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Evapotranspiration flux estimations using combined

satellite measurements

Bing Lin1, Qilong Min2, and Wenbo Sun3

1NASA Langley Research Center, USA2State University of New York at Albany, USA

3Hampton University, USA

The LandFlux Workshop Toulouse, France

May 28 ~ June 1, 2007

Title

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BackgroundoTurbulent fluxo Land surface emissivityoCombined satellite technique

MW-VIS-IR For Land Surfaceo EDVI, NDVI, & ET fluxo Vegetation development: growth season

Current Issues & Potential ApproachesoPhysical relation: EDVI leaf propertiesoSW, LW, storage, & turbulent fluxes

Summary

Outline

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Land surface fluxesVegetation Canopies & Atmosphere

oET & SH fluxes: turbulent, statisticaloUnderstanding water & energy cycle

•large biases in annual means oSite measurements (no large-scale

obs.) Indirect Satellite Approaches

oNDVI: PAR, limited by clouds & aerosols•high spatial resolution, daytime only

oMW emissivity VWC, cover, T, structure•day & night, multiple sensors•low spatial resolution

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Forests

canopy emissivity C, crown layer ()

emission from soil & trunk (s)

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Land surface emissivity C=1–(1–s)exp(-2)

(1exp(-))[1+(1–s)exp(-)]

EDVIp = 2(19p – 37

p)/(19p + 37

p)

NEDVI =(EDVI – EDVIonset)/ (EDVImax

– EDVIonset)

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Simplified scheme (VWC vs EDVI)

0 1.50.5 1.51 0 0.5 1

37GHz

19GHz

Em

issi

vity

V

0.91

0.93

0.95

0.97

VWC (kg/m2)

ED

VI V

0.01

0.02

0.03

0E19 = 0.945E19 = 0.960E19 = 0.975

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Retrieval techniquefull MW RTatmos. abs. correctionLWP & WV: VIS-IRiteration

surface site:SW, LW, PARvege. statusturbulent obs.

collocation

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Harvard Forest (1990)E

F0

0.2

0.4

0.6

0.8

1.0

00.

20.

40.

60.

81.

0

0 0.005 0.015 0.025 0 0.005 0.015 0.025EDVIV EDVIH

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NDVI vs EDVIE

F0

0.2

0.4

0.6

0.8

1.0

00.

20.

40.

60.

81.

0

0.005 0.01 0.015NDVI EDVIV

0. 5 0.6 0.7

R=0.52 R=0.86

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ET flux

ET

(W/m

2 )

0 5 10 15 20

010

020

030

040

0

EDVIV*SW

R=0.95

solid: clear skydaytime

indicatingsome saturation at high end

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Growth Season (1990-2000)

150 200 250 300 150 200 250 300

0.1.

00.

5

NE

DV

I &

leaf

am

oun

t

.005

.01

0.02

0.0

15E

DV

I

day of year (1999) day of year (2000)

bars: onset & end-of-season days of sfc obs.vertical lines: NEDVI decided days

solid: leaf amount

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Issues in this estimationEmpirical technique

oPhysical relation between EDVI & EF oEmissivity vs vegetation physical

properties•VWC, scattering albedo, phase function

oEDVI*SW vs ET fluxes Land surface energy balance

oSW, LW, Net, LH, SH, and storageoTs, Tv and Ta

Spatial/temporal resolution: NDVI/EDVI

No RT theoretical solutionsHeat storage, SH & other

parameters?

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Potential Approachessatellite data

MW, VIS, IR, etc

EDVI/NDVI

surface rad.

assimilationheat storage

weather status

partitioning LH & SHbalancing heat budgetevaluating vegetation

status

theoretical cal. vs VWC etc.

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Surface Net Radiation

SRB

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Current energy balance

To estimate SH, storage terms are also needed even with net radiation

LH

SH

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Calculation: FDTD3D Finite Difference Time Domain

oUniaxial perfectly matched layer(absorbing boundary conditions)

Scattering of elliptic disc (simulated leaf)o5, 2.5, 0.10 cm; half horizontal sizeoRandom orientation

Vegetation propertyoVegetation water content (VWC): 0% ~

70 %oDry vegetation density (0.33 g/cm3)oDielectric constant: dual-dispersion

modeloAt 19 and 37 SSM/I frequencies

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Calculation: geometry

E ●H

● ●

E

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Single scattering albedo

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

VWC (%)

sin

gle

sca

tter

ing

alb

edo

19G_FS 37G_FS 19G_HS 37G_HS

VWC: vegetation water content (%)

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Effective efficiencies (full size)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

VWC (%)

effe

ctiv

e ef

f

ext 19 sca 19 abs 19 ext 37 sca 37 abs 37

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phase function (37G)p

has

e fu

nct

ion

scattering angle (°)

half size

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phase function (diff size 37G)

ph

ase

fun

ctio

n

scattering angle (°)

vwc = 58%

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phase function (19G vs 37G)p

has

e fu

nct

ion

scattering angle (°)

vwc = 58%similar size parameter

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Combining MW, VIS and IR, EDVI & NDVI values can be estimated.

Sampling rates will be increased with a combination of EDVI & NDVI.

EDVI is empirically related to ET fluxes and canopy changes during growth seasons.

Assimilation data of surface parameters are needed for LH and SH partitions.

Scattering calculations potentially provide critical links between vegetation properties and microwave land surface emissivity.

Summary

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Acknowledgement

This study is supported by the NASA NEWS and Radiation programs.

We would like to thank Y. Hu and G. Gibson of Langley for their many helps.

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Potential ApproachesTheoretical calculation

o Leaf scattering properties: single scattering•VWC vs scattering albedo, phase function

oVegetation emissivity: multiple scattering vs VWC (size, shape, type & other para.)

oSatellite estimation: SW + LW fluxes •EDVI-like vs ET fluxes?

Assimilation dataoHeat storageoTs, Tv, Ta, and other parametersoPartitioning between LH & SH

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Approximate: VIE and MoM

Koh and Sarabandi (2005, IEEE Trans. AP)

3cm square

0.2mm thickness

10GHz

vertical incident

(90deg)

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Asymmetry factor

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

VWC (%)

asym

met

ry f

acto

r

19G_FS 37G_FS 19G_HS 37G_HS

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Effective efficiencies (half size)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

VWC (%)

effe

ctiv

e ef

f

ext_19 sca_19 abs_19 ext_37 sca_37 abs_37

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phase function (19G)p

has

e fu

nct

ion

scattering angle (°)

size: 5cm, 2.5cm and 0.1cm