Module 3 Evaporation

8/11/2019 Module 3 Evaporation

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Lecture Notes Hydrology

Department of Civil EngineeringSchool of Engineering, University of Nairobi

Dulo S.O

E-mail: [email protected]


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Department of Civil EngineeringSchool of Engineering, University of Nairobi

Evaporation is a type of vaporization ofa liquid that occurs only on the surface ofa liquid.
http://en.wikipedia.org/wiki/Vaporizationhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Interface_(chemistry)http://en.wikipedia.org/wiki/Interface_(chemistry)http://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Vaporization


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Evaporation

Energy Balance MethodConsider the evaporationfrom an evaporation pan

An evaporation pan is acircular tank containingwater.

The rate of evaporation ismeasured by the rate of fall of

water surface.

A control surface is drawnaround the pan enclosingboth the water in the pan and

the air above it.

Evaporation Pan


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Evaporation Measurement

Class A PanEvaporation Pan Method, E = Kp.EpE = Evaporation Rate (mm/day)Kp = Pan CoefficientEp = Measured Evaporation (mm/day)


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Evaporation

Aerodynamic Method

Evaporation from an open water surface


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EvaporationSummary of Equations

Energy Balance Method

Aerodynamic Method

When)daymm(R.E nr 03530

)mW(RadiationNetRn 2

)daymm()ee(BE aasa

)Pa.daymm(

)]z

z

[ln(

u.B2

0

2

21020

Where

)Pa()T.

T.exp(e as 3237

2717

611

)Pa(eRe asha


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Evaporation

Combined Method

Priestley-Taylor Method

Summary of Equations

)daymm

(EEE ar

)C

Pa()T.(

eo

as23237

4098

)

C

Pa(. o866

r E)(

31 .

Evaporation PanMethod

pp EKE


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Evapotranspiration

The term Evapotranspiration combines two words1) Evaporation of water from the soil2) Transpiration of water from plants into the air

Evapotranspiration means the total loss of water from acrop into the air

Water evaporates from any moistsurface into the air unless the air issaturated Water surfaces incontact with air, such as lakes,plant leaves, and moist soils, all

evaporate water


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Evapotranspiration

Evaporation from land surface comprised1) evaporation directly from soil

and vegetation surface2) transpiration through leaves

The processes of evaporation from land surfaceand transpiration from vegetation are collectivelytermed Evapotranspiration .

Evapotranspiration is influenced by

1) the supply of energy to provide the latent heat ofvaporization Energy Supply2) the ability to transport the vapor away from the

evaporative surface Vapor Transport3) the supply of moisture at the evaporative surface

Moisture


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Evapotranspiration

Crop water is important because it determines how muchwater must be provided by irrigation or rain

-If there is too little water, the crop yield can diminish-If there is too much irrigation, then it will waste

energy,water, and nutrients and unnecessarily deplete theaquifer


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Reference Crop ET

Calculations of the rate of evaporatranspiration

For given climatic conditions, the basic rate is the reference cropevapotranspiration , this being the rate of evapotranspirationfrom an extensive surface of 8-15 cm tall green grass cover of

uniform height, actively growing, completely shading the groundand not short of water

Reference Crop Evapotranspiration Equations:1) Penman Equation

2) Penman-Monteith Equation3) FAO-Penman Monteith Equation4) Doorenbos-Pruitt Equation5) Bladney-Criddle Equation6) others

oET

Climatic conditions are given


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Actual ET and Reference crop ET o

The potential evapotranspiration of another crop growing underthe same conditions as the reference crop is calculated bymultiplying the reference crop evapotranspiration, ETo by a cropcoefficient, kc .

The value of kc changes with the stage of growth of the crop.

The actual evopotranspiration, Et is found by multiplying thepotential evapotranspiration by a soil coefficient, ks (0 ks 1 )

Potential ET

Actual ET

The values of the crop coefficient, kc vary over a range of about0.2 kc 1.3

ocsc ETk k ETocp ETk ET


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Actual ET

ETo

ETp

ETc

Reference CropEvapotranspiration

PotentialEvapotranspiration

ActualEvapotranspiration


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ETo Measurement

Kc=ETc/ETo

Crop ET

Reference ET

Crop ET vs Reference ET


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ETo Measurement

Lysimeter


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Developing Crop Coefficient

Evapotranspiration (ET) is an energydriven process.ET increases with temperature, solarradiation and wind.

ET decreases with increasing humidity.

Humidity

Temperature

Measure climatic conditions and calculate ETo.

Developing crop coefficients todetermine actual crop ET


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Measure ETc for the crop stage of growth.


Crop of Interest

WeightingLysimeter

Early



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Measure ETc for the crop stage ofgrowth.

WeightingLysimeter

PhysiologicallyMature




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Example

The monthly values of reference crop evapotranspirationEtr, calculated using the combination method foraverage condition in Silistra, Bulgaria.

Month Apr May Jun Jul Aug Sep Oct Apr-Oct total

ETo (mm/day) 4.14 5.45 5.82 6.60 5.94 4.05 2.34 34.3 mm

Kc 0.38 0.38 0.69 1.00 1.00 0.78 0.55

ET (mm/day) 1.57 2.07 4.02 6.60 5.94 3.16 1.29 24.7 mm

Crop coefficient for corn, k1=0.38, k2=1.00, k3=0.55, t1=Apr, t2=Jun,t3=Jul,t4=Sep, t5=OctCalculate the actual evapotranspiration from this crop assuming awell-watered soil.

t1 t2 t3 t4 t5


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Example

The Crop water use for maize in selected studydistricts of Kenya.

District ETo(mm)

ETmax(mm)

Kiambu 492.4 329.9Makueni 565.0 378.6

Vihiga 495.0 331.7

Kwale 662.8 444.0

Migori 588.3 394.1

Laikipia 476.2 319.0


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Example

1-Initial stage (less than 10% ground cover).2-Development state (from initial stage to attainment

of effective full ground cover (70-80%)).3-Mid-season stage (from full ground cover to

maturation).

4-Late season state (full maturity and harvest).

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Module 3 Evaporation

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