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ESTIMATION OF EVAPOTRANSPIRATION USING MODIFIED PENMAN METHOD

ESTIMATION OF EVAPOTRANSPIRATION USING MODIFIED PENMAN METHOD. BY R.MADHANA SUPRIYA. ESTIMATING EVAPOTRANSPIRATION FROM CLIMATOLOGICAL DATA.

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ESTIMATION OF EVAPOTRANSPIRATION USING MODIFIED PENMAN METHOD

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  1. ESTIMATION OF EVAPOTRANSPIRATION USING MODIFIED PENMAN METHOD BY R.MADHANA SUPRIYA

  2. 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.

  3. FORMULAE IN ESTIMATING ET • Blaney-Cridle method • Thornthwaite method • Penman method • Christiansen method

  4. 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.

  5. 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

  6. 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

  7. 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)(0.25+0.50n/N) Rnl – the net longwave radiation Where Rnl = f(t).f(ed).f(n/N)

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