Evapotranspiration-ET- definition and measurement

Et

1. Evapotranspiration

2. Evapotranspiration
•Evapotranspiration(ET=Evaporation+Transpiration)
Evaporation
•Evaporationisadiffusiveprocessbywhichwaterfromnaturalsurfaces,
suchasfreewatersurface,baresoil,fromliveordeadvegetationfoliage
(interceptedwater,dewfalletc)islostintheformofvapourtothe
atmosphere.Itisoneofthebasiccomponentsofhydrologiccycle.
Transpiration
•Transpirationisaprocessbywhichwaterislostintheformofvapour
throughplantsurfaces,particularlyleaves.Inthisprocesswateris
essentiallyabsorbedbytheplantrootsduetowaterpotentialgradients
anditmovesupwardthroughthestemandisultimatelylostintothe
atmospherethroughnumerousminutestomataintheplantleaves

4. Evapotranspirationprocess
•Thus,evapotranspirationisacombinedlossofwaterfromthesoil(evaporation)
andplant(transpiration)surfacestotheatmospherethroughvaporizationofliquid
water,andisexpressedindepthperunittime(forexamplemm/day).
Quantificationofevapotranspirationisrequiredinthecontextofmanyissues:
1.Managementofwaterresourcesinagriculture
2.Designingofirrigationprojectsonsoundeconomicbasis
3.Fixingcroppingpatternsandworkingouttheirrigationrequirementsofcrops
4.Schedulingofirritations
5.Classifyingregionsclimatologicallyforagriculture

5. Factors influencing evapotranspiration

6. Climatic or environmental factors
The meteorological factors, which have a significant bearing on the ET,
a) Solar radiation: Solar or thermal energy is necessary to evaporate the water from
both soil and plant surfaces. Thus, of the total solar radiation intercepted by the
leaf, only 1to 5% is used for photosynthesis and 75 to 85% is used for radiating the
canopy surface i.e., leaves and for transpiration. Hence, increased solar radiation
increases atmospheric demand and in turn evapotranspiration.
b) Ambient temperature: increasing the temperature increases the capacity of air to
hold water i.e., vapour pressure deficit is high, which means a greater atmospheric
demand i.e., greater ET.
c)Relative humidity: The greater the water content of air, the higher the air water
potential which means that atmospheric demand decreases resulting in low ET
with increasing relative humidity.

7. Climaticfactorsinfluencingevapotranspiration…………………
d)Wind:Whenwindturbulenceremovesthewatervapourintheleaf,thedifference
inwaterpotentialinsideandimmediatelyoutsidethestomatalopeningis
increasedandnetwaterdiffusionfromtheleafisincreased.
e)Soilmoisture:GenerallyspeakingtheETfromacroppedfieldwillincreasewith
increaseinsoilwatercontentduetoenhancedsoilevaporation.However,the
effectofsoilwatercontentonevapotranspirationvarieswithcropandis
conditionedprimarilybytypeofsoilanditswaterholdingproperties,croprooting
characteristicsandthemeteorologicalconditionsdeterminingthelevelof
evapotranspiration.

8. Managementfactors
ThefollowingmanagementfactorsinfluencetheETcrop.
a)Methodofirrigation:Differentmethodsimplydifferentratesofwaterapplication
duetolargevariationsinthewaterbalancecomponents.Theadvantagesofone
methodoveranotherarethereforedeterminedbydifferencesintotalirrigation
waterappliedandtheeffectivenesswithwhichcroprequirementscanbemet.
Irrigationwaterrequirementsaregenerallyhigherwithsurfaceirrigationmethods
viz.,furrow,borderstrip,checkbasinetcincomparisontooverheadsprinkler,
microsprinkleranddripmethodsofirrigation.
b)Fertilizers:TheuseoffertilizershasonlyalittleeffectonETcrop,unlesscropgrowth
waspreviouslyadverselyaffectedbylowsoilnutritiondelayingfullcropcanopy
cover.Irrigationimposesagreaterdemandonfertilizernutrients.Adequately
fertilizedsoilsproducemuchhigheryieldsperunitofirrigationwaterthandopoor
soils,providedthefertilizerisatthelevelinthesoilprofilewheresoilwateris
extractedbytheplant.Themovementofsolublenutrientsandtheiravailabilityto
thecropishighlydependentonmethodandfrequencyofirrigation.

9. c)Windbreaks:ArtificialandvegetativewindbreaksReducewindvelocities
downwardfromit.ThismayreduceETcropbyabout5%to30%underwindy,
warm,dryconditionsdependingonthehorizontaldownwarddistancefromthe
windbreak.However,inmostcasesshrubsandtreesareusedand,dueto
transpirationofthevegetativebarrier,overallETcropmaybemore.
d)Anti-transpirants:Theuseofanti-transpirants,naturalorartificiallyinduced
variationsinplantfoliagepropertiesandsoilconditionerstoreduceETcropare
foundtobeeffective,butcheapandpracticallysuitablematerialsforfield
applicationareyetbeidentified.
e)Salinity:TheETcropcanbeaffectedbysoilsalinitysincethesoilwateruptakeby
theplantcanbedrasticallyreducedduetohigherosmoticpotentialofsaline
water.Reducedwateruptakeundersalineconditionsisshownbysymptoms
similartothosecausedbydrought,suchasearlywilting,leafburning,abluish-
greencolourinsomeplants,reducedgrowthandsmallleaves.Thesamelevelof
soilsalinitycancausemoredamageunderhighthanunderlowevaporative
conditions.Thenegativeeffectofsoilsalinitycanbepartlyoffsetbymaintaininga
highersoilwaterregimeintherootzonetomaintaintheoptimalETcropvalues.

10. Consumptiveuse
•Thetermconsumptiveuse(Cu)isusedtodesignatethesumoflossesdueto
evaporation+transpirationfromthecroppedfieldaswellasthatwaterutilizedbythe
plantsinitsmetabolicactivitiesforbuildingupoftheplanttissues.Sincethewater
usedintheactualmetabolicprocessesisinsignificant(about1%ofevapotranspiration
losses)thetermconsumptiveuseisgenerallytakenequivalenttoevapotranspiration.
ItisexpressedsimilartoETasdepthofwaterperunittimei.e.,mm/dayorcm/day.
a)Dailyconsumptiveuse:ItisthetotalamountofthewaterusedinETpluswaterused
inmetabolicactivitiesbyacropduringasingledayor24-hoursperiodandis
expressedinmm/dayorcm/day.
b)Seasonalconsumptiveuse:ThetotalamountofwaterusedbythecropinETand
metabolicactivitiesforbuildingupofplanttissuesduringitstotalgrowingseason.It
isexpressedasdepthofwaterinmmorcmperseason.Seasonalconsumptiveuse
valuesareneededtoevaluateanddetermineseasonalirrigationwatersuppliesand
irrigatedcropacreages.
c)Peakperiodconsumptiveuse:TheaveragedailywateruseratesintermsofETplus
thatconsumedinmetabolicprocessduringthehighestconsumptiveuseperiod(6–
10days)oftheseasoniscalledpeakperiodconsumptiveuserate.Thisisthedesign
ratetobeusedinplanninganirrigationsystem.Thepeak-useconsumptiveperiod
generallyoccurswhenthecropisstartingtobuildupitsharvestableproduce,wherein
thecanopyareaismaximumandcapableofinterceptingmaximum
photosyntheticallyactiveradiationandatmosphericdemandishigh.

11. Potential evapotranspiration (PET)
•Penman (1948) introduced the concept of PET and he defined it as “the amount of
water transpired in a unit time by short green crop of uniform height, completely
covering the ground and never short of water”.
•PETcanbeestimatedbyseveraltechniquesviz.,lysimetricmethods,energy
balance,aerodynamicapproach,combinationofenergybalanceandempirical
formulaeetc.,andirrigation’scanbescheduledconvenientlybasedonthe
knowledgeofPETorwateruseratesofcropsovershorttimeintervalsofcrop
growth.

12. 1) Lysimeter
•By isolating the crop root zone from its environment and controlling the processes
that are difficult to measure, the different terms in the soil water balance equation
can be determined with greater accuracy. This is done in lysimeters where the crop
grows in isolated tanks filled with either disturbed or undisturbed soil (Fig.).

13. •Differenttypesoflysimetersvarywidelyintheaccuracywithwhichchangesinsoil
wateraredetected.Inprecisionweighinglysimeters,wherethewaterlossis
directlymeasuredbythechangeofmass,evapotranspirationcanbeobtainedwith
anaccuracyofafewhundredthsofamillimetre,andsmalltimeperiodssuchasan
hourcanbeconsidered.
•Innon-weighinglysimeterstheevapotranspirationforagiventimeperiodis
determinedbydeductingthedrainagewater,collectedatthebottomofthe
lysimeters,fromthetotalwaterinput.
•Arequirementoflysimetersisthatthevegetationbothinsideandimmediately
outsideofthelysimeterbeperfectlymatched(sameheightandleafareaindex).
Lysimetersprovidethedirectestimatesofwaterbalancecomponentsviz.,E,T,ET,
drainage,runoff,effectiverainfalletcinacroppedfieldagainstwhichother
methodscanbetestedandcalibrated.Aslysimetersaredifficultandexpensiveto
constructandastheiroperationandmaintenancerequirespecialcare,theiruseis
limitedtospecificresearchpurposes.

14. 2) Cumulative pan evaporation
•Earlier investigations have shown that transpiration of a crop is closely related to
free water evaporation from an open pan evaporimeter (Fig. 19.2). Thus, the open
pan evaporimeter being simple and as they incorporate the effects of all climatic
parameters into a single entity i.e., pan evaporation could be used as a guide for
scheduling irrigation’s to crops.

15. 3) Field experimental plots
•The seasonal water requirements are computed by adding measured quantities of
irrigation water, the effective rainfall received during the season, and the
contribution of moisture from the soil.Field water balance may be expressed by the
following relationship:
In which
WR = Seasonal water requirement (mm)
IR = Total irrigation water applied (mm)
ER = Seasonal effective rainfall (mm)
M
bi= Moisture percentage at the beginning of the season in the i
th
layer of the soil
M
ei= Moisture percentage at the end of the season in the i
th
layer of the soil
A
i= Apparent specific gravity of the i
th
layer of the soil
D
i= depth of the i
th
layer of the soil with in the root zone (mm)
n = number of soil layers in the root zone D.



n
i
ii
eibi
DA
MM
ERIRWR
1
..
100

16. 4) Water balance method:
•The water balance method, also called the inflow-outflow method, is suitable for
large areas (watersheds) over long periods. It is represented by the hydrological
equation:
•Precipitation = Evapotranspiration + surface runoff + sub-surface drainage + change
in soil water contents.
•The value of evapotranspiration is computed from the measured data.

17. •I. Definition of Evapotranspiration (ET)
•Crop water requirements, or evapotranspiration (ET), are determined by the climatic evaporative
potential, plant characteristics, and all of the factors that influence growth and development of
the crop.
•ET is a process in which water undergoes a phase change from liquid to vapor
•The phase change requires energy, involving both sensible and latent heat
•Sensible heat involves temperature changes, but latent heat involves phase changes at nearly
constant temperature
•It takes more energy per unit mass to convert water to vapor than to convert ice to water
•Almost all of the energy comes from solar radiation (directly & indirectly)
liquid
vapor
energy
energy

18. Energy balance equation:
R
n= ET + H + G
Where,
R
n= net solar radiation at the ground and plant surfaces
ET = latent head energy used in the evapotranspiration process
H = sensible heat transfer to the atmosphere
G = sensible heat transfer to (or from) the ground
R
nincludes short-wave and long wave radiation

19. •R
ncanbemeasuredusinganetradiometer,butthiskindofinstrumentisexpensive
(severalhundredtomorethanathousanddollars)
•Netradiometerscanalsobedifficulttocalibrateandmaintain
•Thus,R
noftenmustbeestimatedfromothermoreavailabledata
•Pyronometersarerelativelyinexpensiveandcanbecalibratedbasedonclear-sky
solarradiationvalues,whichareabout75%ofextraterrestrialsolarradiation(see
Lecture18)
•Mostpyronometershaveawhitecircular"dot"surroundedbyaflatblackregion,
pointingupwardtowardthesky
•Thetemperaturedifferentialbetweenthewhiteandblackregionsiscalibrated
againsttheincomingshort-waveradiation
•IfyoumeasureR
n,HandG,youcansolvetheaboveequationforET
•YoucanmeasureGdirectlyusingsoilheatfluxplates,buttheseareexpensiveand
mostlyusedinresearch(Gcanbeestimatedbasedonairtemperature)
•Netradiationisbasicallytheamountofincomingshortwaveradiationminusoutgoing
longwaveradiation
•Thisinvolvesweatherconditions(e.g.cloudiness)andthedeterminationofsurface
reflectance,oralbedo

20. •Surface conditions and crop types are factors in the magnitude of the albedo, α,
which is typically between 0.23 and 0.30 (the fraction that gets reflected)
•Incoming short wave radiation can be measured with a radiometer
•Common units for radiation are MJ/m
2
/day, or Langleys/day(this is "power per
unit surface area"; note that a J/s is a watt)
•A Langley is a calorie per cm
2
•0.0419 MJ/m
2
= 1.0 cal/cm
2
•ET is commonly given units of equivalent water depth (mm or inches) per day,
week, month or year
•For purposes of irrigation planning, design and management, computations are
usually made to determine reference evapotranspiration (ET
o)
•ET
ois multiplied by a crop coefficient (Kc) to determine the evapotranspiration of a
particular crop at a given growth stage

23. Evapotranspiration -Definition
Total evaporationfrom all water, soil, snow, ice,
vegetation, and other surfaces plus
transpiration
water becoming water vapor
consumptive use by plants

24. Processes
•Evaporationof precipitation intercepted by
plant surfaces
•Evaporation of moisture from plants through
transpiration
•Evaporation of moisture from soil (ground)
surface

25. Evaporation
•Evaporationistheprocesswherebyliquidwaterisconvertedtowatervapour
(vaporization)andremovedfromtheevaporatingsurface(vapourremoval).Water
evaporatesfromavarietyofsurfaces,suchaslakes,rivers,pavements,soilsandwet
vegetation.
•Energyisrequiredtochangethestateofthemoleculesofwaterfromliquidto
vapour.Directsolarradiationand,toalesserextent,theambienttemperatureof
theairprovidethisenergy.Thedrivingforcetoremovewatervapourfromthe
evaporatingsurfaceisthedifferencebetweenthewatervapourpressureatthe
evaporatingsurfaceandthatofthesurroundingatmosphere.Asevaporation
proceeds,thesurroundingairbecomesgraduallysaturatedandtheprocesswillslow
downandmightstopifthewetairisnottransferredtotheatmosphere.The
replacementofthesaturatedairwithdrierairdependsgreatlyonwindspeed.
Hence,solarradiation,airtemperature,airhumidityandwindspeedare
climatologicalparameterstoconsiderwhenassessingtheevaporationprocess.

26. Transpiration
•Transpirationconsistsofthevaporizationofliquidwatercontainedinplant
tissuesandthevapourremovaltotheatmosphere.Cropspredominately
losetheirwaterthroughstomata.Thesearesmallopeningsontheplant
leafthroughwhichgasesandwatervapourpass(Figure1).Thewater,
togetherwithsomenutrients,istakenupbytherootsandtransported
throughtheplant.Thevaporizationoccurswithintheleaf,namelyinthe
intercellularspaces,andthevapourexchangewiththeatmosphereis
controlledbythestomatalaperture.Nearlyallwatertakenupislostby
transpirationandonlyatinyfractionisusedwithintheplant.
•Transpiration,likedirectevaporation,dependsontheenergysupply,
vapourpressuregradientandwind.Hence,radiation,airtemperature,air
humidityandwindtermsshouldbeconsideredwhenassessing
transpiration.Thesoilwatercontentandtheabilityofthesoiltoconduct
watertotherootsalsodeterminethetranspirationrate,asdo
waterloggingandsoilwatersalinity.Thetranspirationrateisalso
influencedbycropcharacteristics,environmentalaspectsandcultivation
practices.Differentkindsofplantsmayhavedifferenttranspirationrates.
Notonlythetypeofcrop,butalsothecropdevelopment,environmentand
managementshouldbeconsideredwhenassessingtranspiration.

27. Evapotranspiration (ET)
•Evaporation and transpiration occur simultaneously and there is no
easy way of distinguishing between the two processes. Apart from
the water availability in the topsoil, the evaporation from a cropped
soil is mainly determined by the fraction of the solar radiation
reaching the soil surface. This fraction decreases over the growing
period as the crop develops and the crop canopy shades more and
more of the ground area. When the crop is small, water is
predominately lost by soil evaporation, but once the crop is well
developed and completely covers the soil, transpiration becomes
the main process. In Figure 2 the partitioning of evapotranspiration
into evaporation and transpiration is plotted in correspondence to
leaf area per unit surface of soil below it. At sowing nearly 100% of
ET comes from evaporation, while at full crop cover more than 90%
of ET comes from transpiration.

28. •Lysimeters
•By isolating the crop root zone from its environment and controlling the processes
that are difficult to measure, the different terms in the soil water balance equation
can be determined with greater accuracy. This is done in lysimeterswhere the crop
grows in isolated tanks filled with either disturbed or undisturbed soil. In precision
weighing lysimeters, where the water loss is directly measured by the change of
mass, evapotranspirationcan be obtained with an accuracy of a few hundredths of
a millimetre, and small time periods such as an hour can be considered. In non-
weighing lysimetersthe evapotranspirationfor a given time period is determined
by deducting the drainage water, collected at the bottom of the lysimeters, from
the total water input.
•A requirement of lysimetersis that the vegetation both inside and immediately
outside of the lysimeterbe perfectly matched (same height and leaf area index).
This requirement has historically not been closely adhered to in a majority of
lysimeterstudies and has resulted in severely erroneous and unrepresentative ET
c
and K
cdata.
•As lysimetersare difficult and expensive to construct and as their operation and
maintenance require special care, their use is limited to specific research
purposes.