A.Olioso, S. Jacquemoud* & F. Baret

UMR Climat, Sol et EnvironnementINRA Avignon, France

* Institut de Physique du Globe de Paris (IPGP)Département de Géophysique Spatiale et Planétaire

Université Paris 7 - Denis Diderot

Adaptation of the leaf optical property model PROSPECT to

thermal infrared

Radiative properties of leaves in the thermal infrared are required for implementing radiative transfer models

ex: => remote sensing studies => fire propagation studies

Model of leaf properties are required for

=> analysing variations of leaf properties (ex. with leaf moisture) => linking leaf properties to plants processes

There is no such model !

=> building a model on the basis of the PROSPECT model (Jacquemoud and Baret 1990) which is working in the solar domain

transmitted + emitted

absorb

ed

Leaf optical properties

reflected + emitted

depend on anatomical leaf structure andbiochemical leaf composition

i

iiCkK

Description of the PROSPECT model

Nidenticallayers

Is

Elementary layer:n: refraction indexK: global absorption coefficient

Surface effects

Hemispheric fluxes

Global absorption:

Specificabsorptioncoefficients

Content inabsorbingmaterial

reflectance

()

() transmittance

Refractive index: n()

n1

n2

1 1

2

1 1 2 2sin sinn n

SCATTERING

Snell’s law

Specific absorption coefficient of constituent i: ki()

d

i ik k C

exp k d

ABSORPTION

Beer law

NCab

Cbp

Cw

Cdm

PROSPECT

()()

leaf structure parameterchlorophyll a+b concentration (g.cm2)brown pigment concentration (g.cm2) equivalent water thickness (cm)dry matter content (g.cm2) N = 1.5, Cab = 50 g.cm2, Cdm = 0.005 g.cm2

PROSPECT

PROSPECT INPUTS

N - Number of layers

Cab - Chlorophyll a+b content

Cbp - Brown pigment content

Cw - Equivalent water thickness

Cdm - Dry matter content

n(λ) - Refractive index

ki(λ) - Specific absorption coefficients of constituants

() – leaf reflectance() – leaf transmittance

PARAMETERS

between 0.4 and 2.5 µm

PROSPECT OUTPUTS

PROSPECT INPUTS

N - Number of layers

Cab - Chlorophyll a+b content

Cbp - Brown pigment content

Cw - Equivalent water thickness

Cdm - Dry matter content

n(λ) - Refractive index

ki(λ) - Specific absorption coefficients of constituants

() – leaf reflectance() – leaf transmittance

PARAMETERS

between 0.4 and 2.5 µm

PROSPECT OUTPUTS

ε () – leaf emissivity

kw(λ) kdm(λ)

between 2.5 and 18 µm

refractive index n(λ) ?

PROSPECT INPUTS

specific absorption coefficient of water kw(λ)

0.4-2.5 µm

PROSPECT INPUTS

* specific absorption coefficient of dry matter: kdm(λ)

-> no info available at the moment

-> to be obtained by inverting PROSPECT against leaf spectrum data (in particular from dry leaf)

* idem for leaf layer refractive index n(λ) (inversion from fresh leaf spectra)

* N, Cw, Cdm may be obtained from library, measurements or from PROSPECT inversion between 0.4 and 1.8 µm

PROSPECT INPUTS

DETERMINATION OF PROSPECT INPUTS:

the only easily available data that made it possible to determine PROSPECT inputs were found in the ASTER spectral library

Solar domain Thermal infraredN, Cw, Cdmkdm(λ), n(λ)

Specific absorption coefficient of dry matter: kdm(λ)

inversion of PROSPECT against ‘ASTER’ dry spectra

result of inversion compared to cellulose and lignin spectra

0.4-2.5 µm

some cellulose and lignin features

but not always specific

Lignin

Specific absorption coefficient of dry matter: kdm(λ)

comparison to water

Difficult zone becauseof high absorption of bothdry matter and H2O

Low absorption zone

Opposite behavior of H2O and dry matter

Determination of the refractive index : n(λ)

inversion of wet spectra gave refrative index

Lowest absorptionzone

COMPARISON OF PROSPECT OUTPUTS / MEASUREMENTS

Data from

-ASTER spectral library -Salisbury and D’Aria 1992

-MODIS spectral library

Comparison of simulated reflectance to data from Salisbury and D’Aria 1992

senescent beech leaf

Comparison of simulated reflectance to data from the MODIS spectra library

3 dry grass spectra

Comparison of simulated reflectance to data from the MODIS spectra library

various fresh leaves

Comparaison de simulations à des mesures

Sensitivity to leaf water content

sensitivity to Cw from 0.0002 cm-1 to 0.0512 cm-1

(0.0002, 0.0008, 0.0032, 0.0128, 0.0512 cm-1)

0.0002

0.0512High transmittance

Sensitivity to leaf water content

sensitivity to Cw from 0.0002 cm-1 to 0.0512 cm-1

(0.0002, 0.0008, 0.0032, 0.0128, 0.0512 cm-1)

Sensitivity to leaf water content

sensitivity to Cw from 0.0002 cm-1 to 0.0512 cm-1

(0.0002, 0.0008, 0.0032, 0.0128, 0.0512 cm-1)

0.0512

0.0002

Emissivity lower than expected fromreflectance

Sensitivity of 8-14 µm emissivity to leaf moisture

fresh leaves and dry leaves don’t have the same internal structure (parameter N = 2 and 4)

different responses average behaviour in situ ?

Sensitivity to leaf surface properties

various components (silica, waxes…) and / or structure (hair, epidermis cell shape…) may affect leaf surface – radiation interactions

introduction of new components use the radiation incident angle of the plate model (set to 59° usualy)

10°

90°

sensitivity to incident angle from 10 to 90° by step of 10°

Conclusion

Encouraging first results

There is a lot of work still to do

acquisition of leaf data for calibrating and testing the model

analysis of the effects of the various components in order to discriminate generic effects and specific effects

investigation of leaf surface effects

investigation of leaf drying impact…

….

implementation in canopy radiative transfer model for the analysis of land surface emissivity spectra acquired from TIR multispectral sensors

The end

S. Knap & N. Knight, 2001, Flora, Harry N Abrams, 80 pages.