STANDARDIZED REFERENCE EVAPOTRANSPIRATION Some slides are from Rick

STANDARDIZED REFERENCE EVAPOTRANSPIRATION Some slides are from Rick Allen, University of Idaho. Kimberly Acknowlegements: Luis Pereira, Univ. Lisbon William Pruitt, UCD (deceased) James L. Wright, USDA-ARS (retired) Terry A. Howell, USDA-ARS (retired) Ayse Kilic, Univ. Nebraska-Lincoln

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 Penman Equation: Rn=net radiation, G = soil heat flux = f (solar radiation) actual vapor pressure = f (humidity) Penman Monteith Equation: John L. Monteith f (crop) surface resistance = f (veg) f (temperature) aerodynamic resistance = f (wind) 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

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 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)

Fixed Vegetation Height Fixed Surface Resistance f (time step, reference type) f (time step, reference type, day/night) 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 (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 Measurements Weighing Lysimeter System at Kimberly, Idaho Dr. James L. Wright, USDA-ARS

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 20 Davis, California CIMIS Station 2008 – 2012 18 24 -h Timestep Ref. ET, mm d-1 16 R 2 = 0. 957 14 Grass Reference 12 10 8 Conclusion: similar accuracy 6 4 2 0 0 2 4 6 8 10 12 14 16 Summed Hourly Reference ET, mm d-1 18 20

ASCE PM -- Daily vs. Hourly Timesteps 20 24 -h Timestep Ref. ET, mm d-1 Davis, California CIMIS Station 2008 – 2012 R 2 = 0. 944 18 16 14 Alfalfa Reference 12 10 8 Conclusion: similar accuracy 6 4 2 0 0 2 4 6 8 10 12 14 16 Summed Hourly Reference ET, mm d-1 18 20

Theoretical Clear Sky Curve QA/QC of Weather Data 24 -hour Solar Radiation Measured 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 Corrected

QA/QC of Weather Data QAQC of Weather Data is 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. Drift radiation and top percentiles of measured data Before Correction Sensor Max Daily RH% • After Correction – No Sensor Drift Max Daily RH%

Calculation of ETref • Use Python script, etc. • Use REF-ET free software: • http: //extension. uidaho. edu/kimberly/2013/04/r ef-et-referenceevapotranspirationcalculator/

QAQC of weather • Use QAQC spreadsheet graphics generator that comes with REF-ET (follows ASCE guidelines): • http: //extension. uidaho. edu/kimberly/2013/04/r ef-et-reference-evapotranspiration-calculator/