The Combination Methods

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Lecture (12)

• The Combination Methods
• Initially developed by Penman (1948)who combined
  the creodynamic and energy budget methods to
  compute ETo. The method has been modified by many
  researchers.
• Doorenbos and Pruitt Equation
• ETo WQn (1-W) (es ea) f (u)
• W a temperature and elevation related
  weighting factor (table 4-1)
• es and ea saturated and air vapor pressures at
  mean air temperature (mb)
• f (u) a wind related fn 0.27 (1u/100)
  where u is the average wind
  speed (km/day)

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• ea es RH
• RH relative humidity
• T mean air temperature, ºC
• FAO Penman Monteith Equation
• G Soil heat flux Gi 0.057 (MTi MTi-1)
• Where Gi is soil heat flux during month i , MTi
  is the mean air temperature of month I and MTi-1
  is the mean air temperature of the previous month

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• Computation of the Net Solar Radiation (Qn)

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• The Empirical Methods
• Much simpler but less accurate than the above 3
  methods
• They are based on one or more of the following
  parameters
• (Ta , RH, U, Qn , ea, .. etc)
• They are usually used when data need for modified
  Penman equation is not available
• Two empirical methods are addressed here
• a. Blany Griddle (Temperature) method
• b. Pan Evaporation method

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• a. Bland Griddle method
• ETO C P (0.46 Tm 8.13) mm/day
• C adjustment factor figure (4-2)
• P mean daily hours as a percentage of the
  total /annual day time hours table
  (4-8)
• Approximate values for RH min, n/N and U may be
  used id fingure (4-2)
• b. Pan Evaporation Method
• Class A
• - 121cm in diameter - 255 cm deep
• - Galvanized iron
• - Mounted on wooden frame
• - Bottom is 15 cm above ground
• ETo Kp Ep
• Ep measured on daily basis
• Kp from table (4-9)