evapotranspiration concepts and irrigation water requirements

July 12, 2005 NRCS IWM Training Course 1

Evapotranspiration Evapotranspiration Concepts and Concepts and

Irrigation Water Irrigation Water RequirementsRequirements

Thomas W. Ley PhD, PEThomas W. Ley PhD, PE

Chief HydrographerChief Hydrographer

Colorado Division of Water Colorado Division of Water ResourcesResources


BackgroundBackground

• EducationEducation• ““ET” related career highlitesET” related career highlites

• Collaborated with Tom Spofford to develop crop water Collaborated with Tom Spofford to develop crop water requirement data for the WA Irrigation Guiderequirement data for the WA Irrigation Guide

• Washington Public Agriculture Weather System Washington Public Agriculture Weather System (PAWS)(PAWS)

• Historical crop water use analyses (KS v. CO)Historical crop water use analyses (KS v. CO)• New lysimeter at CSU Rocky Ford AVRCNew lysimeter at CSU Rocky Ford AVRC• Member ASCE ET in Irrigation and Hydrology Technical Member ASCE ET in Irrigation and Hydrology Technical

Committee and Crop Coefficient Task CommitteeCommittee and Crop Coefficient Task Committee

• What’s a hydrographer?What’s a hydrographer?


ObjectivesObjectives

• Discuss irrigation water Discuss irrigation water requirements and need for crop requirements and need for crop water use informationwater use information

• Define evapotranspiration (ET) and Define evapotranspiration (ET) and consumptive use (CU)consumptive use (CU)

• Overview of the physics of ET and Overview of the physics of ET and factors affecting ETfactors affecting ET

• Methods of determining/estimating Methods of determining/estimating ETET



• “…“…the quantity, or depth, of irrigation the quantity, or depth, of irrigation water in addition to precipitation water in addition to precipitation required to produce the desired crop required to produce the desired crop yield and quality and to maintain an yield and quality and to maintain an acceptable salt balance in the root acceptable salt balance in the root zone.” (NEH, Part 623, Chap 2, Irrigation zone.” (NEH, Part 623, Chap 2, Irrigation Water Requirements)Water Requirements)

• affected by crop types, climate affected by crop types, climate conditions, soil conditionsconditions, soil conditions



• Needed day-to-dayNeeded day-to-day• irrigation schedulingirrigation scheduling• other operational and management other operational and management

decisionsdecisions

• Needed seasonally Needed seasonally • sizing of irrigation system components sizing of irrigation system components

(pipes, valves, ditches)(pipes, valves, ditches)• planning and development of irrigation planning and development of irrigation

projectsprojects• water rights issueswater rights issues• hydrologic studieshydrologic studies


Soil-Water BalanceSoil-Water Balance

IWR=ETIWR=ETcc + DP + RO - P + DP + RO - P SW - GW + SW - GW + LL

ETc

P IWR

DP GW

RO

SWSW

L

Crop root zone


Evapotranspiration and Evapotranspiration and Consumptive UseConsumptive Use

• In general, one and the sameIn general, one and the same• Crop water requirement is an Crop water requirement is an

equivalent termequivalent term• Consumptive use includes water Consumptive use includes water

retained in plant tissue at harvest, retained in plant tissue at harvest, but this is generally minor relative but this is generally minor relative to amount of ETto amount of ET


EvapotranspirationEvapotranspiration

• Combination of two separate Combination of two separate processesprocesses

Evaporation from the soil Evaporation from the soil surface surface

Transpiration by the cropTranspiration by the crop


Physics of EvapotranspirationPhysics of Evapotranspiration

• Evaporation is the process where Evaporation is the process where liquid water is converted to water liquid water is converted to water vaporvapor

Evaporation is predominant when crop Evaporation is predominant when crop is small and water loss is primarily by is small and water loss is primarily by soil evaporation, or under high soil evaporation, or under high frequency wetting when soil frequency wetting when soil evaporation and evaporation of free evaporation and evaporation of free water from plant surfaces can be highwater from plant surfaces can be high


Physics of EvapotranspirationPhysics of Evapotranspiration• Transpiration is the vaporization of Transpiration is the vaporization of

liquid water in plant tissues and vapor liquid water in plant tissues and vapor removal to the atmosphereremoval to the atmosphere

--Vaporization occurs in intercellular spaces --Vaporization occurs in intercellular spaces of the plant tissue, while exchange with the of the plant tissue, while exchange with the atmosphere occurs through and is atmosphere occurs through and is controlled by plant stomata. controlled by plant stomata.

--Transpiration is predominant once the --Transpiration is predominant once the crop has developed and the canopy shades crop has developed and the canopy shades more and more of the surfacemore and more of the surface


Physics of EvapotranspirationPhysics of Evapotranspiration

• ET is an energy controlled process ET is an energy controlled process requiring the conversion of requiring the conversion of available radiation energy available radiation energy (sunshine) and sensible energy (sunshine) and sensible energy (heat contained in the air) into (heat contained in the air) into latent energy (energy stored in latent energy (energy stored in water vapor molecules)water vapor molecules)


Energy BalanceEnergy Balance

RRnn = = ET + H + GET + H + G

RRnn is the net short and longwave radiation at the is the net short and longwave radiation at the surface from sun and sky (main energy source)surface from sun and sky (main energy source)

ET is latent heat flux (energy used in the ET process)ET is latent heat flux (energy used in the ET process)H is sensible heat flux (transfer) to the airH is sensible heat flux (transfer) to the airG is sensible heat flux (transfer) to the ground or soilG is sensible heat flux (transfer) to the ground or soil

RRnn

ETET HH

GG


Why not compute ET directly Why not compute ET directly from the Energy Balance?from the Energy Balance?

ET = RET = Rnn - H – G - H – G

ProsPros: R: Rn n and G can be directly measured or reliably and G can be directly measured or reliably estimated from climatic dataestimated from climatic data

ConsCons: Only vertical fluxes are considered, and the net : Only vertical fluxes are considered, and the net rate at which energy is transferred horizontally, rate at which energy is transferred horizontally, advectionadvection, is ignored (a major problem in many , is ignored (a major problem in many areas of CO and elsewhere). Thus, this approach areas of CO and elsewhere). Thus, this approach can only be applied to large, extensive surfaces of can only be applied to large, extensive surfaces of homogeneous vegetation. Measurement of sensible homogeneous vegetation. Measurement of sensible heat flux, H, is complex and not easily obtained.heat flux, H, is complex and not easily obtained.


Penman Combination Penman Combination EquationEquation

• Penman (1948) developed the well-Penman (1948) developed the well-known combination equation, known combination equation, combining the energy balance with an combining the energy balance with an aerodynamic function to account for aerodynamic function to account for heat and vapor exchange with the airheat and vapor exchange with the air

E

+ + )G - R(

+ = E an


Penman Combination Penman Combination EquationEquation

)u b +a( W

)e - e( W E

2f

ao

f a

• Vapor transport flux term, EVapor transport flux term, Ea

• empirical wind function, Wempirical wind function, Wff

• vapor pressure deficit, (evapor pressure deficit, (e - e - eaa))


Factors Affecting Factors Affecting EvapotranspirationEvapotranspiration

• WeatherWeather• Crop characteristicsCrop characteristics• ManagementManagement• Environmental conditionsEnvironmental conditions


WeatherWeather

• Solar radiationSolar radiation• Air temperatureAir temperature• Relative humidityRelative humidity• Wind speedWind speed


Crop CharacteristicsCrop Characteristics

• Crop type and varietyCrop type and variety• Height, roughness, stomatal control, Height, roughness, stomatal control,

reflectivity, ground cover, rooting reflectivity, ground cover, rooting characteristicscharacteristics

• Stage of developmentStage of development


ManagementManagement

• Irrigation method Irrigation method • Irrigation managementIrrigation management• Cultivation practicesCultivation practices• Fertility managementFertility management• Disease and pest controlDisease and pest control


Environmental ConditionsEnvironmental Conditions

• Soil type, texture, water-holding Soil type, texture, water-holding capacitycapacity

• Soil salinitySoil salinity• Soil depth and layeringSoil depth and layering• Poor soil fertilityPoor soil fertility• Exposure/shelteringExposure/sheltering


Methods of Methods of Determining/Estimating ETDetermining/Estimating ET

• Direct measurementDirect measurement• Compute ET using a wide variety Compute ET using a wide variety

of empirical, semi-empirical, and of empirical, semi-empirical, and physically-based equations using physically-based equations using climate and weather dataclimate and weather data


Direct Measurement of ETDirect Measurement of ET

• LysimetryLysimetry• Soil water depletionSoil water depletion• Energy balance and micro-Energy balance and micro-

meteorological methods—research meteorological methods—research applications onlyapplications only

• Mass transfer / Bowen ratioMass transfer / Bowen ratio• Vertical gradients of air temp and water vaporVertical gradients of air temp and water vapor

• Eddy correlationEddy correlation• gradients of wind speed and water vaporgradients of wind speed and water vapor


LysimetryLysimetry

• Crop of interest grown under natural Crop of interest grown under natural conditions in an isolated tank in large field conditions in an isolated tank in large field of same cropof same crop

• Disturbed or undisturbed soilDisturbed or undisturbed soil• Terms in the soil water balance that are Terms in the soil water balance that are

difficult to measure are carefully controlled difficult to measure are carefully controlled and measuredand measured

• Many types of lysimeters: non-weighing Many types of lysimeters: non-weighing drainage, non-weighing water table, drainage, non-weighing water table, weighing typeweighing type


LysimetryLysimetry

• Direct measurement of Direct measurement of ETET

• Precision weighing Precision weighing lysimeters most accurate lysimeters most accurate (resolution of 0.05 ET (resolution of 0.05 ET mm per hour or better)mm per hour or better)

• Soils inside and outside Soils inside and outside the tank must be similarthe tank must be similar

• Vegetation inside and Vegetation inside and outside the tank must outside the tank must perfectly match (height, perfectly match (height, leaf area, density, vigor)leaf area, density, vigor)


LysimetryLysimetry

• Difficult and expensive to constructDifficult and expensive to construct• Require careful operation and Require careful operation and

maintenancemaintenance• Primarily research applicationPrimarily research application• Primary tool for evaluating weather Primary tool for evaluating weather

effects on ET and evaluation of effects on ET and evaluation of estimating methodsestimating methods


Soil-Water DepletionSoil-Water Depletion

ETETcc)1)122 = (I - DP – RO - L) + P = (I - DP – RO - L) + Pee + + SWSW1122 + GW + GW

ETc

P I

DP GW

RO

SWSW

L

Crop root zone


ET ConceptsET Concepts

• Reference ET (ETReference ET (ETrefref))• ET rate from a reference vegetative ET rate from a reference vegetative

surface, actively growing, not short surface, actively growing, not short of waterof water

• measure of evaporative demand measure of evaporative demand under current climate conditionsunder current climate conditions

• Crop ET under standard conditionsCrop ET under standard conditions• Crop ET under non-standard conditionsCrop ET under non-standard conditions



• Reference ET (Reference ET (ETETrefref))

• Crop ET under standard conditionsCrop ET under standard conditions• ET of disease-free, well-fertilized crop not short of ET of disease-free, well-fertilized crop not short of

water achieving full productionwater achieving full production

ETETcc = crop coefficient x ET = crop coefficient x ETrefref

• Crop coefficients are determined experimentally by Crop coefficients are determined experimentally by lysimeter or soil water balance methods as the lysimeter or soil water balance methods as the ratio of measured crop ET (under optimal growing ratio of measured crop ET (under optimal growing conditions) to reference crop ET across the growing conditions) to reference crop ET across the growing seasonseason

• Crop ET under non-standard conditionsCrop ET under non-standard conditions



• Reference ET (Reference ET (ETETrefref))• Crop ET under standard conditionsCrop ET under standard conditions

• Crop ET under non-standard Crop ET under non-standard conditionsconditions

• ET of crop considering “real-world” ET of crop considering “real-world” growing conditions (diseases, pests, growing conditions (diseases, pests, fertility problems, salinity effects, water fertility problems, salinity effects, water stress, management, etc.)stress, management, etc.)

• Use a water stress coefficient, KUse a water stress coefficient, Kss, and , and adjust crop coefficients for other stressesadjust crop coefficients for other stresses


breakbreak


Estimating ETEstimating ET

• wide variety of empirical, semi-wide variety of empirical, semi-empirical, and physically-based empirical, and physically-based equations/modelsequations/models

• generally categorized as:generally categorized as:• temperature methodstemperature methods• radiation methodsradiation methods• combination methodscombination methods• pan evaporation methodspan evaporation methods


Modified Blaney-Criddle Modified Blaney-Criddle MethodMethod

• Originally developed in the 1920’s and 1930’s; Originally developed in the 1920’s and 1930’s; modified in 1945, 1950, 1952, 1960, 1965, modified in 1945, 1950, 1952, 1960, 1965, 19701970

• ET of an actively growing crop with adequate ET of an actively growing crop with adequate soil moisture varies directly with the product of soil moisture varies directly with the product of mean monthly air temperature and monthly mean monthly air temperature and monthly percentage of annual daytime hourspercentage of annual daytime hours

U = (kf) = [(0.0173 t - 0.314) kc (t p/100)]


Modified Blaney-Criddle Modified Blaney-Criddle MethodMethod

• Simple, easy to useSimple, easy to use• Minimal data requirements—mean Minimal data requirements—mean

monthly air temperaturemonthly air temperature• Wide application across western Wide application across western

USUS• Widely used in CO water rights Widely used in CO water rights

proceedingsproceedings


Precautions/LimitationsPrecautions/Limitations

• Not a reference ET methodNot a reference ET method• Crop growth stage coefficient, kCrop growth stage coefficient, kcc

• is specific to this methodis specific to this method• not a true crop coefficient, i.e., shown to be not a true crop coefficient, i.e., shown to be

dependentdependent on climate/location on climate/location• Should not be used to compute ET on Should not be used to compute ET on

less than a monthly time stepless than a monthly time step• Underpredicts in arid climates, and Underpredicts in arid climates, and

under windy or high advection under windy or high advection conditionsconditions


1985 Hargreaves Method1985 Hargreaves Method

• Originally developed in 1975Originally developed in 1975• solar radiation and temperature data inputssolar radiation and temperature data inputs

• Updated in 1982 and 1985 Updated in 1982 and 1985 • solar radiation estimated from solar radiation estimated from

extraterrestrial radiationextraterrestrial radiation• Grass reference ET (ETGrass reference ET (EToo))

• Can be used to compute daily estimatesCan be used to compute daily estimates

amean5.0

minmaxo R)8.17T()TT(0023.0ET


1985 Hargreaves Method1985 Hargreaves Method

• Simple, easy to useSimple, easy to use• Minimal data requirements—Minimal data requirements—

maximum and minimum air maximum and minimum air temperaturetemperature

• Better predictive accuracy in arid Better predictive accuracy in arid climates than modified Blaney-Criddleclimates than modified Blaney-Criddle• Max-min temperature differenceMax-min temperature difference• Extra-terrestrial radiationExtra-terrestrial radiation



• Grass reference ET methodGrass reference ET method• Convert to alfalfa basis before using alfalfa Convert to alfalfa basis before using alfalfa

reference crop coefficients reference crop coefficients • Adds another level of uncertainty to crop ET Adds another level of uncertainty to crop ET

estimatesestimates

• Accuracy improves when used over Accuracy improves when used over longer intervals, i.e., 10-days, monthlylonger intervals, i.e., 10-days, monthly

• Still underpredicts in arid climates, and Still underpredicts in arid climates, and under windy or high advection conditionsunder windy or high advection conditions


1982 Kimberly Penman1982 Kimberly Penman

• Developed at Kimberly IDDeveloped at Kimberly ID• Alfalfa reference ET (ETAlfalfa reference ET (ETrr) )

• Calibrated wind function (varies daily) Calibrated wind function (varies daily) attempts to account for seasonally- varying attempts to account for seasonally- varying local and regional advection and daylengthlocal and regional advection and daylength

• Calibrated net radiation function (varies daily)Calibrated net radiation function (varies daily)

/) e - e( ) u b + a( +

K + )G - R( +

= ET as2wwwn

r


1982 Kimberly Penman1982 Kimberly Penman

• May be used for hourly or daily ET May be used for hourly or daily ET estimatesestimates

• Good predictive accuracy across a Good predictive accuracy across a wide range of climates often wide range of climates often ranking second only to the ranking second only to the Penman-Monteith (ASCE Manual Penman-Monteith (ASCE Manual 70)70)

• Widely used across the western USWidely used across the western US


Crop CoefficientsCrop Coefficients

• Specifically developed using the Specifically developed using the 1982 Kimberly Penman method1982 Kimberly Penman method

• Wright (1982) Wright (1982) • ASCE Manual 70 Tables 6.6 and 6.9ASCE Manual 70 Tables 6.6 and 6.9

• Basal and “mean” coefficientsBasal and “mean” coefficients• Transferability of “mean” crop Transferability of “mean” crop

coefficients requires assessment of coefficients requires assessment of irrigation and rainfall patternsirrigation and rainfall patterns



• Same weather data requirements as Same weather data requirements as any other Penman-based equationany other Penman-based equation

• Wind and net radiation functions Wind and net radiation functions calibrated to Kimberly ID climatecalibrated to Kimberly ID climate

• Aerodynamic term may lose accuracy in Aerodynamic term may lose accuracy in climates windier or more advective than climates windier or more advective than those experienced at Kimberlythose experienced at Kimberly

• Net radiation function bias—underpredicts Net radiation function bias—underpredicts on high Ron high Rnn days days


Penman-Montieth EquationPenman-Montieth Equation(ASCE Full-Form)(ASCE Full-Form)

• ET of a well-watered cropET of a well-watered crop• Physically-based, theoretically sound modelPhysically-based, theoretically sound model

• Neutral atmospheric stability Neutral atmospheric stability • Logarithmic wind profileLogarithmic wind profile

• Most accurate on an hourly basisMost accurate on an hourly basis• Standard of comparison for evaluating other modelsStandard of comparison for evaluating other models

/

r

r1

r

)e(ecG)(R

ET

a

s

a

aspan

ref

timeK



• Often used in a reference crop approach Often used in a reference crop approach (alfalfa, grass) due to limited data on bulk (alfalfa, grass) due to limited data on bulk canopy surface resistance of other cropscanopy surface resistance of other crops

• Same weather data requirements as any Same weather data requirements as any other Penman-based equationother Penman-based equation

• Empirical simplifications are introduced Empirical simplifications are introduced when using daily weather datawhen using daily weather data

• diurnal distributions of humidity, wind speed and diurnal distributions of humidity, wind speed and net radiationnet radiation


ASCE Manual 70 (1990) ASCE Manual 70 (1990) StudiesStudies

• 19 estimating methods evaluated19 estimating methods evaluated• Carefully screened lysimeter data from 11 Carefully screened lysimeter data from 11

worldwide locations representing a range of worldwide locations representing a range of climatic (arid to humid) conditionsclimatic (arid to humid) conditions

• Penman-Monteith found to be most Penman-Monteith found to be most accurate and consistent across all climates accurate and consistent across all climates on both monthly and daily basison both monthly and daily basis

• 1982 Kimberly Penman ranked second at 1982 Kimberly Penman ranked second at arid sites and at all locations and ranked arid sites and at all locations and ranked second in the evaluation of daily estimatessecond in the evaluation of daily estimates


ASCE Standardized Penman-ASCE Standardized Penman-Montieth EquationMontieth Equation

• ET for hypothetical standardized reference cropET for hypothetical standardized reference crop• ETETosos, short reference crop, like 12 cm tall grass , short reference crop, like 12 cm tall grass

with bulk surface resistance of 70 s/mwith bulk surface resistance of 70 s/m

• ETETrsrs, tall reference crop, like 50 cm tall alfalfa , tall reference crop, like 50 cm tall alfalfa with bulk surface resistance of 45 s/mwith bulk surface resistance of 45 s/m

)1(

)(273

)(408.0

2

2

uC

eeuTC

GRET

d

asn

n

sz


ASCE Task Committee ASCE Task Committee Evaluation of the Evaluation of the

Standardized P-M EquationStandardized P-M Equation

• Evaluated the predictive accuracy of 13 reference Evaluated the predictive accuracy of 13 reference equations (including the standardized equation) equations (including the standardized equation) at 49 sites across the USat 49 sites across the US

• Standard of comparison was the (ASCE full form) Standard of comparison was the (ASCE full form) Penman-Monteith equationPenman-Monteith equation

• ASCE standardized P-M equation performed well ASCE standardized P-M equation performed well on hourly and daily basison hourly and daily basis• simplifications and standardized computations simplifications and standardized computations

included in the ASCE standardized P-M equation included in the ASCE standardized P-M equation considered acceptableconsidered acceptable


Penman Methods with Penman Methods with Limited Climate DataLimited Climate Data

• Penman-type ET estimates using Penman-type ET estimates using limited climate data and limited climate data and estimation procedures for missing estimation procedures for missing data are considered more accurate data are considered more accurate than estimates computed using than estimates computed using less data-intensive ET methodsless data-intensive ET methods


Temperature DataTemperature Data

• Minimum data requirements are maximum and Minimum data requirements are maximum and minimum air temperatureminimum air temperature

• Predict/estimate dewpoint temperature from Predict/estimate dewpoint temperature from minimum air temperatureminimum air temperature

TTdewdew = T = Tminmin – K – Koo

where Kwhere Koo ~ 2 – 4 ~ 2 – 4 °C in dry climates, and°C in dry climates, and ~ 0 ~ 0 °C in humid climates°C in humid climates


Solar RadiationSolar Radiation

• Estimate solar radiation fromEstimate solar radiation from• a regional station, or,a regional station, or,• from max/min tempsfrom max/min temps


Wind SpeedWind Speed

• Use data from a nearby stationUse data from a nearby station• Estimate mean monthly wind speedEstimate mean monthly wind speed

DescriptionDescription Mean Wind Speed ( at 2 m) Mean Wind Speed ( at 2 m)

light windslight winds 1.0 m s 1.0 m s-1-1

light to moderate windslight to moderate winds 1-3 m s1-3 m s-1-1

moderate to strong windsmoderate to strong winds3-5 m s3-5 m s-1-1

strong windsstrong winds 5 m s 5 m s-1-1

(adapted from FAO-56)(adapted from FAO-56)



• Minimum data requirements are Minimum data requirements are max/min air temperaturemax/min air temperature

• must be representative of, or measured must be representative of, or measured in an irrigated areain an irrigated area

• Using data from nearby stationsUsing data from nearby stations• climate conditions, physiographic climate conditions, physiographic

features, etc. at both locations should be features, etc. at both locations should be similar, i.e., region should be similar, i.e., region should be homogeneoushomogeneous



• Validate at regional level by comparing Validate at regional level by comparing reference ET calculated using a full data reference ET calculated using a full data set and a limited/estimated data set set and a limited/estimated data set

• Not recommended for daily estimates, Not recommended for daily estimates, better suited for longer interval (10-better suited for longer interval (10-days,monthly)days,monthly)

• Best reserved for filling in intervals of Best reserved for filling in intervals of missing data or data of suspect quality at missing data or data of suspect quality at sites where all variables are measuredsites where all variables are measured


CalibrationCalibration


Why Consider Calibration?Why Consider Calibration?

• Period of record of electronic weather Period of record of electronic weather station (EWS) data may be limitedstation (EWS) data may be limited

• Period of record at NOAA Coop Observer Period of record at NOAA Coop Observer network (max/min/precip) stations often network (max/min/precip) stations often much longermuch longer

• With minimum 3 years of overlapping With minimum 3 years of overlapping record (better if 5-7 years) it is desirable to record (better if 5-7 years) it is desirable to calibrate the less data-intensive methods calibrate the less data-intensive methods to compute more accurate historical crop to compute more accurate historical crop ET estimatesET estimates


ApproachApproach• Compute calibration coefficients for Compute calibration coefficients for

some specific time interval during some specific time interval during growing season (10-days, monthly)growing season (10-days, monthly)

Penman Crop ETPenman Crop ETCalibration coeff. = Calibration coeff. =

Crop ET by method to Crop ET by method to be be calibratedcalibrated

• Compute average values for Compute average values for overlapping period of recordoverlapping period of record



• Calibration coefficients should be Calibration coefficients should be computed by pairing each computed by pairing each individual NOAA station with an individual NOAA station with an electronic weather stationelectronic weather station

• Extent of areal representation of Extent of areal representation of calibration coefficients limited by calibration coefficients limited by that of the EWS datathat of the EWS data


Coefficients for one EWS-NOAA station pair Coefficients for one EWS-NOAA station pair generally not applicable at other NOAA stations generally not applicable at other NOAA stations when conditions at the NOAA sites are dissimilarwhen conditions at the NOAA sites are dissimilar


breakbreak



• dual crop coefficient approachdual crop coefficient approach

ETETc) actualc) actual = (K = (Kss K Kcbcb + K + Kee ) ET ) ETrefref

KKss is a water stress coefficient used to account for is a water stress coefficient used to account for effects of water stress on crop transpirationeffects of water stress on crop transpiration

KKcbcb is the basal crop coefficient and is the ratio of crop is the basal crop coefficient and is the ratio of crop ET to reference ET when the soil is dry and the crop is ET to reference ET when the soil is dry and the crop is transpiring at potential ratestranspiring at potential rates

KKee is a coefficient for wet soil evaporation is a coefficient for wet soil evaporation

• use when daily crop ET estimates are neededuse when daily crop ET estimates are needed



• single crop coefficient approachsingle crop coefficient approach

ETETc) actualc) actual = K = Kss K Kcc ET ETrefref

KKss is a water stress coefficient used to account for effects of is a water stress coefficient used to account for effects of water stress on crop transpirationwater stress on crop transpiration

KKcc is “average” or “mean” crop coefficient incorporating is “average” or “mean” crop coefficient incorporating crop characteristics and averaged effects of soil evaporationcrop characteristics and averaged effects of soil evaporation

• for normal irrigation planning and management, for normal irrigation planning and management, hydrologic studies, etc., mean crop coefficients are hydrologic studies, etc., mean crop coefficients are applicable and easier to apply than the dual crop applicable and easier to apply than the dual crop coefficient approachcoefficient approach



• Alfalfa or grass reference basisAlfalfa or grass reference basis• Method specificMethod specific• Geographical transferabilityGeographical transferability


Crop CoefficientsCrop Coefficients• Alfalfa or grass reference basisAlfalfa or grass reference basis

• Crop coefficients for the two references are not Crop coefficients for the two references are not interchangeable without adjustmentinterchangeable without adjustment

• ASCE Manual 70 used a ratio of 1.15 for alfalfa to ASCE Manual 70 used a ratio of 1.15 for alfalfa to grass reference ET to allow the extensive grass reference ET to allow the extensive comparisons between methods and lysimeter sitescomparisons between methods and lysimeter sites

• More recent work (Wright et al. 2000) indicates this More recent work (Wright et al. 2000) indicates this ratio is climate, season and location dependent and ratio is climate, season and location dependent and should be determined on at least a monthly basisshould be determined on at least a monthly basis

• Method specificMethod specific• Geographical transferabilityGeographical transferability


Crop CoefficientsCrop Coefficients• Alfalfa reference basisAlfalfa reference basis

• ASCE Manual 70 Table 6.6 basal crop coefficientsASCE Manual 70 Table 6.6 basal crop coefficients• ASCE Manual 70 Table 6.9 “mean” crop ASCE Manual 70 Table 6.9 “mean” crop

coefficientscoefficients

• Grass reference basisGrass reference basis• FAO 56: Crop Evapotranspiration, Guidelines for FAO 56: Crop Evapotranspiration, Guidelines for

Computing Crop Water RequirementsComputing Crop Water Requirements

• Method specificMethod specific• Geographical transferabilityGeographical transferability



• Alfalfa or grass reference basisAlfalfa or grass reference basis

• Method specificMethod specific• Generally thought that KGenerally thought that Kcc values developed for values developed for

one method can be used with another method one method can be used with another method without adjustment, as long as the reference without adjustment, as long as the reference basis is the same and the two methods produce basis is the same and the two methods produce equivalent reference ET valuesequivalent reference ET values

• ASCE Standardized P-M method with a fixed crop ASCE Standardized P-M method with a fixed crop height yields different alfalfa reference ET values height yields different alfalfa reference ET values than the 1982 Kimberly Penmanthan the 1982 Kimberly Penman

• Geographical transferabilityGeographical transferability


0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

Ratio

3 4 5 6 7 8 9 10Month

1.21

1.12

1.02

0.97 0.971.01

1.08

1.18

Kimberly, Idaho 1966 - 1998Ratio of ASCE PM ETr to 1982 Kimb. Pen


Monthly ratios of 1982 Kimberly Penman to ASCE Monthly ratios of 1982 Kimberly Penman to ASCE standardized Penman-Monteith alfalfa reference ET standardized Penman-Monteith alfalfa reference ET (adapted from Allen, 2001 unpublished paper).(adapted from Allen, 2001 unpublished paper).

MonthMonth

VinelandVineland1994-20001994-2000

AvondaleAvondale1994-20001994-2000

Rocky FordRocky Ford1993-20001993-2000

MarMar 0.770.77 0.730.73 0.760.76

AprApr 0.850.85 0.810.81 0.850.85

MayMay 0.920.92 0.900.90 0.940.94

JunJun 1.011.01 0.980.98 1.001.00

JulJul 1.051.05 1.001.00 1.031.03

AugAug 1.051.05 0.980.98 1.011.01

SepSep 0.950.95 0.900.90 0.950.95

OctOct 0.840.84 0.810.81 0.850.85



• On a seasonal basis differences On a seasonal basis differences between the two methods “average between the two methods “average out” out”

• On a monthly basis the differences are On a monthly basis the differences are large enough to warrant adjustment of large enough to warrant adjustment of the 1982 Kimberly Penman coefficients the 1982 Kimberly Penman coefficients prior to use with the ASCE std P-Mprior to use with the ASCE std P-M

• Allen and Wright (2002) conversionAllen and Wright (2002) conversion


Crop CoefficientsCrop Coefficients• Alfalfa or grass reference basisAlfalfa or grass reference basis• Method specificMethod specific

• Geographical transferabilityGeographical transferability• ET of well-watered crops is mainly dependent on ET of well-watered crops is mainly dependent on

available energyavailable energy• Requires assessment of, and often adjustment for Requires assessment of, and often adjustment for

differences in growing period conditions between differences in growing period conditions between development and application sitesdevelopment and application sites

• Transferability of “mean” crop coefficients requires Transferability of “mean” crop coefficients requires assessment of irrigation and rainfall patternsassessment of irrigation and rainfall patterns

• Climate differences between sites; primarily wind, Climate differences between sites; primarily wind, humidity and advection considerations impact humidity and advection considerations impact transferability transferability


Weather Data ConsiderationsWeather Data Considerations

• Detailed weather data requirementsDetailed weather data requirements• solar radiationsolar radiation• air temperatureair temperature• relative humidityrelative humidity• wind speed at 2 mwind speed at 2 m

• Weather data qualityWeather data quality• Data collection environmentData collection environment• Weather station location and densityWeather station location and density


Solar radiationSolar radiation

Air temperatureAir temperature

Relative Relative humidityhumidity

Wind speedWind speed

Wind Wind directiondirection

RainfallRainfall



• Detailed weather data requirementsDetailed weather data requirements

• Weather data qualityWeather data quality• All data need quality assessmentAll data need quality assessment• Detailed QA/QC procedures available Detailed QA/QC procedures available

(e.g. EWRI, 2002; Allen, 1996)(e.g. EWRI, 2002; Allen, 1996)

• Data collection environmentData collection environment• Weather station location and densityWeather station location and density


Original data plotted Original data plotted with clear sky solar with clear sky solar radiation enveloperadiation envelope

Calibration correction Calibration correction factor = 1.155factor = 1.155

““Re-calibrated” data Re-calibrated” data plotted with clear sky plotted with clear sky solar radiation solar radiation envelopeenvelope


RH sensor degradationRH sensor degradation

Used periods of good RH data to develop regression Used periods of good RH data to develop regression relationship of dewpoint temperature with daily relationship of dewpoint temperature with daily minimum temperature. Then used regression to minimum temperature. Then used regression to estimate dewpoint temperature during periods of poor estimate dewpoint temperature during periods of poor RH sensor performance.RH sensor performance.


Sheltering of weather station by corn crop causing Sheltering of weather station by corn crop causing low measured daily wind run values (days 190 – low measured daily wind run values (days 190 – 280)280)



• Detailed weather data requirementsDetailed weather data requirements• Weather data qualityWeather data quality

• Data collection environmentData collection environment• Weather data intended for reference ET Weather data intended for reference ET

estimation should be collected at weather estimation should be collected at weather stations sited over well-watered, clipped stations sited over well-watered, clipped green grass surfaces in open, irrigated green grass surfaces in open, irrigated settingssettings

• Green, irrigated fetch in the primary wind Green, irrigated fetch in the primary wind directiondirection

• Weather station location and densityWeather station location and density



• Detailed weather data requirementsDetailed weather data requirements• Weather data qualityWeather data quality• Data collection environmentData collection environment

• Weather station location and densityWeather station location and density• Various studies suggest weather station Various studies suggest weather station

spacing of 20-40 miles to maintain 0.90 spacing of 20-40 miles to maintain 0.90 spatial cross-correlation for reference ET spatial cross-correlation for reference ET estimatesestimates

• Highly dependent on topography, Highly dependent on topography, prevailing weather patterns, etcprevailing weather patterns, etc..


SummarySummary

• ET is key component to determining ET is key component to determining irrigation water requirementsirrigation water requirements

• Most direct methods have limited Most direct methods have limited practical applicationpractical application

• Climate based ET estimationClimate based ET estimation• Penman-based ET methods:Penman-based ET methods:

• carefully screened, good quality weather data,carefully screened, good quality weather data,• collected under irrigated reference conditions,collected under irrigated reference conditions,• spatially representative of the area of interestspatially representative of the area of interest

• Crop coefficientsCrop coefficients

evapotranspiration concepts and irrigation water requirements

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