Surface Energy Balance and
Evapotranspiration remote sensing
Lecture 2
Outline
• 1. Energy balance
• 2. ET and in situ measurements
• 3. Remote sensing ET using energy
balance method
• 4. Remote sensing ET using other
methods
radiation
Longwave
(1 - ε0)Lin
1.
Energy
Balance
Longwave Shortwave Latent Heat
( E ) Sensible Heat
(Lout)
(1- α)Sin
Longwave
(H)
(Lin)
Energy Balance
Net radiation (Rn )
Ground Heat
(G)
Rn  (1   )  Sin  ( Lin  Lout )  (1   0 )  Lin
Rn

0
(Radiance Balance)
. Net radiation absorbed by the land
. Albedo
. Emissivity
Units : W/m2
Rn  G  E  H (Heat Balance)
Latent Heat Flux
GE .. Heat
transfer to the ground (soil)
Rn-G called available
energy (or Qa)
H
. Sensible heat flux loss to the air due to convection and
conduction
Surface energy balance and
micrometeorology
E
E
Hornberger et al., 1998
2. Evapotranspiration
• Evaporation: Liquid water is converted to water vapor and removed
from evaporating surface (lake, river, pavement, soils, and wet
vegetation)
• Transpiration: Vaporization of liquid water contained in plant tissue and
the vapor removal to the atmosphere through the stomata
• Evaporation and transpiration occur simultaneously and there is no
easy way of distinguishing between the two processes. They are often
measurement together.
• Potential ET (PET): The evapotranspiration of a surface with optimal
water condition (both on the surface and in the root zone) for E and T. It
depends on micrometeorological condition, as well as the surface
characteristics. It is the largest water availability (or loss) from a
vegetated land surface under given climate and weather condition but
not limited by soil moisture.
• Reference ET is the PET of the reference (clipped) grass (h= 0.12m) or
the (full-covered) alfafa (h>0.3 m).
Quantifying Evapotranspiration
Coupling of energy balance and water balance equations
Water balance method
Energy balance method
(e.g., bowen ratio - energy balance)
Eddy covariance method
Scintillometry method
Aerodynamic method
Combination approach (energy balance + aerodynamic)
Penman-Monteith equation
Coupling energy balance and water balance
Water balance: Evaporation pan
Evaporation Pan -- In hydrologic terms, a pan used to hold water during
observations for the determination of the quantity of evaporation at a given location.
Such pans are of varying sizes and shapes, the most commonly used being circular
or square. 121cm diameter x 24cm deep
Write water balance equation for
the pan.
What variables do we measure to
determine E
What do you think of the pan
evaporation measurement?
Can it be a good estimate for
lake E?
Can you think of a better way to
install evaporation pan?
http://www.ametsoc.org/amsedu/WES/WESSlides/WESThumbnailHyperlinks/ch10/s041evaporation_pan.htm
Data: http://www.wrcc.dri.edu/htmlfiles/westevap.final.html
Water balance: Lysimeter
Left and center, construction of the two large weighing
lysimeters at KREC began in 1986. Each lysimeter consists of
an underground chamber that houses a balancebeam
weighing system with a rectangular “flower pot” measuring 6.5
feet wide by 13 feet long by 6.5 feet deep. Right, researchers
enter the completed lysimeter.
http://calag.ucop.edu/0502AMJ/pdfs/Lysimeters.pdf
• The Kearney lysimeters have
been in constant use since 1987.
One lysimeter measures water
use in peaches (top), the other
‘Thompson Seedless’ grapes (
bottom).
• The lysimeters have provided
more accurate evapotranspiration
values for peach trees and grape
vines, information necessary in
irrigation management decisions
that ultimately affect plant health
and fruit quality.
Energy Balance: Bowen Ratio (EBBR)
Rn  G  E  H
Rn  G  E  E 
Rn  G  E (1   )
Rn  G
E 
1 
H

E
EBBR
EF (evaporate _ factor) 
E
Qa

E
Rn  G
5 over semiarid area, 0.5 over grassland, 0.2
over irrigated orchards, and 0.1 over sea

E
E ((1   )

1
1 
Eddy covariance method
H  C p w' T '
E   w' C '
http://horton.engr.uconn.edu/gamboa/images/IMG_0796-1.jpg
ET from EBBR is usually 10% larger than that from Eddy covariance method
Scintillometry method
Path length L, Aperture D, Wavelength 
• Measurement of spatially average fluxes of sensible heat and
water vapor (evaporation) on km scale
• Measurements of spatially avergae rainfall rate
Aerodynamic approach
1
H   a c p (T0  Ta )
ra
1 ac p
E 
(e0  ea )
ra 
T0  Ta
H


E
e0  ea
ρa is air density, cp is the heat capacity of air, ra is the aerodynamic
resistance, T0 is the aerodynamic surface temperature, Ta is the nearsurface air temperature, e0 is the vapor pressure (Pa) at the aerodynamic
surface, ea is the vapor pressure at near surface, γ is the psychrometric
constant (≈ 66 Pa/K)
Combination approach: PM equation
Penman equation
Penman-Monteith
equation
( Rn  G ) 
PE 
( Rn  G ) 
PT 
ac p
ra
 
ac p
ra
   (1 
(es (Tz )  ez )
(es (Tz )  ez )
rs
)
ra
γ is the psychrometric constant (≈ 66 Pa/K)
Field ET tower
measurements
Eddy Covariance ET Tower (2-3m)
And Bowen Ratio ET tower both
in the Sevilleta National WildLife
Refuge
Kurc and Small 2004, Small and Kurc 2001
Components of the SEB at the grassland site using Bowen Ratio ET system.
Left side shows a wet day (JD 233) and right side shows a dry day (JD 254).
Hours from midnight are plotted on the x-axis, with time shifted one hour
forward from the solar cycle due to daylight savings time.
Small and Kurc 2003
Small and Kurc 2001
• ET in this semi-arid region is only related to top 5 cm soil
moisture and to root zone soil moisture as other climate
regions
Kurc and Small, 2004
Large Aperture Scintillometry (LAS)
network NMT
Gomez, D. thesis, 2008
Dr. Jan Hendrickx
3. Remote sensing ET using
energy balance method
E  Rn  G  H
Rn  (1   )  Sin  ( Lin  Lout )  (1   0 )  Lin
recap emissivity
M B ,
2hc 2 / 5

exp[( hc) /( kT )]  1
2hc 2 / 5
M 
exp[( hc) /( kTB )]  1
where h=Planck constant, 6.6261E-34 J.sec
k= Baltzmann constant, 1.38E-23 J/K,
c= speed of light, 3E8 m/sec
M
 
M B ,
M B  T
4
M  T
4
SEBAL-METRIC model
Developed by
Wim Bastiaanssen, The Netherlands
Richard Allen, University of Idaho
Based on Landsat imagery
User Manual Can be downloaded from here:
\\129.115.25.240\XIE_misc\EES5093
for learning purpose not for any other purpose.
Estimating
Net radiation
Estimating
Short-wave Albedo
Satellite Sensor
Sun
Reflectance
at air
Top of Atmosphere
Solar Radiation
Reflectance at Land Surface
Air
Land Surface
Estimating
Short-wave Albedo
Values for α of path_radiance range between 0.025 and 0.04 and for SEBAL. It is
recommended a value of 0.03 based on Bastiaanssen (2000).
Can we test it?
Estimating
Incoming short-wave
radiation
Can we test the transmissivity function for San Antonio area?
Estimating
surface emitted
long-wave radiation
Why are two emmissivities used?
Explain Rc function
Estimating
Incoming long-wave
radiation
Estimating
ground heat flux
How can we test the empirical
function for G?
Why do deep water and snow
have larger G/Rn?
Estimating
sensible heat
1
zd
rH 
ln(
)
ku *
zH
1
zd
rH 
ln(
)
ku *
zH
ET (mm/d) estimation from Energy Balance Method
June 16 2002
ET(mm
/d)
ET(mm /d)
0.0
0.0
< 1.0
< 2.0
< 1.0
< 3.0
< 4.0
< 2.0
< 5.0
path/row: 33/36
< 3.0
< 6.0
< 7.0
< 4.0
< 8.0
< 5.0
< 6.0
< 7.0
< 8.0
4. Remote sensing ET using other
methods
• Priestley-Tayloy’s equation
• Granger's complementary relationship and
Priestley–Taylor's equation
• …
One paper for you to read if you are interested in