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

What is Reference ET? Reference ET (ET ref ) is: ET from a well-defined surface of dense vegetation that has reproducible ET that can be predicted using weather data ET ref represents a near upper limit on ET based on weather ET ref 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 (ET o ) Cool season grass (fescue or perennial ryegrass) Mowed to 8 to 15 cm height Extensive cover (~ 50 m or more) Full-cover Alfalfa (ET r ) 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 ρ a is density, c p is specific heat, λ is latent heat of vaporization, Δ is slope of sat. vapor pressure curve, γ is psychrometric constant Penman Equation: 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 Monteith Equation: surface resistance = f (veg)

History of Standardized Reference ET ASCE PM initially promoted by ASCE Manual 70 in Adoption of ASCE PM for ET o 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 ET o has a long history of application in urban areas and for agriculture in much of the U.S. Alfalfa ET r has a long history for agricultural application in the midwest and northwestern U.S. Alfalfa ET r 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) Fixed Vegetation Height Fixed 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 (FAO-56 PM ET o ) (ASCE PM ET r 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, r s of 30 s/m for daytime hourly, r s of 45 s/m for 24-hours fixed grass height of 0.12 m, LAI of 3.0, r s of 50 s/m for daytime hourly, r s 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 Periods of Full Cover SEE = 1.0 mm d -1

ASCE PM -- Daily vs. Hourly Timesteps Davis, California CIMIS Station 2008 – 2012 Grass Reference Conclusion: similar accuracy

ASCE PM -- Daily vs. Hourly Timesteps Davis, California CIMIS Station 2008 – 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 hour Solar Radiation Measured Corrected Theoretical Clear Sky Curve 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 radiation and top percentiles of measured data Max Daily RH% Before Correction – Sensor Drift After Correction – No Sensor Drift QA/QC of Weather Data

Calculation of ET ref Use Python script, etc. Use REF-ET free software: ref-et-reference- evapotranspiration- calculator/

QAQC of weather Use QAQC spreadsheet graphics generator that comes with REF-ET (follows ASCE guidelines): ef-et-reference-evapotranspiration-calculator/