8/12/2019 Evapotranspiration - reference or potential.pdf

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ABE 343

Evapotranspiration: Potential or Reference? 1Suat Irmak and Dorota Z. Haman 2

1. This document is ABE 343, one of a series of the Agricultural and Biological Engineering Department, Florida Cooperative Extension Service, Instituteof Food and Agricultural Sciences, University of Florida. Original publication date June 06, 2003. Reviewed October 2011. Visit the EDIS website athttp://edis.ifas.u.edu .

2. Suat Irmak, Post-Doctoral Research Associate, and Dorota Z. Haman, Professor, Agricultural & Biological Engineering Department, Institute of Foodand Agricultural Sciences (IFAS), Florida Cooperative Extension Service, University of Florida, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only toindividuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, nationalorigin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A&M University CooperativeExtension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean

Te objective o this article is to dene two commonly usedevapotranspiration (E ) concepts: potential evapotranspira-tion (E p) and re erence evapotranspiration (E o); and toprovide insight on the differences between the two terms.A common understanding o these widely used concepts inagricultural communities will help to make communicationeasier between armers/growers, extension agents, andresearchers in the academic environment.

Te process known as evapotranspiration (E ) is o greatimportance in many disciplines, including irrigation systemdesign, irrigation scheduling, and hydrologic and drainagestudies. In a broad denition, the evapotranspiration isa combined process o both evaporation rom soil andplant sur aces and transpiration through plant canopies.In the evapotranspiration process, the water is trans erredrom the soil and plant sur aces into the atmosphere inthe orm o water vapor. In practice, the estimation o theevapotranspiration rate or a specic crop requires rst

calculating potential or re erence evapotranspiration andthen applying the proper crop coefficients (K c) to estimateactual crop evapotranspiration (E a).

Te objective o dening potential or re erence evapo-transpiration is to eliminate the crop specic changes inthe evapotranspiration process. In the potential evapo-transpiration denition this is attemped by assuming theconstant crop conditions. However, in this denition, there erence crop is not very well specied and this may createa problem in total elimination o crop component. Sincere erence evapotranspiration is based on hypothetical

crop, the process o elimination o crop specic changes ismuch easier.

Potential evapotranspiration (ET p ): Te potentialevapotranspiration concept was rst introduced in thelate 1940s and 50s by Penman and it is dened as theamount o water transpired in a given time by a short greencrop, completely shading the ground, o uni orm heightand with adequate water status in the soil prole. Notethat in the denition o potential evapotranspiration, theevapotranspiration rate is not related to a specic crop.Te main con usion with the potential evapotranspirationdenition is that there are many types o horticulturaland agronomic crops that t into the description o shortgreen crop. So, scientists may be con used as to which cropshould be selected to be used as a short green crop becausethe evapotranspiration rates rom well-watered agriculturalcrops may be as much as 10 to 30% greater than thatoccurring rom short green grass.

Reference evapotranspiration (ET o ): Re erence evapotrans-piration is dened as the rate o evapotranspiration roma hypothetical re erence crop with an assumed crop heighto 0.12 m (4.72 in), a xed sur ace resistance o 70 sec m -1 (70 sec 3.2f -1) and an albedo o 0.23, closely resemblingthe evapotranspiration rom an extensive sur ace o greengrass o uni orm height, actively growing, well-watered,and completely shading the ground. In the re erenceevapotranspiration denition, the grass is specicallydened as the re erence crop and this crop is assumed tobe ree o water stress and diseases. In the literature, the

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terms re erence evapotranspiration and re erence cropevapotranspiration have been used interchangeably andthey both represent the same evapotranspiration rate roma short, green grass sur ace.

Te re erence evapotranspiration concept was introducedby irrigation engineers and researchers in the late 1970sand early 80s to avoid ambiguities that existed in the

denition o potential evapotranspiration. By adoptinga re erence crop (grass), it has become easier and morepractical to select consistent crop coefficients and to makereliable actual crop evapotranspiration (E a) estimates innew areas. Introduction o the re erence evapotranspirationconcept also helped to enhance the trans erability o thecrop coefficients rom one location to another. In addition,with using re erence evapotranspiration, it is easier to selectconsistent crop coefficients and to calibrate evapotranspira-tion equations or a given local climate.

Historically two main crops have been used as the re erencecrop, grass and al al a. In Florida, the re erence crop isgrass since al al a is not commonly grown. It is generallyaccepted that the grass re erence crop is the type o grasswith physiological and structural characteristics similarto perennial ryegrass ( Lolium perenne L.) or alta escue(Festuca arundinacea Schreb . Alta). Although al al a has thephysical characteristics (lea area index, roughness, etc.)closer to many agronomic crops than the grass, research-ers generally agree that a clipped grass provides a betterrepresentation o re erence evapotranspiration than doesal al a. Tis is mainly because o the two reasons: (1) the

characteristics o the grass are better known and dened,(2) the grass crop has more planting areas than al al athroughout the world and the measured evapotranspirationrates o the grass are more readily available and accessible ascompared to the measured al al a evapotranspiration rates.

One o the other important differences between thepotential and re erence evapotranspiration is that theweather data collection site is well dened in the re erenceevapotranspiration denition. It is important to note in there erence evapotranspiration denition that the climatedata that are used to estimate re erence evapotranspirationneed to be collected in a well-dened (re erence) environ-ment. Tere ore, based on the denition, the weather dataor the re erence evapotranspiration estimations shouldbe collected in a well-irrigated and well-maintained grassarea. Te irrigated grass area o the weather data collectionsite should be airly large [(approximately two hectares)(4.94 acres)] because the quality o the weather data willultimately affect the nal estimated re erence evapotranspi-ration value. For example, in a hot, dry month the average

air temperature may be as much as 5 to 6 oC (9 to 10.8 oF)higher in a dryland (non-irrigated) than or a well-irrigatedland. Te differences in the air temperature will also affectthe relative humidity and vapor pressure decit values andthese differences will ultimately cause differences in there erence evapotranspiration calculated using the weatherdata collected rom the two sites (dry versus well-irrigated).

Te re erence evapotranspiration concept has been gainingsignicant acceptance by the engineers and scientiststhroughout the world since its introduction. Specicequations and standardized procedures are being recom-mended or re erence evapotranspiration estimates. TeInternational Commission or Irrigation and Drainage(ICID) and the Food and Agriculture Organization o theUnited Nations (FAO) Expert Consultation on Revisiono FAO Methodologies or Crop Water Requirementsrecommended that the Food and Agriculture Organizationo the United Nations Paper No. 56 Penman-Monteithequation (FAO56-PM) be used as the standard method toestimate E o. Tis equation has been increasingly gainingacceptance and used throughout the world or re erenceevapotranspiration estimations. It is recommended thatthe grass-re erence evapotranspiration concept be used orirrigation scheduling and water management, hydrologicstudies, and drainage researches in Florida in order toestablish a common and standard ground between thegrowers/ armers and their advisors and between theresearchers in Florida and other states.

ReferencesPenman, H.L. 1948. Natural evaporation rom open water,bare soil and grass. Proceedings o the Royal Society oLondon, A193: 120-146.

Allen, R.G., Pereiro, L.S., Raes, D. and Smith, M. 1998.Crop evapotranspirtion: Guidelines or computing croprequirements. Irrigation and Drainage paper No. 56. FAO,Rome.