2.3 Calculation of Reference Evapotranspiration
The measured ETref datasets were unavailable in Bangladesh, so, we use the FAO-56 Penman-Monteith (PM) method to compute ETref, that is highly acknowledged and widespread exercise when ETref calculations are unavailable. This method is suggested by FAO and many other studies i.e. Allen et al. (1998) and Feng et al. (2017). FAO suggested this method as the ideal tool to calculate ETref, which was performed fit for different climate settings and time step computations without any calibration and validation (Valiantzas 2013; Feng et al. 2017; Islam et al. 2019). The PM method is expressed in the following Eq. (1)-
\(\text{ET}_{o}=\frac{0.408(R_{n}-G)+\gamma\frac{900}{\left(T+273\right)}u_{2}\left(e_{s}-e_{a}\right)}{+\gamma(1+0.34u_{2})}\)……… …………………………. ……………(1)
Where ETref is reference evapotranspiration (mm d1); Rn is net radiation (MJ m2d1); G is soil heat flux density (MJ m2 d1); Tmean is mean air temperature (C); es is saturation vapor pressure, (kPa); ea is actual vapor pressure, (kPa); D is slope of the saturation vapor pressure function (kPa C1); g is psychometrics constant (kPa C1); U2 is wind speed at 2 m height (m s1).
Due to the lack of U2 data, U10 data was used for calculation by using the logarithmic vertical wind speed profile, the Eq. (2) is recommended by Allen et al. (1998):
\(U_{2}=U_{z}\frac{4.87}{In(67.8z-5.42)}\)……………… ………………………………………………..(2)
Where z is the height of measurement above ground surface (m), Uz is measured wind speed at z m above ground surface (m s1), U2 is the wind speed at a 2 m height (m s1).
The computation of the selected stations required in analyzing ETref followed the standard tools (Allen et al. 1998). The periodical and yearly ETref were computed by the daily ETref dataset. It should be noted that a season is denoted in the ideal climatological pathway: Pre-monsoon is represented as happening from March to May, monsoon season from June to August, post-monsoon season from September to November, and from December to February is known as winter (Islam et al. 2019).