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The new SMOS-IC L-band vegetation index (L-VOD):
Overview and application to monitoring vegetation biomass at global scale
CCI Biomass Workshop, Paris, Spt. 25-26, 2018
J-P Wigneron, L. Fan, et al.With Bordeaux group: A. Al-Yaari, J. Swenson, F. Frappart, X. Li.
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VOD (definition)
SMOS-IC L-VOD product (new , simplified, well-suited to applications)
Applications of L-VOD to vegetation monitoring
CCI Biomass Workshop, Paris, Spt. 25-26, 2018
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SMOS (Soil Moisture and Ocean Salinity), PI Y Kerr
Spatial resolution: ~ 35-50kmRevisit time: Max. 3 daysSensitivity ~ 2K over land Goal of accuracy in SM: ~ 0.04 m3/m3
Retrieval algorithm: using multiangular and dual polarization TB
Soil moisture & vegetation opacity (VOD)
based on the inversion of L-MEB, (L-band Microwave Emission of the Biosphere)Wigneron et al., 2000-2018 (algorithm in ESA proposal, L-MEB, SMOS-IC product)
Launch : Dec. 2009: ~ a 9-year data set
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Extinction
ATMOSPHERE
SOIL
VEGETATION
eS TS TB* S TB*
Tb*(z=H)VegetationEmission(1- )(1-ω)
Tb*(z=0)Soil reflectivityS
Soilemission
Extinction
VEGETATION
Sol
v
RADAR
s / 2
PASSIVE
Extinction
‐soil moisture (SM), determines smooth soil reflectivity smooth‐biomass (VOD), determines vegetation extinction γ = exp(‐VOD/cos())
‐temperature (TB = emissivity x temperature)
‐canopy type (ω), soil roughness (rough= C . smooth), and texture
The Brightness temperature (TB) observations are sensitive to:
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L-MEB (L-band Microwave Emission of the Biosphere model)
VOD (nadir) = b . VWC Jackson and Schmugge, 1991
with,
VWC = vegetation water content (kg/m2)
b~0.12 (0.1-0.2 for crops)
For vegetation, L-MEB is based on a zero order solution of radiative transfer equations (- model):
- VOD = KE. H, accounts for extinction effectsKExtinction = KAbsorption + KScatteringKAbsorption = KEmission
- accounts for scattering effects (KScattering / KExtinction)
TBveg=(1-e-VOD/cos())(1-)Tveg(1+soile-VOD/cos())radiometer
SOIL
VEG
ATMSKY
H=Height of Crop
Theory
Experimental
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In situ:• EMIRAD (TUD) at the INRA Avignon test site (soybean, corn)• Smosrex, 2003-2010, Toulouse, with Lewis (CESBIO, CNRM, INRA, ONERA), soil, fallow• Landes forest, 2004-2007 (INRA), with EMIRAD-1 (TUD), coniferous forest
• Elbara, 2004-2006 (ETH, U. of Bern), grass, deciduous forest• Elbara -2, 2010 (ESA funded) at the Munich, VAS, Sodankyla sites, grass, mattoral, forest
and airborne• Carols, 2007-2010 (Cnes, ESA), Smosmania (France) and Vas sites (Spain), …
L-MEB algorithm development /evaluation
LEWIS at Smosrex site
MELBEX- EMIRADINRA’01 - EMIRAD
EMIRAD, Landes forest
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Examples of results : SOYBEAN (INRA-1991) [Wigneron et al., RSE, 1995-2007]
Retrieved soil moisture
Retrieved VOD, VWC and LAI = f(time)
Retrieved VOD
R2=0.96 R2=0.88
VWC LAItime
VOD VOD VOD
VWC
LAI
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-multi-angular observations ➡ simultaneous retrievals of SM and VOD (Wigneron et al., 1995, 2000)
-in SMOS-IC SM retrievals, no need of optical indices to estimate VOD (and vice versa)
SMAP: In the SCA algorithm, VOD is estimated from NDVIAMSR-E: iterative approach based on only 2 observations (LPRM algorithm)
-L-band (1.4GHz, ~ 30 cm): higher sensing capabilitiesthrough dense vegetation than C- (6 GHz, ~ 5 cm) and X- bands
-passive observations are much less sensitive to structural effects of vegetation (row, vertical structure), soil (roughness, surface geometry), topography, etc. than radar observations
the b and parameter are relatively constant for varying vegetation conditions
= 0.07 for forests = 0.1 for other vegetation types
(no need for parameter tuning)
Key features of SMOS to retrieve VOD:
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Dynamics of carbon stocks in Africa over 2010-2016
X-VOD
SMOS-IC L-VOD
Biomass(Baccini map2007-2008)
-spatial calibration L-VOD / Biomass in 2011
-’space’ for time substitution: L-VOD is used to monitor time changes in carbon stocks in Africa
Biomass(Baccini map2007-2008)
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F. Tian,, et al. "Coupling of ecosystem-scale plant water storage and leaf phenology observed by satellite", Nature EE
Pre-rain Miombo forest© C. Ryan, University of Edinburgh
A high temporal decoupling between plant water storage and LAI in dry Tropical forests(especially in Miombo)
L-VOD
LAI
Time variation in L-VOD and LAI (Miombo)© F. Tian, University of Copenhagen
Time lag between L-VOD and LAI© F. Tian, University of Copenhagen
Miombo
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THANK YOU!
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Green vegetation:gv =f°(LAI) =f°(VWC) (gv =b•VWC, b~0.1)
Retrieving of the # components of optical depth:[Saleh et al., RSE, 2006]
retrieving VOD_GV (standing vegetation):
for dry conditions : dry litter and no interception
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Nadir path
Satellite
Spacecraftvelocity
d N
Swath1000 km
30°
= 55°
Local incidenceangle
Earth
m
SMOS : le Système d’Observation:
Champ de vue (FOV)
visées multi-angulaires: un point au sol est vu sous différents angles de visée au fur et à mesure que le satellite avance:
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18SMOS Brightness Temperature (L1C product), Browse product at 42.5°
SMOS : un Système d’Observation Multiangulaire
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Le principe:
Mesure de la réflectivité du sol = f(constante diélectrique ε)
sCte diélectrique
εHumidité du sol SM (m3/m3)
Texturestructure du sol
s_lisseMesure radarou passifCorrection
rugosité
SM
fort contraste sol sec (ε~5) et sol humide (ε~30)
Soil dielectric constant = f(SM) à 1.4GHZ
Ulaby et al. (1986)
Soil dielectric
constant ε
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Mesures dans les Micro‐ondes• Le principe: mesure de la reflectivité du sol =
f(constante dielectrique ε)Il existe un fort contraste entre la constante dielectrique d’un sol sec (ε~5) et celle d’un sol humide (ε~30)
• Mesures dites ‘Actives’ (Radar) ou ‘Passives’• actif →reflectivité• passif →émissivité (= 1-Réflectivité)
• Mesures dans le domaine ‘basse fréquence’ ~ 1.4 Ghz (bande L) et 10 Ghz (bande X):-faible sensibilité aux effets atmosphériques:(mesure tout temps & corrections précises)-faible atténuation du signal sol par la végétation
Soil dielectric constant = f(SM) à 1.4GHZ
Ulaby et al. (1986)
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Nadir path
Satellite
Spacecraftvelocity
d N
Swath1000 km
30°
= 55°
Local incidenceangle
Earth
m
SMOS : Multi‐angular observation system
Field of view (FOV)
multi-angular observations:
-a given site on Earth is seen at different incidence angles as the satellite moves ahead.
-larger angle ranges for sites close to the sub-satellite track
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soil
vegetation
atmosphere
vegetationemission
Reflected (Гs)vegetationemission
soil emission : (1-Гs) Ts
TB* Гs TB* (1-Гs) Ts
TB
TB*(z=0)
TB*(z=-d)
diffusion absorption
()
diffusion absorption
()
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Vegetation attenuation increases as frequency increases
Saturation of Biomass = f(VOD) comes quicker at high frequencies
X-band (~10 GHz, ~3 cm), AMSR-EC-band (~5 GHz, ~6 cm), AMSR-E, ASCAT… ~100 t/haL-band (1.4 GHz, ~30 cm), ESA/SMOS, NASA/SMAP …, ~250 t/haP-band (~0.4 GHz, ~75 cm), ESA/Biomass…, ~ no saturation ?
Quicker saturation for active vs passive systems
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Time variations in L-VOD, SM, EVI,
rainfall
(Most) Dense tropical forests in the Amazon basin (Guyana) = an extreme case
-SM can be clearly related to rainfall
Tian et al., 2017