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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
INTRODUCTION



The ability to estimate short-term fluxes of water
vapor from a growing crop are necessary for
validating estimates from high resolution remote
sensing techniques, such as NASA’s Advanced
Thermal and Land Applications Sensor (ATLAS).
On February 11th, 2004, the ATLAS was used to
evaluate the Urban Heat Island Effect within the
San Juan Metropolitan area.
To validate energy flux estimates from ATLAS, a
ground study was conducted at the University of
Puerto Rico Experiment Station in Rio Píedras
(located within the metropolitan area).
Objectives



To support modeling efforts related to the Urban
Heat Island problem.
To obtain ground-based measurements and/or
estimates of energy fluxes to validate the ATLAS
estimates.
The specific objective of this presentation is to
present estimates of reference evapotranspiration
during the ATLAS fly-over.
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
Reference Evapotranspiration
900 

0.408   Rn  G   
u2  es  ea

T  273 

ET o 
    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.
Vapor Flux Equation
  a cp   VD0.2  VD2
q

    w   400  Rs 
 u2

q = vapor flux
ρa = density of air
ρw = density of water
VD0.2 = absolute vapor density at 0.2 m
VD2 = absolute vapor density at 2 m
Rs = reference grass stomatal resistance
u2 = wind velocity at 2 m
.
Results
One-second reading of RH
Instrument is at 30 cm Height
Instrument is at 200 cm Height
Relative Humidity Differences
RH for a single sensor at 30 cm and 200 cm from the ground
February 11, 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
75
31
70
RH %
60
29
55
50
28
45
35
30
25
10:09 AM
27
RH
TEMP
40
26
25
11:21 AM
12:33 PM
1:45 PM
2:57 PM
Time (Hr)
4:09 PM
5:21 PM
Temp (C)
30
65
Air Temperature Differences
Air Temperature for a single sensor at 30 cm and 200 cm from the ground
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 Fly-Over
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
2:48 PM
4:00 PM
5:12 PM
Time
Soil Heat Flux on the 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 300 cm and 30 cm above the ground
February 11, 2004
8
7
Wind Speed (m/s)
6
5
300 cm
4
3
2
20 cm
1
0
10:00 AM
11:12 AM
12:24 PM
1:36 PM
2:48 PM
4:00 PM
5:12 PM
Time
Soil Temperature on the Day of the Fly-Over
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
Reference Evapotranspiration
Penman-Monteith
Reference Evapotranspiration
February 11, 2004
Vapor Flux
Equation
ETo and q (mm/hr)
1.000
0.800
0.600
0.400
0.200
0.000
10:00
11:12
12:24
Time of ATLAS fly-over
13:36
14:48
Time(hr)
16:00
17:12
18:24
Future Work Related to ATLAS


Latent and sensible heat fluxes will be
estimated by several methods for
comparison with the ATLAS estimates.
The ATLAS ground surface temperature
data are expected to be available in
September 2004.