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Evapo-Transpiration

and its Estimation

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Evapo-Transpiration and its Estimation

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method of Evapo transpiration

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Evapo –Transpiration data are frequently needed for,

I. Project planning

II. Designing of Irrigation schedule

Therefore scientist & specialist have found some methods to calculate

evapo –Transpiration (ET0)

There are lot f method to estimate Evapo-Transpiration

Blaney-criddle method

Radiation method

Modified penman method

Pan evaporation method

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To achieve satisfactory result the method required,

•The radiation method-humidity of climate

• Pan method (Pan evaporation method)-wind speed & air humidity

• blaney criddle method-air temperature

• penman method-wind

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Energybalance

Masstransfer

Penmanmethod

Therefore the penman method is also called as combination method

Penman method

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Standard

climatological record

Penman method

Standard climatological record

Temperature

Humidity

Sunshine

Wind speed

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Penman-Monteith Equation

Where,

Rn-the net radiation

G-the soil heat flux

(es - ea)- represents the vapour pressure deficit of the aira-the mean air density at constant pressure

cp-the specific heat of the air

rs and ra-the (bulk) surface and aerodynamic resistances.

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Aerodynamic resistance (ra)

The transfer of heat and water vapour from the evaporating surface into the air above thecanopy is determined by the aerodynamic resistance:

Where,

Ra- aerodynamic resistance [s m-1],

Zm- height of wind measurements [m],

Zh- height of humidity measurements [m],

D- zero plane displacement height [m],

Zom- roughness length governing momentum transfer [m],Zoh- roughness length governing transfer of heat and vapour [m],

k -von Karman's constant, 0.41 [-],

Uz- wind speed at height z [m s-1].

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(Bulk) surface resistance (rs)

The 'bulk' surface resistance describes the resistance of vapour flow through the

transpiring crop and evaporating soil surface. Where the vegetation does not completely

cover the soil.

Where,

rs -(bulk) surface resistance [s m-1],

rl -bulk stomatal resistance of the well-illuminated leaf [s m-1],

LAIactive -active (sunlit) leaf area index [m2 (leaf area) m-2 (soil surface)]

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What is FAO Penman method?

•As a new standard for reference evapo-transpiration.

FAO penman method provides standard to,

1) Compare the ETo at different period of year or other regions.

2) Be related the ETo of otherops.

Requirement penman method,

1) Air temperature

2) Humidity

3) Radiation

4) Wind speed data

For daily/weekly/monthly calculation.

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