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HISTORICAL EVOLUTION OF ET ESTIMATING METHODS. “A Century of Progress” by Marvin E. Jensen. INTRODUCTION. This a partial condensation of a previous paper: Jensen & Allen (2000), 2000 ASAE National Irrig . Symposium it includes developments since 2000

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    1. HISTORICAL EVOLUTION OF ET ESTIMATING METHODS “A Century of Progress” by Marvin E. Jensen ARS/CSU ET Workshop

    2. INTRODUCTION • This a partial condensation of a previous paper: • Jensen & Allen (2000), 2000 ASAE National Irrig. Symposium • it includes developments since 2000 • In this paper, I also place more emphasis on my involvement • reviewing old docs, early est methods , and current literature • added pictures of key personnel • methods based on theory and European methods • dissemination and adoption of new U.S. estimating methods • new models based on satellite-imagery (SEBAL and METRIC) • development of the ASCE Manual 70, FAO-56, the ASCE Standardized Reference ET Equation, and the 2nd edition of ASCE Manual 70 ARS/CSU ET Workshop

    3. EARLY ESTIMATING METHODS • Early need for ET estimates • Early estimating methods • initially, estimates were only for seasonal estimates • then estimates were made for monthly periods • Mean air temperature was the primary weather variable, or temperature above 32 F • Secondary variables were: • relative humidity • percent of daylight hours ARS/CSU ET Workshop

    4. EARLY IRRIGATION STUDIES • Irrigation development began the mid-1800s • deliveries to farms varied widely and exceeded CU • “over-irrigation was the first and most serious mistake made by early settlers” (Buffam, 1892) • Irrigation investigations began around 1900 • Duty of Water measurements were made • involving water delivered to canals • and water delivered to farms • units were acres per 1 cfs, or depth applied ARS/CSU ET Workshop

    5. RESULTS OF MY EARLY WORK • My involvement began in 1960-monthly k-values • From about 100 values for full crop cover, • ETp = (0.014 T – 0.37) Rs • approximated ET from a crop like alfalfa • 1962 preliminary report, estimating ET from Rs • Publication Jensen and Haise (1963), objective • encourage engineers, soil scientists and agronomists to begin thinking about solar energy as a primary variable in estimating ET (and to motivate) rather than only air temp ARS/CSU ET Workshop

    6. EARLY WATER USE STUDIES • 1897Joint USDA-Exp. Stn investigations started • and continued under various departments • investigations were transferred to SCS in 1935 • 1890-1920 measurements of “Duty of water” • acres that could be irrigated with a constant flow • or depth of water applied • at the farm turnout, it was called net duty of water • Progress report, ASCE Duty of Water committee • presented in 1927 by O.W. Israelson • published in 1930 (Anonymous, 1930) ARS/CSU ET Workshop

    7. EARLY TRANSPIRATION STUDIES • 1902 USDA Bur. of Plant Industry established • Briggs, biophysicist, and Shantz, plant physiologist, conducted classic studies in eastern Colorado from 1913 to 1916 • they recognized that solar radiation was the primary cause of cyclic changes in environmental factors • developed hourly equations of “T” as a function of Rs and temperature rise; and Rs and VP deficit ARS/CSU ET Workshop

    8. EARLY ESTIMATING METHODS • 1924 Hedke proposed a method based on “heat available” defined as degree-days (temp x days) • 1942 Blaney and Morin proposed a method based on mean temperature, daylight hours and humidity • 1942 Lowry and Johnson proposed a method based on maximum temperature above 32F • 1948 Thornthwaite proposed a method based on mean monthly temperature. He recognized its limitations – such as why PET at a given temperature is not the same everywhere ARS/CSU ET Workshop

    9. Blaney-Criddle Method • Most widely known method (1945, 1950, 1952, 1962) up to the 1970s • U = KF = ∑kf • “k” monthly CU coefficient based on temperature • “f” = t ∙ p/100 • where t = mean monthly temperature, °F, and “p” = mean monthly percent of annual daytime hours ARS/CSU ET Workshop

    10. Harry F Blaney 1966Receiving ASAE John Deere Medal ARS/CSU ET Workshop

    11. ICID Vice President – 1970-1973 ARS/CSU ET Workshop

    12. Why Not Ra instead of “%DL”?%DL/%DL(max) and Ra/Ra(max) ARS/CSU ET Workshop

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    16. Transition Methods in the U.S. • Temperature and solar radiation • Jensen-Haise (1963) • ETalfalfa ref= 0.014 (Tf – 0.37) Rs (Western USA) • Stephens and Steward (1963) • ETgrass ref = 0.0082 (Tf – 0.19) Rs (Florida) • Hargeaves and Samani (1985); Hargreaves et al. (1985) • ETgrassref = 0.0023 TD0.5 (Tc + 17.8) Ra ARS/CSU ET Workshop

    17. Methods Based on Theory • The Bowen Ratio, BR (Bowen, 1926) (BR = ΔT/Δe ) • Was used for estimate evaporation from water in the 1930s, but was not used to estimate ET from land until Penman (1948) • Penman applied physics and [ET = (Rn – G)/(1 + BR)] • the Bowen Ratio, energy balance along with the • the rates of sensible heat and vapor transfer • Penman laid the foundation for later development of improved estimating methods (Penman, 1948, 1956, and 1963) • Surface resistance was added to the Penman equation (Penman and Long 1960; Monteith 1965; & Rijtema 1965) • Today, ET measurement methods include eddy covariance, boundary layer theory, remote sensing and energy balance or a combination of these methods ARS/CSU ET Workshop

    18. Penman Equation (1948) • E = (Δ Rn + γλ Ea )/(Δ + γ) (Eq. 16) • E = evaporation • Δ = the slope of saturated vapor pressure curve • γ = psychrometric constant • Rn = net radiation at crop surface • λ = latent heat of vaporization • Ea was an empirical wind function (like Rohwer’s (1931) equation ARS/CSU ET Workshop

    19. Howard L PenmanPres., Royal Meteor. Soc. 1961-63 ARS/CSU ET Workshop

    20. John L MonteithPres., Royal Meteor. Soc. 1978-80 ARS/CSU ET Workshop

    21. Other Estimating Methods • The European engineers progressed more rapidly • Makkink (1957) developed a formula for potential ET based on solar radiation & air temp • Turc (1960, 61) developed a formula based on 10-day mean air temperature and solar radiation • Rijtema (1958) a formula for individual crops using crop factors and length of growing season • Olivier (1961), a formula for monthly CU using wet bulb depression and clear sky radiation factors • Jensen (1968) proposed estimating ET using alfalfa ref x Kc-- known as the two-step process ARS/CSU ET Workshop

    22. Some details of my involvement • USDA-SCS Request to ARS-USDA • calculate crop coefficients for the B-C formula • Howard Haise and Harry Blaney solicited data from ARS researchers • the calculated monthly coefficients varied widely • I prepared a new questionnaire • basic soil water and climate data were requested • criteria were established for screening the data • new calculations, resulted in ~1000 ET rates for short time periods along with estimated solar radiation ARS/CSU ET Workshop

    23. RESULTS • From about 100 values of ET for full crop cover • ET = (0.014 T – 0.37) Rs • approximated ET from a crop with full cover like alfalfa with 30 to 50 cm (12-20 in.) of growth • A 1962 preliminary report, estimating ET from Rs, prepared for a workshop on estimating ET • Publication by Jensen and Haise (1963) had as its objective to encourage (& motivate) engineers soil scientists and agronomists to consider solar energy as the primary variable controlling ET ARS/CSU ET Workshop

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    25. Adopting & Disseminating New Estimating Methods • “Though the physicist still has some problems he can solve by himself, much of his future contribution to understanding evaporation in agriculture must be in collaboration with the biologist and soil scientists” (Penman, 1956) • In 1966 we organized a conference of ET specialists in Chicago • In 1966, I was also asked to chair the ASCE committee on Water Requirements • New members were added to the ASCE WR committee • Control members were Burman, U of Wyo; Collins, SCS; Gibbs, USBR; and Johnson, USGS • Harry Blaney remained on the committee ARS/CSU ET Workshop

    26. Dissemination Continued • An ASCE report on CU was prepared in 1973, it was widely distributed (Jensen ed. 1974) and was the start of ASCE Manual 70 (Jensen et al. 1990) • I left the committee for three years while serving on the ASCE I&D executive committee • Committee members continued work on the manual • In 1986 an ASCE Manual subcommittee was formed consisting of Allen, Burman, Blatchley, Jensen (chm), Johns, Stone and Wright, eventually resulting in the • ASCE Manual 70 “Evapotranspiration and Irrigation Water Requirements” (Jensen et al. 1990) ARS/CSU ET Workshop

    27. Dissemination Continued • Rick Allen published a detailed paper in the Agronomy Journal (Allen et al. 1989) • FAO organized a consultation for revising FAO-24 Crop Water Requirements with participants from seven countries (Smith et al. 1991) • (Allen, Jensen and Pruitt represented the U.S.) • Draft copies of ASCE Manual 70 was a key reference at this 1990 conference ARS/CSU ET Workshop

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    33. SATELLITE-IMAGERY MODELS • In the Netherlands, the surface energy balance model for land was developed by Bastiaanssen et al. in 1998 (SEBAL) • In the U.S., a model was developed by Allen et al. (2007) using some of the basic principles in SEBAL, called a model for Mapping Evapotranspiration at high Resolution using Internalized Calibration (METRIC) ARS/CSU ET Workshop

    34. Reference Crop x Crop Coefficient • Known as the two-step method – widely used • Was proposed in 1968 with alfalfa as the ref crop • Crop coefficient -- Similar to that proposed by Wijk and de Vries (1954) • Results can be very robust – requires judgment in selecting and applying crop coefficients • Example: Comparison with SEBAL for IID for water-year 1998, +2% all areas to +5% agr. land • (Jensen & Walter 2002) ARS/CSU ET Workshop

    35. Recent Methods • The Irrigation Association requested the ASCE ET Committee to recommend a single procedure for est. reference ET in the U.S. • An ASCE task committee developed such a procedure described in 2000 and published a report in 2005 (ASCE-EWRI 2005) • Remote sensing using satellite data (SEBAL and METRIC) • Remote sensing used to estimate Kc (grd, aircraft, satellite) • procedures for estimating Kc from ground cover and height expanded the FAO Kc database (Allen and Pereira 2009) • A one-step estimating method theory (Shuttleworth 2006) was tested in Australia (Shuttleworth & Wallace 2009) • Also known as Direct Penman-Montheith method • The 2nd ed. of Manual 70 is nearing completion ARS/CSU ET Workshop

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    37. SUMMARY AND CONCLUSIONS • This brief presentation summarizes a century of progress in the development of modern methods of estimating seasonal, daily and hourly ET (Only two centuries ago Dalton first described the hydrologic cycle) • Most of the progress in the U.S. was made during the last third of a century. ET is a complex process involving plants, soils, local weather such as wind speed and humidity, and solar and long-wave radiation • Many scientists and engineers were involved in the development of current, modern ET estimating technology • My association with development process was a learning experience • contacts with leading U.S & int’l scientists and engineers was rewarding • I have emphasized my involvement – its been my work for the past 50 yrs • I may have missed some recent developments–I tried to highlight main items • For a recent and more detailed review, see Farahani et al. (2007) • (Photos follow, time permitting) ARS/CSU ET Workshop

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