Page 1
1
LaRC
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
Page 2
2
LaRC
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
Page 3
3
LaRC
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
Page 4
4
LaRC
Forests
canopy emissivity C, crown layer ()
emission from soil & trunk (s)
Page 5
5
LaRC
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)
Page 6
6
LaRC
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
Page 7
7
LaRC
Retrieval techniquefull MW RTatmos. abs. correctionLWP & WV: VIS-IRiteration
surface site:SW, LW, PARvege. statusturbulent obs.
collocation
Page 8
8
LaRC
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
Page 9
9
LaRC
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
Page 10
10
LaRC
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
Page 11
11
LaRC
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
Page 12
12
LaRC
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?
Page 13
13
LaRC
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.
Page 14
14
LaRC
Surface Net Radiation
SRB
Page 15
15
LaRC
Current energy balance
To estimate SH, storage terms are also needed even with net radiation
LH
SH
Page 16
16
LaRC
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
Page 17
17
LaRC
Calculation: geometry
E ●H
● ●
E
Page 18
18
LaRC
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 (%)
Page 19
19
LaRC
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
Page 20
20
LaRC
phase function (37G)p
has
e fu
nct
ion
scattering angle (°)
half size
Page 21
21
LaRC
phase function (diff size 37G)
ph
ase
fun
ctio
n
scattering angle (°)
vwc = 58%
Page 22
22
LaRC
phase function (19G vs 37G)p
has
e fu
nct
ion
scattering angle (°)
vwc = 58%similar size parameter
Page 23
23
LaRC
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
Page 24
24
LaRC
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.
Page 25
25
LaRC
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
Page 26
26
LaRC
Approximate: VIE and MoM
Koh and Sarabandi (2005, IEEE Trans. AP)
3cm square
0.2mm thickness
10GHz
vertical incident
(90deg)
Page 27
27
LaRC
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
Page 28
28
LaRC
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
Page 29
29
LaRC
phase function (19G)p
has
e fu
nct
ion
scattering angle (°)
size: 5cm, 2.5cm and 0.1cm