Development of a Procedure for Estimating Crop

Evapotranspiration over Short Periods

Dr. Jorge Gonzalez, ProfessorDept. of Mechanical Engineering

Santa Clara University.

Eric Harmsen, Associate Professor Richard Diaz, Research Assist.Dept. of Agr. and Biosystems Eng. Civil Engineering Department

University of Puerto Rico University of Puerto Rico

Acknowledgements NASA-EPSCoR (NCC5-595), NOAA-CREST, USDA-

TSTAR, NASA-URC, and UPRM-TCESS.

Individuals: Javier Chaparro, Antonio Gonzalez, Jose Paulino-Paulino, and Dr. Ricardo Goanaga of the USDA Tropical Agricultural Research Station in Mayaguez, PR.

The ATLAS Sensor was provided by NASA Stennis Space Center and the Lear Jet Plane was provided by NASA Glenn Research Center.

 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It is estimated that 69% of all water withdrawn on a global basis is used for agriculture.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.

ATLAS

Objective

To describe a relatively inexpensive method for estimating short-term (e.g., hourly) actual evapotranspiration.

Present validation results for the method

Present application example results from two field studies conducted in Puerto Rico.

Combine

Humidity Gradient

and Generalized

Penman-Monteith Methods

Methodology

Generalized Penman-Monteith Method (GPM)

ET = evapotranspirationΔ = slope of the vapor pressure curve Rn = net radiationG = soil heat flux densityρa = air densitycp = heat capacity of airλ = psychrometric constant T = mean daily air temperature at 2 m heightu2 = wind speed at 2-m heightes = saturated vapor pressure and ea is the actual vapor pressurers and ra = bulk surface resistance and aerodynamic resistance, respectively.

ET

Rn G a cpes ea

ra

1rs

ra

.

Simplified representation of the (bulk) surface resistance and aerodynamic resistances for water vapor flow (from Allen et al., 1989).

Aerodynamic Resistance (ra)

ra = aerodynamic resistancezm = height of wind measurement zh = height of the humidity measurementd = zero plane displacementh = crop heightk = von Karman’s constantuz = wind speed at height z

ra

u2

.ra

lnzm d 0.123h

lnzh d

0.0123h

k2u2

.

Humidity Gradient Method (HGM)

ET = evapotranspirationρa = density of aircp = heat capacity of airρw = density of waterρvL = water vapor density at height LρvH = water vapor density at height Hrs = bulk surface resistancera = aerodynamic resistance = ζ / u2

u2 = wind velocity at 2 m

ETa cp

w

vL vH ra rs

.

Elevator Device

Method Validation

Eddy-Covariance System

Eddy Covariance System

Eddy Covariance System

ET Station

ET Results - UF Agr. Experiment Station - April 6th, 2005

0.0

0.2

0.4

0.6

0.8

8:30 AM 10:54 AM 1:18 PM 3:42 PM 6:06 PM

Time

Eva

po

tran

spir

atio

n (m

m/h

r) .

Penman-Monteith Method

Humidity Gradient Method

RH results over a 15-minute periodUF Agr. Experiment Station April 6th, 2005

0.5

1

1.5

2

2.5

2:15 PM 2:17 PM 2:20 PM 2:23 PM 2:26 PM 2:29 PM

Time

Squ

are

Wa

ve

40

44

48

52

56

60

Re

lativ

e H

um

idty

(%

) .

SW

RH

Actual Vapor Pressures and Actual Vapor Pressure DifferencesUF Agr. Experiment Station - April 6th, 2005

0.0

0.1

0.2

0.3

0.4

0.5

8:00 AM 10:24 AM 12:48 PM 3:12 PM 5:36 PM

Time

Act

ua

l Va

po

r P

ress

ure

D

iffe

ren

ce (

KP

a)

.

0.0

0.5

1.0

1.5

2.0

2.5

Act

ua

l Va

po

r P

ress

ure

(K

Pa

) .Actual Vapor Pressure Difference

Up Position Actual Vapor Pressure

Dow n Position Actual Vapor Pressure

Comparison: eddy covariance system and ET station

U of F Agr. Experiment Station April 5th and 6th, 2005

1

Date Method

Daily ET

(mm) Kc ζ rs

(s/m)

PM - ETo 4.37 208 70 Eddy Covariance 3.92 0.90 4/5/2005

ET station 4.11 0.94 191 157

PM - ETo 4.06 208 70 Eddy Covariance 3.78 0.93 4/6/2005

ET station 3.66 0.90 191 160 2

Comparison: eddy covariance system and ET station

U of F Agr. Experiment Station April 5th, 2005

0.0

0.2

0.4

0.6

0.8

8:30 AM 10:54 AM 1:18 PM 3:42 PM

Time

Eva

po

tra

nsp

ira

tion

(m

m/h

r) .

EToEddy Covariance SystemPM ET

Comparison: eddy covariance system and ET station

U of F Agr. Experiment Station April 6th, 2005

0.0

0.2

0.4

0.6

0.8

1.0

8:30 AM 10:54 AM 1:18 PM 3:42 PM 6:06 PM

Time

Eva

potr

ansp

iratio

n (m

m/h

r) . ETo

Eddy Covariance System

ET Station

0

50

100

150

200

250

300

1.5 1.7 1.9 2.1 2.3 2.5

Actual Vapor Pressure (KPa)

He

igh

t ab

ove

gro

un

d (

cm)

.

Grass

Sugar Cane

Monteith and Unsworth, 1990

Vapor Pressure

0

50

100

150

200

250

300

27.0 28.0 29.0 30.0 31.0 32.0

Temperature (oC)

He

igh

t a

bo

ve

gro

un

d (

cm

) .

Sugar cane

Grass

Monteith and Unsworth, 1990

Temperature

Application Example No. 1

Estimation of ET and aerodynamic resistancefor sugarcane, Lajas, PR

Estimated ET for a sugarcane plot November 9, 2004, Lajas, PR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

7:00 AM 8:12 AM 9:24 AM 10:36 AM 11:48 AM 1:00 PM 2:12 PM 3:24 PM 4:36 PM 5:48 PM

Time

Eva

potr

ansp

iratio

n (m

m/h

r) .

HG method

GPM method

Average of HG and GPMmethods

ET = 1.3 mm/day and ζ = 305

Estimated surface and aerodynamic resistances for a sugarcane plot November 9, 2004, Lajas, PR

y = 12564x2 - 12139x + 3065.7

0

200

400

600

800

1000

1200

7:00 AM 9:24 AM 11:48 AM 2:12 PM 4:36 PM 7:00 PM

Time

Re

sist

an

ce (

s/m

) .

Aerodynamic Resistance

Surface Resistance

Poly. (Surface Resistance)

Measured Net Radiation for a sugarcane plot Oct. 31st and Nov. 9, 2004 Lajas, PR

-200

-100

0

100

200

300

400

500

600

700

6:00:00 8:24:00 10:48:00 13:12:00 15:36:00 18:00:00

Time

Net

Rad

iatio

n (W

/m2)

.

31-Oct-04

9-Nov-04

Application Example No. 2The 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.

The ATLAS Mission

Estimated ET for a grass-covered field Nov. 11, 2004, Rio Piedras, PR

0.0

0.2

0.4

0.6

0.8

1.0

1.2

10:00 AM 11:55 AM 1:50 PM 3:45 PM 5:40 PM

Time

ET

(m

m/h

r)

GPM method

HG method

Average of GPM and HG methods

ET = 3.7 mm/day and ζ = 208 and rs = 90 sm-1

0.53 mm/hr

Time of fly-over 2:25 PM

Remote Sensing ET Equation

ETa cp

v vs rs

.

ρv is the vapor density of the air measured at the ground surface.

ρvs is the actual vapor pressure based on the corrected remotely sensed surface temperature

ATLAS-Estimated Surface Temperature for a grass-covered field Nov. 11, 2004, Rio Piedras, PR

1

2

60

52

46

39

33

26

19

12

6

Degrees oC

Pixel No.

Surface Temperature

(C)1 122 198 32.502 124 200 32.463 122 201 33.164 121 201 32.505 119 199 32.506 120 200 32.507 123 198 31.038 119 197 33.469 123 199 33.94

10 120 202 32.9811 118 199 36.2212 123 203 32.94

Average = 33.02

Pixel Location

Area of the station

ATLAS surface temperature correction: 33.0 oC – 29.8 oC= 4.2 oC

60

52

46

39

33

26

19

12

6

Degrees oC

Areal Photo Surface

Temperature

31.78 oC

33.95 oC

4

5

y = -0.1542x + 33.496

R2 = 0.4706

28

29

30

31

32

33

34

35

0 5 10 15 20

Distance from Ocean (ft)

Air

Tem

per

atu

re (

C)

Temperature and ET variation with distance from the ocean

The estimated ET varied 0.1 mm/hr over the 20 km transect.

0 4 8 12 16 2024

26

28

30

Corrected Surface TemperatureAir Temperature

Distance from Ocean (km)

Tem

pera

ture

(C

)

0 5 10 15 200.3

0.45

0.6

0.75

Distance from Ocean (km)E

T (

mm

/hr)

Relative Cost Comparison of Direct ET Methods

WeighingLysimeter

EddyCovariance

Bowen Ratio Systemdescribed in

this paper

Evapotranspiration Measurement System

Re

lati

ve

Co

st

+

 Future Work Additional method validation. Utilize the ET station to estimate

evapotranspiration rates and factors (ra, rs, Kc, Ks).

Deploy numerous stations around Puerto Rico to validate/calibrate remote sensing estimates of surface temperature, ET and the surface energy balance.