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).