asm/swes 404/504 - university of arizona · pdf fileevapotranspiration definition also known...
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EvapotranspirationEvapotranspiration
ASM/SWES 404/504ASM/SWES 404/504
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Crop Water RequirementsCrop Water RequirementsRate of water use depends on kind of crop, Rate of water use depends on kind of crop, maturity, and atmospheric conditions.maturity, and atmospheric conditions.Seasonal water requirement is needed toSeasonal water requirement is needed to
match crops with available water supplymatch crops with available water supplyvariation within season needed for irrigation variation within season needed for irrigation schedulingscheduling
What determines whether irrigation is What determines whether irrigation is economically feasible in humid and some economically feasible in humid and some subhumid regions?subhumid regions?
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EvapotranspirationEvapotranspirationDefinitionDefinition
also known as consumptive water useexpressed as inches/day, inches/month, inches/season
Factors affecting Factors affecting evaporationevaporation
Factors affecting Factors affecting transpirationtranspiration
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EvapotranspirationEvapotranspirationMethods for determining ETMethods for determining ET
tank and lysimeter experimentstank and lysimeter experimentsexperimental field plotsexperimental field plotssoilsoil--water studieswater studiesanalysis of climatological dataanalysis of climatological dataintegration methodsintegration methodsinflowinflow--outflow methodoutflow method
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Nonweighinglysimeter
Water tablelysimeter
Weighinglysimeter
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EvapotranspirationEvapotranspiration
Methods for predicting ETMethods for predicting ETMass transferMass transferEnergy balance Energy balance
Penman Penman JensenJensen--HaiseHaise
EmpiricalEmpiricalBlaneyBlaney--CriddleCriddle
ETc = (Kc)(ETo)ETc = (Kc)(ETo)
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BlaneyBlaney--Criddle MethodCriddle MethodOne the oldest methods of estimating ETOne the oldest methods of estimating ET
a bit simplistica bit simplisticmean monthly temperaturemean monthly temperaturemonthly percent of annual daylight hoursmonthly percent of annual daylight hourscrop coefficientcrop coefficient
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BlaneyBlaney--Criddle MethodCriddle MethodEquationsEquations
monthly: monthly: u = ktp/u = ktp/100100 = kf= kfwhere: where: uu = monthly ET, inches= monthly ET, inches
kk = monthly ET coefficient= monthly ET coefficienttt = mean monthly temp, = mean monthly temp, ooFFpp = monthly % annual daytime hours= monthly % annual daytime hoursff = (= (tptp)/100 = monthly ET factor)/100 = monthly ET factor
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BlaneyBlaney--Criddle ExampleCriddle Example
Compute the evapotranspiration for corn Compute the evapotranspiration for corn at Yuma (valley) for the month of May at Yuma (valley) for the month of May using the Blaneyusing the Blaney--Criddle method. Criddle method. Assume a Assume a kk of 0.7.of 0.7.
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BlaneyBlaney--Criddle MethodCriddle MethodEquationsEquations
seasonal: seasonal: UU = = KFKF = = KK ∑∑ff = = ∑∑k fk fwherewhereUU == seasonal water use, inchesseasonal water use, inchesKK == seasonal ET coefficientseasonal ET coefficientFF == sum of monthly ET factors sum of monthly ET factors ff for the periodfor the periodff == ((tptp)/100 = monthly ET factor)/100 = monthly ET factortt == mean monthly temperature, °Fmean monthly temperature, °Fpp == monthly % of annual daytime hoursmonthly % of annual daytime hours
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BlaneyBlaney--Criddle ExampleCriddle Example
Compute the evapotranspiration for corn Compute the evapotranspiration for corn at Yuma (valley) for the growing season at Yuma (valley) for the growing season using the Blaneyusing the Blaney--Criddle method.Criddle method.
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BlaneyBlaney--Criddle MethodCriddle MethodAdvantages and disadvantages?Advantages and disadvantages?
advantagesadvantages
disadvantagesdisadvantages
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Penman MethodPenman MethodEnergy Balance and Mass Transfer ApproachEnergy Balance and Mass Transfer Approach
Rn = E + A + S + CRn = net radiant energy available at earth’s surfaceE = energy used in evaporating waterA = energy used in heating airS = energy used in heating the waterC = energy used in heating the surroundings of the water
E = Rn - A
Estimates ET for wellEstimates ET for well--watered short grass, i.e., watered short grass, i.e., estimates ETestimates EToo
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Penman MethodPenman MethodEquationsEquations8ETo
ETo
ETETcc
Actual ET for crop:Actual ET for crop:ETETcc = (= (KKcc))ETEToo
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Penman MethodPenman MethodAdvantages and Disadvantages?Advantages and Disadvantages?
AdvantagesAdvantages
DisadvantagesDisadvantages
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JensenJensen--Haise MethodHaise MethodEnergy Balance (solar radiation) ApproachEnergy Balance (solar radiation) ApproachSimpler than PenmanSimpler than PenmanBased on extensive field dataBased on extensive field dataEstimates ETEstimates ETrr
88ETETr = = CCtt ((TT -- TTxx) ) RRss
88ETETr = alfalfa-based reference ETRs = solar radiation = (0.35 + 0.61 n/N)RsoT = mean air temperature for calculation periodCt = calc. value based on elevation and vapor
pressures
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JensenJensen--Haise MethodHaise MethodCrop ET estimated from Kc for alfalfa-based reference ET
ETETcc = (= (KKcc))ETEToo
grass-based Kc … alfalfa-based Kc
Advantages and disadvantagesadvantages
disadvantages
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Pan Evaporation MethodPan Evaporation MethodETEToo = (K= (Kpp)(E)(Epanpan))Pan coefficient, KPan coefficient, Kpp
determined by avg daily wind speed and mean determined by avg daily wind speed and mean relative humidityrelative humidity
Advantages and disadvantagesAdvantages and disadvantagesadvantages: advantages: realreal--time evaporation rates, ontime evaporation rates, on--site site data, relatively easydata, relatively easy
disadvantages: disadvantages: data influenced by pan data influenced by pan placement and type, climate; water in pan stores placement and type, climate; water in pan stores and releases water differently than cropand releases water differently than crop
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Reference evapotranspiration Reference evapotranspiration and crop coefficientand crop coefficient
Reference evapotranspiration, EToReference evapotranspiration, EToobtained directlyobtained directlycomputed through empirical methodscomputed through empirical methodsInternet (http://Ag.Arizona.Edu/azmet/etrain.htm)Internet (http://Ag.Arizona.Edu/azmet/etrain.htm)
Reference ET is converted to crop ET with a Reference ET is converted to crop ET with a seasonal crop coefficient.seasonal crop coefficient.
crop coefficient dependent on stage of growth and crop coefficient dependent on stage of growth and locationlocation
ReliabilityReliability
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How is ET data used?How is ET data used?
Reference ET converted to crop ET to Reference ET converted to crop ET to estimate how much water is needed for estimate how much water is needed for the crop during a time periodthe crop during a time periodKnowing the irrigation efficiency, Knowing the irrigation efficiency, determine the inches of water to applydetermine the inches of water to applyIrrigation Water Requirement, inches = Irrigation Water Requirement, inches =
ETc / Irrigation Efficiencyrigation Efficiency