ESTIMATION OF EVAPOTRANSPIRATION USING MODIFIED PENMAN METHOD

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ESTIMATION OF EVAPOTRANSPIRATION USING MODIFIED
PENMAN METHOD

• BY
• R.MADHANA SUPRIYA

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ESTIMATING EVAPOTRANSPIRATION FROM CLIMATOLOGICAL
DATA

• Due to difficultly in obtaining accurate direct
  measurement of pan evaporation under field
  conditions, ET is predicted on the basis of
  climatological data.

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FORMULAE IN ESTIMATING ET

• Blaney-Cridle method
• Thornthwaite method
• Penman method
• Christiansen method

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Modified penman method

• Based on
• Intensive studies of the climatic data,
• Measured grass evapotranspiration data.
• Required values from weather station
• wind velocity.
• temperature.
• humidity.
• sunshine.

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• Modified Penman Formula
• ET0 W.Rn (1-W).f(u).(ea-ed)
• in which,
• W.Rn Radiation term
• (1-W).f(u).(ea-ed) Aerodynamic term

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• ET0 the reference crop ET in mm/day.
• ea saturation vapour pressure in mbar at the
  mean air temperature in 0C.
• ed mean actual vapour pressure of the air in
  mbar.
• F(u) a wind related function.
• (1-W) a temperature and elevation related
  weighting factor for the effect of wind and
  humanity on ET0.
• W weighing factor for radiation on ET0.
• Rn net radiation

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• ed ea x RH mean /100
• RH relative humidity, determined by dry and
  wet bulb temperature.
• Rn Rns Rnl
• Rns net incoming shortwave solar radiation
• Where Rns Ra(1-a)(0.250.50n/N)
• Rnl the net longwave radiation
• Where Rnl f(t).f(ed).f(n/N)