STANDARDIZED REFERENCE EVAPOTRANSPIRATION

Acknowlegements:Luis Pereira, Univ. LisbonWilliam Pruitt, UCD (deceased)James L. Wright, USDA-ARS (retired)Terry A. Howell, USDA-ARS (retired)Ayse Kilic, Univ. Nebraska-Lincoln

Some slides are from Rick Allen, University of Idaho-Kimberly

What is Reference ET?

• Reference ET (ETref) is:

• ET from a well-defined surface of dense vegetation that has reproducible ET that can be predicted using weather data

• ETref represents a near upper limit on ET based on weather

• ETref includes the major weather-based effects on ET and can be based on

physics , a

• If there were no Reference ET:• We would have to use a specific ET equation for:

• each crop

• each stage of growth

• much wasted effort in parameterizations

What is Reference ET?

• Two Current Vegetation Types for Reference ET:• Clipped Grass (ETo)

• Cool season grass (fescue or perennial ryegrass)• Mowed to 8 to 15 cm height• Extensive cover (~ 50 m or more)

• Full-cover Alfalfa (ETr) • Dense stand with no cutting effects• 30 to 70 cm height• Extensive cover (~ 50 m or more)

Monteith (of Scotland) inserted the surface resistance term in the Penman (of England) Combination equation

ρa is density, cp is specific heat, λ is latent heat of vaporization, Δ is slope of sat. vapor pressure curve, γ is psychrometric constant

/

)e(e)(G)(RET as2n

uba wwPenman Equation:

/

r

r1

r

)e(ecG)(R

ET

a

s

a

aspan

timeK

Rn=net radiation, G = soil heat flux= f (solar radiation)

f (temperature) aerodynamic resistance = f (wind)

actual vapor pressure = f (humidity)

f (crop)

John L. Monteith

Penman MonteithEquation:

surface resistance = f (veg)

History of Standardized Reference ET

• ASCE PM initially promoted by ASCE Manual 70 in 1990.

• Adoption of ASCE PM for ETo by FAO in 1990 and 1998 with reduced form format

• Publication of standardized ASCE PM by ASCE task committee in 2005

Considerations in Reference ET Type• Two different reference crops, alfalfa and clipped

grass are used, with usage generally divided among western States • Grass reference ETo has a long history of application in

urban areas and for agriculture in much of the U.S.

• Alfalfa ETr has a long history for agricultural application in the midwest and northwestern U.S.

• Alfalfa ETr is taller and ‘rougher’ and leafier than clipped grass and better represents an ‘upper’ bound on ET that is set by energy availability for ET

Families of crop coefficients have been developed (and are required) for each reference type.

• Theoretical arguments for both short and tall reference crops have been made by Perrier (1980) and Pereira et al. (1999)

f (time step, reference type, day/night)

f (time step, reference type)

)(

)()(.

2d

as2n

n

sz uC1

eeu273T

CGR4080

ET

Fixed Vegetation HeightFixed Surface Resistance

ASCE has same Reduced Form as the FAO-56 Penman-Monteith

Standardized ASCE Reference ET

American Society of Civil Engineers

The single standardized Penman-Monteith equation is applied to

a) grass and alfalfa and b) for daily or hourly timesteps

)uC1(

)ee(u273T

C)GR(408.0

ET2d

as2n

n

ref

Calculation Time Step Short Reference,ETo

Tall Reference,ETr

Units for ETo,ETr

Units for Rn, G

Cn Cd Cn Cd

Daily or monthly 900 0.34 1600 0.38 mm d-1 MJ m-2 d-1

Hourly during daytime 37 0.24 66 0.25 mm h-1 MJ m-2 h-1

Hourly during nighttime 37 0.96 66 1.7 mm h-1 MJ m-2 h-1

(FAO-56 PM ETo )

(ASCE PM ETr hourly)

Fixed Settings for Stdzd ASCE PM• albedo = 0.23 for both grass and alfalfa reference settings• fixed alfalfa height of 0.5 m, LAI of 4.5, rs of 30 s/m for

daytime hourly, rs of 45 s/m for 24-hours

• fixed grass height of 0.12 m, LAI of 3.0, rs of 50 s/m for daytime hourly, rs of 70 s/m for 24-hours

• Soil heat flux, G, = 0.04 Rn for alfalfa and 0.1 Rn for grass for hourly; G = 0 for 24-hours

• The standardized Rn method of ASCE (2005) and FAO (1998) is recommended.

• Weather data should be collected in an agricultural area

Comparisons with MeasurementsWeighing Lysimeter System at Kimberly, Idaho

Dr. James L. Wright, USDA-ARS

-0.10

0.10

0.30

0.50

0.70

0.90

1.10 E

T, m

m/h

our

010003000500070009001100130015001700190021002300Time of Day

Etr Lys . 2 al fal fa

Kimberly Lysimeters - September 4,1990Data from Dr. J .L Wright

-0.10

0.10

0.30

0.50

0.70

0.90

1.10

ET, m

m/h

our

010003000500070009001100130015001700190021002300Time of Day

Etr Lys . 2 alfalfa

Kimberly Lysimeters -September 7, 1990

ASCE Stdzd. PM (tall reference)at Kimberly, Idaho

-hourly timestep-shows same response as measurements

Kimberly, Idaho, Daily Alfalfa ET 1969-1971

Periods of Full Cover

SEE = 1.0 mm d-1

ASCE PM -- Daily vs. Hourly Timesteps

Davis, CaliforniaCIMIS Station2008 – 2012

Grass Reference

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

12

14

16

18

20

f(x) = 1.00488597302306 x + 0.182281071994687R² = 0.956802579185958

Summed Hourly Reference ET, mm d-1

24

-h T

ime

ste

p R

ef.

ET

, mm

d-1

Conclusion: similar accuracy

ASCE PM -- Daily vs. Hourly Timesteps

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

12

14

16

18

20

f(x) = 1.02473777317142 x + 0.433837216221479R² = 0.943756337156299

Summed Hourly Reference ET, mm d-1

24

-h T

ime

ste

p R

ef.

ET

, mm

d-1

Davis, CaliforniaCIMIS Station2008 – 2012

Alfalfa Reference

Conclusion: similar accuracy

Corrected by multiplying by 1.14 for day 90 to day 250 for year 1992 x 1.16 for day 90 to day 240 for year 1993

24-hour Solar Radiation

Measured

Corrected

Theoretical Clear Sky CurveQA/QC of Weather Data

QAQC of Weather Datais a Must!

Example of Relative humidity degradation and QAQC for Max. Daily RH% at UC Davis CIMIS station

• Base adjustments on ratios between theoretical clear sky solar radiation and top percentiles of measured dataM

ax

Da

ily R

H%

Ma

x D

aily

RH

%Before Correction – Sensor Drift

After Correction – No Sensor Drift

QA/QC of Weather Data

Calculation of ETref

• Use Python script, etc.• Use REF-ET free software:

• http://extension.uidaho.edu/kimberly/2013/04/ref-et-reference-evapotranspiration-calculator/

QAQC of weather• Use QAQC spreadsheet graphics generator that comes with REF-ET (follows ASCE guidelines):

• http://extension.uidaho.edu/kimberly/2013/04/ref-et-reference-evapotranspiration-calculator/