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) W = a temperature and elevation related weighting factor (table 4-1) e s and e a = 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)

e a = e s × RH RH = relative humidity T = mean air temperature, º C FAO Penman – Monteith Equation G = Soil heat flux; G i = (MT i – MT i-1 ) Where G i is soil heat flux during month i, MT i is the mean air temperature of month I and MT i-1 is the mean air temperature of the previous month

Computation of the Net Solar Radiation: (Q n )

The Empirical Methods Much simpler but less accurate than the above 3 methods They are based on one or more of the following parameters: (T a, RH, U, Q n, e a,.. 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

a. Bland – Griddle method ET O = C [P (0.46 T m )] 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 cm deep - Galvanized iron - Mounted on wooden frame - Bottom is 15 cm above ground ET o = K p E p E p = measured on daily basis K p = from table (4-9)