For the estimation of Penman-Monteith ETo computer model CROPWAT 8.0 was applied which was suggested by FAO (Food and Agriculture organization). The input metrological data needed was minimum and maximum atmospheric temperature, relative humidity, wind speed and sunshine hour’s data. The monthly ETo was estimated by applying computer model (FAO CROPWAT 8.0, 2009) the following PM method applied as recommended by (Allen et al., 1998):
Where, ETo = reference evapotranspiration (mm day-1); Rn = net radiation at the crop surface (MJm-2 /day); G =soil heat flux density (MJ m2/day); T = mean daily air temperature at 2 m height (°C); U2 = wind speed at 2 m height (m/s); es =saturation vapour pressure (kPa); ea = actual vapour pressure (kPa); es-ea = saturation vapour pressure deficit (kPa); ? = slope vapour pressure curve (kPa/ °C); ? is the psychometric constant (kPa (°C)?1) The computation of all data required for the calculation of the ETo followed the method of (Allen et al., 1998).