EvapotranspirationEvapotranspiration

ASM/SWES 404/504ASM/SWES 404/504

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?

EvapotranspirationEvapotranspirationDefinitionDefinition

also known as consumptive water useexpressed as inches/day, inches/month, inches/season

Factors affecting Factors affecting evaporationevaporation

Factors affecting Factors affecting transpirationtranspiration

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

Nonweighinglysimeter

Water tablelysimeter

Weighinglysimeter

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)

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

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

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.

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

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.

BlaneyBlaney--Criddle MethodCriddle MethodAdvantages and disadvantages?Advantages and disadvantages?

advantagesadvantages

disadvantagesdisadvantages

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

Penman MethodPenman MethodEquationsEquations8ETo

ETo

ETETcc

Actual ET for crop:Actual ET for crop:ETETcc = (= (KKcc))ETEToo

Penman MethodPenman MethodAdvantages and Disadvantages?Advantages and Disadvantages?

AdvantagesAdvantages

DisadvantagesDisadvantages

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

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

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

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

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