The experiments were conducted in a micro-lysimeter at the research farm of Bangladesh Agricultural Research Institute, Gazipur, during the rabi season of 2015 - 2016. It was a warm-dry climate, with an average temperature of 22.50°C. The average relative humidity, sunshine hour, and evaporation were 79.76 %, 5.15, and 3.00 mm day-1. The highest rainfall was recorded during the month of March but the crop received only 38% of it. The soil characteristic of the experimental field was silty clay loam, with field capacity and bulk density, 29.5%, and 1.5 g.cc-1. Soybean (variety: BARI Soybean-6) was sown in a 1×1m micro-lysimeter at a spacing of 30 × 10 cm on 19 November 2015. Also, the same crop was sown in the land adjacent to the lysimeter tank with the objective of creating a favorable atmospheric condition. Recommended fertilizer, intercultural operation, and other necessities were as per requirement. The crop was harvested on 21 March 2016. The following treatments were used in this experiment.
T1 = Irrigation at 10-days interval allowing drainage
T2 = Irrigation at 15-days interval allowing drainage
T3 = Irrigation at 20-days interval allowing drainage
T4 = Irrigation at 25-days interval allowing drainage
The micro–lysimeter contains 4-tanks, with an area of 1 sq. meter. In this lysimeter, the crop is grown in a completely controlled environment (Khan et al., 1993). This system was also used by Islam and Hossain, 2010 by estimating the crop coefficient value of hybrid maize at Gazipur. The crop was irrigated according to the design of the experiment. The measured quantity of water was applied to the tank ensuring drainage. The adjacent plot outside the tank was also irrigated as per requirement. Drainage water from the lysimeter was measured by the graduated cylinder inside the tank and ETc was calculated by following water balance equation (Eq. 1). Prior to irrigation, soil moisture was measured to determine the depleted soil water. The part of rainfall collected as drainage and the stored soil moisture during the period under consideration were subtracted from the total rainfall to obtain crop evapotranspiration (ETc). The following formula was used to calculate the crop evapotranspiration for the specific period.
ETct = Wat – (Dwt ± ΔSst) -------------------------- (1)
Where,
ETct = Crop evapotranspiration in mm for time, t
Wat = Applied water + rainfall, mm, for time, t
Dwt = Drainage water, mm, for time, t
ΔSst = Stored soil moisture, mm, for time, t
Reference evapotranspiration (ETo) was estimated by using CROPWAT software only by inserting location information (latitude, longitude, and elevation), and climatic data (such as maximum, and minimum temperature, air humidity, sunshine hour, and wind speed).
Finally, crop coefficient (kc) was estimated by using the following formula.
Kc = ETcrop / ET0……………………………(2)