Crop evapotranspiration - Guidelines for computing

AB.W1Cropevapotranspiration-Guidelinesforcomputingcropwaterrequirements-FAOIrrigationanddrainagepaper56ByRichardG.AllenUtahStateUniversityLogan,Utah,USALuisS.PereiraInstitutoSuperiordeAgronomiaLisbon,PortugalDirkRaesKatholiekeUniversiteitLeuvenLeuven,BelgiumMartinSmithWaterResources,DevelopmentandManagementServiceFAOFAO-FoodandAgricultureOrganizationoftheUnitedNationsRome,1998ThedesignationsemployedandthepresentationofmaterialinthispublicationdonotimplytheexpressionofanyopinionwhatsoeveronthepartoftheFoodandAgricultureOrganizationoftheUnitedNationsconcerningthelegalstatusofanycountry,territory,cityorareaorofitsauthorities,orconcerningthedelimitationofitsfrontiersorboundaries.©FAO1998AB.W2Evapotranspiration(ET)Thecombinationoftwoseparateprocesseswherebywaterislostontheonehandfromthesoilsurfacebyevaporationandontheotherhandfromthecropbytranspirationisreferredtoasevapotranspiration(ET).EvaporationEvaporationistheprocesswherebyliquidwaterisconvertedtowatervapour(vaporization)andremovedfromtheevaporatingsurface(vapourremoval).Waterevaporatesfromavarietyofsurfaces,suchaslakes,rivers,pavements,soilsandwetvegetation.TranspirationTranspirationconsistsofthevaporizationofliquidwatercontainedinplanttissuesandthevapourremovaltotheatmosphere.Cropspredominatelylosetheirwaterthroughstomata.ThesearesmallopeningsontheplantleafthroughwhichgasesandwatervapourpassEvapotranspiration(ET)Evaporationandtranspirationoccursimultaneouslyandthereisnoeasywayofdistinguishingbetweenthetwoprocesses.Apartfromthewateravailabilityinthetopsoil,theevaporationfromacroppedsoilismainlydeterminedbythefractionofthesolarradiationreachingthesoilsurface.Thisfractiondecreasesoverthegrowingperiodasthecropdevelopsandthecropcanopyshadesmoreandmoreofthegroundarea.Whenthecropissmall,waterispredominatelylostbysoilevaporation,butoncethecropiswelldevelopedandcompletelycoversthesoil,transpirationbecomesthemainprocessFactorsaffectingevapotranspirationWeatherparameters,cropcharacteristics,managementandenvironmentalaspectsarefactorsaffectingevaporationandtranspiration.TherelatedETconceptspresentedinFigure3arediscussedinthesectiononevapotranspirationconcepts.WeatherparametersTheprincipalweatherparametersaffectingevapotranspirationareradiation,airtemperature,humidityandwindspeed.Theevaporationpoweroftheatmosphereisexpressedbythereferencecropevapotranspiration(ETo).Thereferencecropevapotranspirationrepresentstheevapotranspirationfromastandardizedvegetatedsurface.CropfactorsThecroptype,varietyanddevelopmentstageshouldbeconsideredwhenassessingtheevapotranspirationfromcropsgrowninlarge,well-managedfields.Differencesinresistancetotranspiration,cropheight,croproughness,reflection,groundcoverandcroprootingcharacteristicsresultindifferentETlevelsindifferenttypesofcropsunderidenticalenvironmentalconditions.ManagementandenvironmentalconditionsFactorssuchassoilsalinity,poorlandfertility,limitedapplicationoffertilizers,thepresenceofhardorimpenetrablesoilhorizons,theabsenceofcontrolofdiseasesandpestsandpoorsoilmanagementmaylimitthecropdevelopmentandreducetheevapotranspiration.OtherfactorstobeconsideredwhenassessingETaregroundcover,plantdensityandthesoilwatercontent.TheeffectofsoilwatercontentonETisconditionedprimarilybythemagnitudeofthewaterdeficitandthetypeofsoil.Ontheotherhand,toomuchwaterwillresultinwaterloggingwhichmightdamagetherootandlimitrootwateruptakebyinhibitingrespiration.AB.W3Referencecropevapotranspiration(ETo)Theevapotranspirationratefromareferencesurface,notshortofwater,iscalledthereferencecropevapotranspirationorreferenceevapotranspirationandisdenotedasETo.Thereferencesurfaceisahypotheticalgrassreferencecropwithspecificcharacteristics.TheuseofotherdenominationssuchaspotentialETisstronglydiscouragedduetoambiguitiesintheirdefinitions.TheonlyfactorsaffectingEToareclimaticparameters.Consequently,EToisaclimaticparameterandcanbecomputedfromweatherdata.EToexpressestheevaporatingpoweroftheatmosphereataspecificlocationandtimeoftheyearanddoesnotconsiderthecropcharacteristicsandsoilfactors.Cropevapotranspirationunderstandardconditions(ETc)Thecropevapotranspirationunderstandardconditions,denotedasETc,istheevapotranspirationfromdisease-free,well-fertilizedcrops,growninlargefields,underoptimumsoilwaterconditions,andachievingfullproductionunderthegivenclimaticconditions.
ETcwillbebetween1to9mm/dayfromcooltowarmaveragetemperature.Theamountofwaterrequiredtocompensatetheevapotranspirationlossfromthecroppedfieldisdefinedascropwaterrequirement.Althoughthevaluesforcropevapotranspirationandcropwaterrequirementareidentical,cropwaterrequirementreferstotheamountofwaterthatneedstobesupplied,whilecropevapotranspirationreferstotheamountofwaterthatislostthroughevapotranspiration.Theirrigationwaterrequirementbasicallyrepresentsthedifferencebetweenthecropwaterrequirementandeffectiveprecipitation.Theirrigationwaterrequirementalsoincludesadditionalwaterforleachingofsaltsandtocompensatefornon-uniformityofwaterapplication.EnergybalanceTheenergyarrivingatthesurfacemustequaltheenergyleavingthesurface