Factors affecting evapotranspiration

several factors affecting evapotranspiration...

1. Factors affecting
evapotranspiration

2. FACTORS AFFECTING
EVAPOTRANSPIRATION
Weatherparameters,cropcharacteristics,management and
environmentalaspectsarefactorsaffectingevaporationandtranspiration.
TherelatedETconceptspresentedinFigure(onnextslide)arediscussed
inthesectiononevapotranspirationconcepts.

4. WEATHER PARAMETERS
Theprincipalweatherparametersaffectingevapotranspirationare:
1.Radiation
2.Airtemperature
3.Humidity
4.Windspeed.
Severalprocedureshavebeendevelopedtoassesstheevaporation
ratefromtheseparameters.Theevaporationpoweroftheatmosphere
isexpressedbythereferencecropevapotranspiration(ET
o).The
referencecropevapotranspirationrepresentstheevapotranspiration
fromastandardizedvegetatedsurface.TheET
oisdescribedindetail
laterinthisChapterandinChapters2and4.

5. CROP FACTORS
Cropfactorsinclude:
1.Thecroptype
2.Variety
3.Developmentstage
Thesefactorsshouldbeconsideredwhenassessingthe
evapotranspirationfromcropsgrowninlarge,well-managedfields.
Differencesinresistancetotranspiration,cropheight,croproughness,
reflection,groundcoverandcroprootingcharacteristicsresultin
differentETlevelsindifferenttypesofcropsunderidentical
environmentalconditions.
Cropevapotranspirationunderstandardconditions(ET
c)referstothe
evaporatingdemandfromcropswhicharegrowninlargefieldsunder
optimumsoilwater,excellentmanagementandenvironmentalconditions,
andachievefullproductionunderthegivenclimaticconditions.

6. MANAGEMENT AND
ENVIRONMENTAL CONDITIONS
Factorssuchassoilsalinity,poorlandfertility,limitedapplicationof
fertilizers,thepresenceofhardorimpenetrablesoilhorizons,the
absenceofcontrolofdiseasesandpestsandpoorsoilmanagementmay
limitthecropdevelopmentandreducetheevapotranspiration.Other
factorstobeconsideredwhenassessingETaregroundcover,plant
densityandthesoilwatercontent.TheeffectofsoilwatercontentonETis
conditionedprimarilybythemagnitudeofthewaterdeficitandthetype
ofsoil.Ontheotherhand,toomuchwaterwillresultinwaterlogging
whichmightdamagetherootandlimitrootwateruptakebyinhibiting
respiration.

7. CONTINUE…
WhenassessingtheETrate,additionalconsiderationshouldbegivento
therangeofmanagementpracticesthatactontheclimaticandcrop
factorsaffectingtheETprocess.Cultivationpracticesandthetypeof
irrigationmethodcanalterthemicroclimate,affectthecrop
characteristicsoraffectthewettingofthesoilandcropsurface.A
windbreakreduceswindvelocitiesanddecreasestheETrateofthefield
directlybeyondthebarrier.Theeffectcanbesignificantespeciallyin
windy,warmanddryconditionsalthoughevapotranspirationfromthe
treesthemselvesmayoffsetanyreductioninthefield.Soilevaporationin
ayoungorchard,wheretreesarewidelyspaced,canbereducedbyusing
awell-designeddriportrickleirrigationsystem.Thedrippersapply
waterdirectlytothesoilneartrees,therebyleavingthemajorpartofthe
soilsurfacedry,andlimitingtheevaporationlosses.Theuseofmulches,
especiallywhenthecropissmall,isanotherwayofsubstantiallyreducing
soilevaporation.Anti-transpirants,suchasstomata-closing,film-forming
orreflectingmaterial,reducethewaterlossesfromthecropandhence
thetranspirationrate.