Evapotranspiration (ET) was calculated using the general water balance equation (Jensen et al., 1990):
P + I + U = R + D ± SW + ET ………………………….(1)
where, P = Effective rainfall, cm ,
I = Irrigation water applied, cm
U = Upward flux, cm
R = Surface runoff, cm
SW = Change in soil moisture storage in the soil profile, cm
D = Water lost by deep percolation, cm
ET = Crop evapotranspiration, cm.
The sum of the terms on the left-hand side of the above equation represents the net addition of water to the soil profile over the time period of interest (sowing to physiological maturity). We assumed negligible upward flux beyond the measured depth, because the level of the water table (piezometric surface) recorded in the observation well near the experimental field was never closer than 2.5 m from the soil surface. Surface runoff was assumed to be zero as irrigation water was protected by 30 cm height bunds. Deep percolation (D) was assumed negligible, because water was applied only to replace soil moisture in the root zone. Thus, equation (1) reduces to the following form for calculating ET:
ET = I + P ± SW ………………........... (2)
- Calculation of reference crop evapotranspiration
The choice of a ET0 method is mainly based on the availability of weather data. The methods for estimating ET0 from the numerous methods reported in the literature were chosen based on the report of Smith et al. (1992). They reported that the P-M method gives the best estimation of ET0, but also recommended the Blaney-Criddle (referred as FAO temperature), Radiation (FAO radiation), and Hargreaves method. In this study, ET0 was calculated according to FAO Penman-Monteith, FAO temperature, FAO radiation and Hargreaves methods.
Calculation of crop coefficient, kc
Crop coefficient (kc) is defined as the ratio of the crop evapotranspiration to the reference crop evapotranspiration and can be calculated by different methods (e.g. single crop coefficient method and dual crop coefficient method). In this study, crop coefficient was calculated according to the single crop coefficient method:
kc = ETc / ET0 .............................................. (3)
where ETc and ET0 are crop evapotranspiration and reference crop evapotranspiration at various growth stages, respectively. The kc described by the above equation includes the effect of evaporation from both plant and soil surfaces. The treatment T9 was considered for adequate water supply and T1 for the deficit control.
Uniformity coefficient for the kc values
The uniformity coefficient (UC) of the yearly kc values were determined as ( Devitt et al., 1992):
UC = 1 – (standard deviation /mean ) ..................... (4)