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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/