahedron face • Needle-Twig angle is constant
Shoot scattering: Forward ray tracing simulation
Optical properties A RED B A NIR B
Twig reflectance 0.05 0.05 0.45 0.45
Needle reflectance 0.05 0.062 0.45 0.65
Needle transmittance 0.05 0.028 0.45 0.25
PARCINOPY(Chelle 1997)
Scot pine shoot
Optical properties
• Directional scattering • Reflectance coefficient• Transmittance coefficient • Albedo
Illumination direction s
Black soil
Shoot ScatteringImpact of needle reflectance and transmittance …
Directional Radiation scattering (N=N )
NIR
SH=0.74
RED
SH=0.056
Hot spot in principal plane around illumination direction
Shoot albedo is quite similar whatever the illumination direction while reflectanceand transmittance coefficients show some differences
N-N Needle effect (NIR)Impact of needle reflectance and transmittance …
Shoot albedo is quite insensitive Reflectance and transmission
coefficients are sensitive up +5% &
-13%
LAI=1.28
LAI=2.56
Canopy scale
Random shoot distribution with
spherical inclination Increase of backscattering and
decrease of forward scattering in the
principal plane No effect around nadir view direction
Shoot scale (illumination s=45)
N-N Needle effect (RED)Impact of needle reflectance and transmittance …
Canopy scale
LAI=1.28
Random shoot distribution with
spherical inclination Increase of backscattering and
decrease of forward scattering in the
principal plane No effect around nadir view direction
LAI=2.56
Shoot albedo is quite insensitive Reflectance and transmission
coefficients are sensitive up 7%
Shoot scale (illumination s=45)
SummaryIllumination direction
NIR(=) RED(=) Smolander RED NIR RED NIR
0 0.49
0.28
0.036
0.021
0.057 0.770.04
40.7945 0.3
80.33
0.026
0.03 0.056 0.71
75 0.42
0.33
0.03 0.024
0.054 0.75
Table4: Reflectance, transmittance and albedo computed for an ‘Accurate’ shoot with illumination directions around (0 45 75 ) considering N=N and Smolander’s albedo.
Shoot albedo is quite similar whatever the illumination direction while reflectanceand transmittance coefficients show some differences
Smolander parameterization agrees with ours in the NIR but not in RED
(differences in shoot structure design)
Outlook
Context
Background
Smolander Approach
Objectives
Impact of needle reflectance and transmittance on shoot
scattering and canopy BRDF
A simple shoot parameterizationA simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF
Conclusion
Method: Simple Shoot A simple shoot parameterization
Decahedron shoot (SD) Projected shoot silhouette area (As) conserved by reducing width (2.88cm) Convex ‘non self shadowing’ element with surface area A=4*As Decahedron sides are lambertian Optical properties of each side are equal to detailed shoot one averaged on
illumination directions (with N=N)
Flat leaf (SF) Lambertian Leaf area equals to 4*As. Optical properties equal to detailed shoot one averaged on illumination directions
Optical properties RED NIR
Reflectance 0.031 0.43
Transmittance 0.025 0.31
Illumination angles 0; 45; 90
SD Shoot ScatteringA simple shoot parameterization
Directional Radiation scattering
NIR
SD=0.74; SD=0.46;
SD=0.28
RED
SD=0.052; SD=0.032;
SD=0.020
Hot spot in principal plane around illumination direction
Albedo is quite close from detailed shoot albedo Some differences in reflectance and transmittance coefficients Backscattering enhancement occurs over a large view directions
Scattering comparisonA simple shoot parameterization
Directional scattering intercomparison (NIR)
• Detailed shoot ( Red line)• Decahedron shoot (Black line)• Lambertian leaf (Dashed line)
s=75, incidence plane
s=0, incidence plane
s=75, perpendicular plane
s=0, perpendicular plane
Both detailed and SD shoots show quite similar scattering profiles
Existence of less shadowing for decahedron shoot induces slow decrease far away from illumination direction less anisotropy in perpendicular plane due to the uniformity of shadow distribution
SummaryIllumination direction
NIR RED
NIR RED
0 0.51 0.25 0.0357
0.0187
0.76 0.054
45 0.447
0.275
0.0306
0.0202
0.72 0.0508
75 0.416
0.317
0.0295
0.023 0.73 0.0525
Average 0.458
0.28 0.032
0.0206
0.738
0.052
Table7: Reflectance, transmittance and albedo computed for‘Decahedron’ shoot with illumination directions around (0 45 75 ).
Outlook Context
Background
Smolander Approach
Objectives
Impact of needle reflectance and transmittance on shoot
scattering and canopy BRDF A simple shoot parameterization
Impact of simple shoot parameterization on Impact of simple shoot parameterization on
canopy BRDFcanopy BRDF Conclusion
Shoot Canopy BRDFImpact of simple shoot parameterization on BRDF
Canopy Characteristics
3 types: detailed shoot (S), Decahedron shoot(SD), Flat shoot (SF) Scene dimensions: 0.5mx0.5mx1m deep (infinite boundary) LAI=[0.64, 1.28, 2.56, 5.12, 10.24]; Random distribution Spherical inclination distribution (SD) canopy have the same positions and inclination than (S) canopy (SF) shoot is described by tree surfaces randomly distributed (3D structure )
Radiative transfert simulation
Surface opt. properties RED NIR
Detailed (S) 0.05 0.05 0.45 0.45
Decahedron (SD) 0.031 0.025 0.43 0.31
Flat (SF) 0.031 0.025 0.43 0.31
PARCINOPY(Chelle 1997)
Shoot canopy
Optical properties
• Directional scattering • Reflectance coefficient• Transmittance coefficient • Albedo
Illumination direction
(45)
Black soil
(SD) and (S) BRDF comparison (NIR)
Impact of simple shoot parameterization on BRDF
• Detailed shoot ( symbol)• Decahedron shoot (line)
scattering contributions (NIR)Impact of simple shoot parameterization on BRDF
Multiple ( Bleu)Single (Red)
• Detailed ( symbol)• Decahedron (line)
Multiple ( Bleu)Single (Red)
(SD) and (S) BRDF comparison (RED)
Impact of simple shoot parameterization on BRDF
• Detailed shoot ( symbol)• Decahedron shoot (line)
Scattering contribution (RED)Impact of simple shoot parameterization on BRDF
• Detailed ( symbol)• Decahedron (line)
Multiple ( Bleu)Single (Red)
(SD) and (S) comparisonImpact of simple shoot parameterization on BRDF
REDNIR
Multiple scattering regime is quite different Compensation between multiple and single scattering in (SD) canopy induces canopy
reflectance and transmittance similar to (S) canopy in NIR Discrepancies remains in RED due to the absence of this compensation
Upward and downward hemispherical canopy fluxes
(SF) and (S)BRDF comparison
Impact of simple shoot parameterization on BRDF
• Detailed shoot ( symbol)• Decahedron shoot (line)
NIR
RED
NIR
RED
Scattering contribution (RED)Impact of simple shoot parameterization on BRDF
Multiple ( Bleu)Single (Red)
• Detailed shoot ( symbol)• Decahedron shoot (line)
Outlook
Context
Background
Smolander Approach
Objectives
Impact of needle reflectance and transmittance on shoot
scattering and canopy BRDF A simple shoot parameterization
Impact of simple shoot parameterization on canopy BRDF
ConclusionConclusion
Conclusion 1- N-N effect
At shoot scale, no effect in albedo but some differencesin directional scattering and reflectance and transmittance coefficients
At canopy scale, N-N has significant effect around hot spot direction(Implication on LAI and clumping index estimation).
2- Can we design a simple shoot? Convex volume albedo is quite close with some differences in transmittance
and reflectance coefficients Convex volume describes directional scattering better than flat leaf
3- Impact of a simple shoot on canopy BRDF? Convex volume: - is inconvenient in RED due to a change in canopy clumping. - doesn’t well describe the canopy radiative regime. Simplification of shoot structure as lambertian flat leaf show significant
discrepancies (using ray tracing over a simplified structure is not realistic :Models assessment).
Lab measurements
Lab measurements
Lab measurements