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.