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Lecture (12)

Lecture (12). The Combination Methods Initially developed by Penman (1948)who combined the creodynamic and energy budget methods to compute ET o . The method has been modified by many researchers. Doorenbos and Pruitt Equation ET o = WQ n + (1-W) (e s – e a ) f (u)

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Lecture (12)

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  1. 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 (1+u/100) ; where u is the average wind speed (km/day)

  2. 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) WhereGiis 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

  3. Computation of the Net Solar Radiation: (Qn)

  4. 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

  5. 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)

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