NOAA-CREST Research Symposium
Hampton University
April 20-22, 2004
Ground-based energy flux measurements for calibration of the
Advanced Thermal and Land Applications Sensor (ATLAS) 1
Eric W. Harmsen and Richard Díaz Román2
ABSTRACT
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 Piedras (located
within the metropolitan area). Short-term
measurements (10-second) of micrometeorological parameters, including soil
heat flux, soil temperature, soil moisture and
net radiation were measured. Wind speed,
relative humidity and air temperature were
measured at two vertical positions above the
ground. Vertical differences in soil water
tension (negative pressure) were also
measured continuously. This paper presents
results from the ground-based study.
Large differences in relative
humidity were observed between 30 and 200
cm heights above the turf grass, whereas
temperature differences were negligible.
Estimates
of
evapotranspiration
are
presented based on the Penman-Monteith
and vapor flux methods.
1
This material is based on research supported by the NOAACREST and NASA-EPSCoR (NCC5-595 ).
2
Associate Professor and Undergraduate Research Assistant,
respectively. Dept. of Agricultural and Biosystems Engineering
University of Puerto Rico, Mayagüez, PR 00681
eric_harmsen@cca.uprm.edu, (787)834-2575
INTRODUCTION
Luvall et al. (1990) have used the ATLAS
instrument to estimate the latent heat flux
within a Costa Rican rainforest. Latent heat
flux was estimated using the following
equation:
(1)
   cp   VDa  VDs

Rs
  
.
where ρ = density of air (g/cm3), cp =
specific heat of air (J kg-1 oC-1), VDa = water
vapor density of the air (g/cm3), VDs =
saturated water vapor density of the air at
the
vegetation
canopy,
temperature
measured from ATLAS channel 4 (g/cm3), γ
= psychrometric constant (KPaoC-1), and Rs
= stomatal resistance (s/m).
LE  
The goals of this study were:
 To support modeling efforts related
to the Urban Heat Island problem,
and
 To
obtain
ground-based
measurements and/or estimates of
energy fluxes to validate the ATLAS
estimates.
METHODOLOGY
Field Measurements
Climatologically data were saved on
a Campbell Scientific (CS) CRX10 data
logger every 10-seconds. Two minute
measurements of air temperature and
relative humidity were measure at 30 cm
and 200 cm, respectively, using a single CS
1
HMP45C sensor.
Net radiation was
measured using a CS NR Lite Net
Radiometer. Wind speed was measured at
30 cm and 300 cm above the ground,
respectively. The upper sensor was a MET
One 034B wind speed and direction sensor.
The lower wind speed was measured using a
HOBO wind speed sensor. Soil water
content was measured using a CS616 Water
Content Reflectometer. Soil temperature
was measured using two TCAV Averaging
Soil Temperature probes, and the soil heat
flux at 8 cm below the surface was measured
using a HFT3 Soil Heat Flux Plate. Soil
heat flux at the soil surface was estimated
using the average soil temperature, soil heat
flux at 8 cm and water content data.
latent heat by use of the latent heat of
vaporization L (0.408 MJ/gm).
Latent Heat Flux Estimates from Humidity
Data
The latent heat flux can be estimated
using a modified form of equation 1:
  a cp   VD0.2  VD2

    w   400  Rs 
 u2

q
 37  u  e  e
 2  s a
 T  273 
0.408   Rn  G   
    1  0.34 u2
.
(2)
where ETo = reference evapotranspiration
(mm/hr) Δ
the vapor pressure
o -1
curve (KPa C ), Rn = net radiation (mm/hr),
G = soil heat flux density (mm/hr), γ =
psychrometric constant (KPaoC-1), T = mean
daily air temperature at 2 m height (oC), u2 =
wind speed at 2 m height (m/s), es is the
saturated vapor pressure and ea is the actual
vapor pressure (KPa). Equation 2 applies
specifically to a hypothetical reference crop
with an assumed crop height of 0.12 m, a
fixed surface resistance of 70 s/m and a solar
reflectivity of 0.23.
Assuming that the conditions at the
field site represent a grass reference surface,
then the reference evapotranspiration ETo
approximates the actual evapotranspiration
ET. Furthermore, we can convert the ET to
(3)
where 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 (70 s/m), and u2 = wind velocity
at 2 m.
Reference Evapotranspiration
The reference evapotranspiration
was estimated using the Penman-Monteith
equations (Allen et al., 1998) at 4-minute
intervals:
ETo 
.
RESULTS AND DISCUSSION
Figure 1 shows the relative humidity
at 1-second intervals. The figure clearly
shows a difference in the relative humidity
between the 30 cm and 200 cm height.
Figure 2 and 3 show the relative humidity
and temperature for the two heights from 10
AM to 6 PM on day of the ATLAS fly-over.
Figures 4 – 7 show the measured wind speed
(at 20 cm and 300 cm, respectively), net
radiation, soil temperature and soil heat flux
(at 8 cm depth). Figure 8 shows the
estimated latent heat flux (in mm/ day) from
equations 2 and 3. The Penman-Monteith
method (equ 2) was less variable than the
vapor flux method (equ. 3). The total
estimated vapor flux for the eight-hour study
period was 3.6 mm/day and 3.7 mm/day
from equations 2 and 3, respectively.
SUMMARY AND CONCLUSIONS
Ground-based measurements of air
temperature, relative humidity, wind speed,
wind direction, net radiation, soil
temperature, soil moisture content and soil
heat flux were measured on February 11,
2004 during a fly-over of the ATLAS
3
RH for a single sensor at 30 cm and 200 cm from the ground
February 11, 2004
70
Relative Humdity (%)
instrument. The measured data were used to
estimate the latent heat flux using an energy
balance-type method (Penman-Monteith)
and a vapor flux method. The overall depths
of water evaporated during the eight hour
study period were in excellent agreement.
These data will be compared to the ATLASbased estimates of vapor flux when they
become available.
65
60
55
50
200 cm
30 cm
45
40
35
10:00 AM
11:12 AM
12:24 PM
1:36 PM
2:48 PM
4:00 PM
5:12 PM
Time
Figure 2. Measured relative humidity from 10
AM to 6 PM on February 11, 2004.
REFERENCES
Allen, R. G., L. S. Pereira, Dirk Raes and M.
Smith, 1998. Crop Evapotranspiration
Guidelines for Computing Crop Water
Requirements. FAO Irrigation and Drainage
Paper 56, United Nations, Rome.
Air Temperature for a single sensor at 30 cm and 200 cm from the ground
February 11, 2004
30
29.5
Relative Humdity (%)
29
28.5
28
27.5
27
200 cm
30 cm
26.5
26
25.5
25
10:00 AM
1 Second Readings of RH (%)
11:12 AM
12:24 PM
1:36 PM
2:48 PM
4:00 PM
5:12 PM
Tim e
Figure 3. Measured air temperature from 10 AM
to 6 PM on February 11, 2004.
Wind Speed at 300 cm and 30 cm above the ground
February 11, 2004
8
7
6
Wind Speed (m/s)
Luvall, J., C., D. Lieberman, M. Lieberman
G. S. Hartshorn and R. Peralta. 1990.
Estimation of Tropical Forest Canopy
Temperatures, Thermal Response Numbers,
and Evapotranspiration Using an Aircraftbased Thermal Sensor. Photogrammetric
Engineering
and
Remote
Sensing,
(56)10:1393-1401.
5
4
3
2
1
0
Instrument is at 30 cm Height
10:00 AM
11:12 AM
12:24 PM
1:36 PM
2:48 PM
4:00 PM
5:12 PM
Time
Figure 4. Measured wind speed from 10 AM to
6 PM on February 11, 2004.
Net Radiation on the Day of the Fly-Over
February 11, 2004
Instrument is at 200 cm Height
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
Figure 1. One-second measurements of relative
humidity.
Figure 5. Measured net radiation from 10 AM to
6 PM on February 11, 2004.
4
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
2:48 PM
4:00 PM
5:12 PM
Time
Figure 6. Measured wind speed from 10 AM to
6 PM on February 11, 2004.
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
2:48 PM
4:00 PM
5:12 PM
Time
Figure 7. Measured soil temperature from 10
AM to 6 PM on February 11, 2004.
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
16:00
17:12
18:24
Time(hr)
Figure 8. Calculated latent heat flux using equ 2
and equ 3 from 10 AM to 6 PM on February 11,
2004.
5