Ground-based energy flux
measurements for
calibration of the Advanced
Thermal and Land
Application Sensor (ATLAS)
Eric Harmsen, Associate Professor
Dept. of Agricultural and Biosystems
Engineering
Richard Diaz, Undergraduate
Research Assistant
Department of Civil Engineering
Water Use


Agriculture is the greatest consumer of
water in society.
It is estimated that 69% of all water
withdrawn on a global basis is used for
agriculture.
Water Losses


Large losses of
irrigation water are
common.
Irrigation efficiencies
on the order of 50%
are typical.


The ability to estimate short-term latent
heat fluxes (i.e., crop water use) from
remotely sensed data is an essential tool
for managing the worlds future water
supply.
However, validation of these sensors is
necessary.
Objective


The objective of the study was to obtain
ground-based estimates of the latent heat
flux for calibration of latent heat flux
estimates from NASA’s ATLAS sensor.
The specific objective of this presentation is
to present ground-based estimates of
evapotranspiration obtained during the ATLAS
fly-over in San Juan, PR.
Definition
Crop Water Use =
Evapotranspiration =
Latent Heat Flux
The ATLAS Mission


On February 11th, 2004, the ATLAS was used
to evaluate the Urban Heat Island Effect
within the San Juan Metropolitan area.
A ground-based study was conducted at the
University of Puerto Rico Agricultural
Experiment Station in Rio Píedras.
Estimating Latent heat flux
from ATLAS
   cp   VDa  VDs
LE  

rs
  
.
ρ = density of air
Cp = specific heat of air
VDa = water vapor density of the air
VDs = saturated water vapor density of the air at the vegetation
canopy, temperature measured from ATLAS channel 4
γ = psychrometric constant, and
rs = stomatal resistance
ATLAS Spectrum
Ground-based methods for
estimating the latent heat flux

Eddy-Covariance System



Accurate
Expensive ($20,000)
Vapor flux and Energy Balance methods*



Easy to use
Require estimates of resistance factors
Less expensive (less than $5,000)
* Methods used in this study
Eddy-Covariance System
Vapor Flux
and Energy
Balance
Methods
Vapor flux method
Vapor Flux Equation
  a cp   VD0.3  VD2
q

ra  rs
  w 
q = vapor flux
ρa = density of air
ρw = density of water
VD0.2 = absolute vapor density at 0.3 m
VD2 = absolute vapor density at 2 m
rs = bulk surface resistance
ra = aerodynamic resistance = 400/u2
u2 = wind velocity at 2 m
Simplified representation of the (bulk) surface
resistance and aerodynamic resistances for
water vapor flow (from Allen et al., 1989).
Energy Balance Method
ET o 
0.408   Rn  G   
900 
u2  e s  e a 

 T  273 
    1  0.34 u2
where
ETo is the Latent heat flux or Reference Evapotranspiration
Δ is the slope of the vapor pressure curve (kPa oC-1),
Rn is net radiation (MJ m-2 d-1), G is the soil heat flux density
(MJ m-2 d-1),
g is the psychrometric constant (kPa-1),
T is mean daily air temperature at 2 m height (oC),
u2 is wind speed at 2-m height,
es is the saturated vapor pressure (kPa-1) and ea is the actual
vapor pressure (kPa-1).
Penman-Monteith Equation
The equation applies specifically to a
hypothetical reference crop with an
assumed crop height of 0.12 m, a fixed
surface resistance of 70 sec m-1 and an
albedo of 0.23.
Results
One-second reading of RH
Instrument is at 30 cm Height
Instrument is at 200 cm Height
RH for a single sensor at 30 cm and
200 cm from ground
RH for a single sensor at 30 cm and 200 cm from the ground
February
11,11,
2004
February
2004
Relative Humdity (%)
70
65
60
55
50
200 cm
30 cm
45
40
35
10:00 AM
11:12 AM
12:24 PM
1:36 PM
Time
2:48 PM
4:00 PM
5:12 PM
Air Temperature Differences for a
single sensor at 30 cm and 200 cm
from the ground
and 200 cm from the ground
cm2004
at 30
Air Temperature for a single sensor
February
11,
February 11, 2004
30
Relative Humdity (%)
29.5
29
28.5
28
27.5
27
200 cm
30 cm
26.5
26
25.5
25
10:00 AM
11:12 AM
12:24 PM
1:36 PM
Tim e
2:48 PM
4:00 PM
5:12 PM
Net Radiation on the Day of the
ATLAS Fly-over
February
11, Day
2004of the Fly-Over
Net Radiation
on the
February 11, 2004
Net Radiation (W/m2)
750
650
550
450
350
250
150
50
-50
10:00 AM
11:12 AM
12:24 PM
1:36 PM
Time
2:48 PM
4:00 PM
5:12 PM
Soil Heat Flux on the Day of the
ATLAS Fly-over
February
Soil Heat
Flux on11,
the2004
Day of the Fly-Over
February 11, 2004
50
Soil Heat Flux (W/m2)
45
40
35
30
25
20
15
10
5
0
10:00 AM
11:12 AM
12:24 PM
1:36 PM
Time
2:48 PM
4:00 PM
5:12 PM
Wind speed at 30 cm and 200 cm
above the ground
February 11, 2004
Wind Speed at 300 cm and 30 cm above the ground
February 11, 2004
8
Wind Speed (m/s)
7
6
300 cm
5
4
3
20 cm
2
1
0
10:00 AM
11:12 AM
12:24 PM
1:36 PM
Time
2:48 PM
4:00 PM
5:12 PM
Net Radiation on the Day of the
ATLAS Fly-over
Soil Temperature
on11,
the 2004
Day of the Fly-Over
February
February 11, 2004
30
Soil Heat Flux (W/m2)
29
28
27
26
25
24
23
22
21
20
10:00 AM
11:12 AM
12:24 PM
1:36 PM
Time
2:48 PM
4:00 PM
5:12 PM
Evapotranspiration
Reference Evapotranspiration
ETo and q (mm/hr)
Penman-Monteith
February 11, 2004
Vapor Flux
Equation
1.000
0.800
0.600
0.400
0.200
0.000
10:00
11:12
12:24
13:36
14:48
Time(hr)
Time of ATLAS fly-over
16:00
17:12
18:24
Future Work



The ATLAS ground surface temperature data
are expected to be available in September
2004.
Latent and sensible heat flux estimates by
several methods will be compared with the
ATLAS estimates.
An automated devices is currently being
developed for obtaining the temperature and
humidity at the two heights.
Future Work – cont.


A study is planned to verify the vapor
flux method by comparing it with the
eddy covariance system.
The vapor flux instrument will be used
to verify flux estimates under tropical
conditions from other sensors, such as
MOTIS.