4.18 The FAO-56 Penman-Monteith method

The FAO-56 PM method is recommended as the standard method for estimating ETo in case of locations where measured lysimeter data is not available. The equation for the estimation of daily ETo can be written as [3]:

$$\rm ET\_o = \frac{0.408\Delta (R\_n - G) + \chi \frac{900}{T + 2\Im 3} \, W\_s (e\_s - e\_d)}{\Delta + \chi (1 + 0.34 \, W\_s)} \tag{23}$$

where ETo = reference evapotranspiration calculated by FAO-56 PM method (mm day�<sup>1</sup> ); Rn = daily net solar radiation (MJ m�<sup>2</sup> day�<sup>1</sup> ); γ = psychrometric constant (kPa <sup>o</sup> C�<sup>1</sup> ); Δ = slope of saturation vapor pressure versus air temperature curve (kPa <sup>o</sup> C�<sup>1</sup> ); es and ea = saturation and actual vapor pressure (kPa), respectively; T = average daily air temperature (°C); G = soil heat flux (MJ m�<sup>2</sup> day�<sup>1</sup> ); Ws = daily mean wind speed (m s�<sup>1</sup> ).

The ETo values obtained from above equation are used as target data in ANN due to unavailability of lysimeter measured values.
