ground-based energy flux measurements for calibration of the advanced thermal and land application...
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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
LE cp
VDa VDs Rs
.ρ = density of airCp = specific heat of airVDa = water vapor density of the airVDs = saturated water vapor density of the air at the vegetation canopy, temperature measured from ATLAS channel 4γ = psychrometric constant, and Rs = stomatal resistance
Reference Evapotranspiration
where ETo is the Latent heat flux or Reference EvapotranspirationΔ 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).
ETo
0.408 Rn G 900T 273
u2 es ea
1 0.34 u2
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
q = vapor fluxρa = density of airρw = density of waterVD0.2 = absolute vapor density at 0.2 mVD2 = absolute vapor density at 2 mRs = reference grass stomatal resistanceu2 = wind velocity at 2 m
q a cp
w
VD0.2 VD2 400
u2Rs
.
Results
One-second reading of RH
Instrument is at 200 cm Height
Instrument is at 30 cm Height
Relative Humidity Differences
RH for a single sensor at 30 cm and 200 cm from the groundFebruary 11, 2004
35
40
45
50
55
60
65
70
10:00 AM 11:12 AM 12:24 PM 1:36 PM 2:48 PM 4:00 PM 5:12 PM
Time
Rel
ativ
e H
um
dit
y (%
)
200 cm
30 cm
25
30
35
40
45
50
55
60
65
70
75
10:09 AM 11:21 AM 12:33 PM 1:45 PM 2:57 PM 4:09 PM 5:21 PM
Time (Hr)
RH
%
25
26
27
28
29
30
31
Tem
p (
C)
RHTEMP
Air Temperature Differences
Air Temperature for a single sensor at 30 cm and 200 cm from the groundFebruary 11, 2004
25
25.5
26
26.5
27
27.5
28
28.5
29
29.5
30
10:00 AM 11:12 AM 12:24 PM 1:36 PM 2:48 PM 4:00 PM 5:12 PM
Time
Rel
ativ
e H
um
dit
y (%
)
200 cm
30 cm
0
5
10
15
20
25
30
35
40
45
50
10:00 AM 11:12 AM 12:24 PM 1:36 PM 2:48 PM 4:00 PM 5:12 PM
Time
So
il H
eat
Flu
x (
W/m
2)
Soil Heat Flux on the Day of the Fly-OverFebruary 11, 2004
-50
50
150
250
350
450
550
650
750
10:00 AM 11:12 AM 12:24 PM 1:36 PM 2:48 PM 4:00 PM 5:12 PM
Time
Net
Rad
iati
on
(W
/m2)
Net Radiation on the Day of the Fly-OverFebruary 11, 2004
0
1
2
3
4
5
6
7
8
10:00 AM 11:12 AM 12:24 PM 1:36 PM 2:48 PM 4:00 PM 5:12 PM
Time
Win
d S
pee
d (
m/s
)
Wind Speed at 300 cm and 30 cm above the groundFebruary 11, 2004
300 cm
20 cm
20
21
22
23
24
25
26
27
28
29
30
10:00 AM 11:12 AM 12:24 PM 1:36 PM 2:48 PM 4:00 PM 5:12 PM
Time
So
il H
eat
Flu
x (
W/m
2)
Soil Temperature on the Day of the Fly-OverFebruary 11, 2004
Reference EvapotranspirationFebruary 11, 2004
0.000
0.200
0.400
0.600
0.800
1.000
10:00 11:12 12:24 13:36 14:48 16:00 17:12 18:24
Time(hr)
ET
o a
nd
q (
mm
/hr)
Reference Evapotranspiration
Time of ATLAS fly-over
Vapor FluxEquation
Penman-Monteith
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.
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