The experiment was conducted at the IWM divisional research field, BARI, Gazipur, during rabi (winter) crop season in 2016 - 2017. The weather during the experimental season was cool and dry with an average minimum and maximum temperature of 15.38°C and 29.2°C, respectively. The average relative humidity, sunshine hour and wind speed were 76.2%, 7.18, and 123.4 km/day. There was no rainfall during this period. The soil characteristic of experimental field was silty clay loam, with field capacity of 29.5% and bulk density of 1.5 gm/cc, respectively. Radish (variety BARI Mula-4) seeds were sown in a 1m × 1m Micro-lysimeter keeping spacing of 30 × 30 cm on 09 November 2016. Also, the same crop was sown adjacent to the lysimeter tank (1m × 1m) with the objective to keep favorable atmospheric condition. Recommended fertilizer, intercultural operation and other necessities were provided. The crop was harvested at 17 March 2017. The following treatments were used in this experiment.
T1 = Irrigation at 7 days interval allowing drainage
T2 = Irrigation at 12 days interval allowing drainage
T3 = Irrigation at 17 days interval allowing drainage
T4 = Irrigation at 22 days interval allowing drainage
The micro–lysimeter contains 4-tanks, with the area of 1 sq. meter. In this system, crop is grown in a completely controlled environment which was shown in the design by Khan et al. (1992). This system was also used by Islam & Hossain (2010); and Biswas et al. (2014) in estimating Crop coefficient value of hybrid maize at Gazipur. The crop was irrigated according to the design of the experiment. Measured quantity of water was applied to the tank allowing drainage; the adjacent plot outside of the tank was also irrigated accordingly. Percolated water from each of the lysimeter tanks were collected at the drainage chamber and measured with graduated cylinder. ETc was calculated by following water balance equation (Eq. 1). Soil samples were collected and soil moisture contents were measured periodically (according to each different treatment) to calculate total stored soil moisture within the tank during the period (ΔSs). Rainfall and other necessary climatic data was collected from the nearby weather station. Eventually, the following formula was used to calculate the crop evapotranspiration (ETc) for the specific time period.
ETc = (I+R) – (P ± ΔS) --------------------------(1)
Where,
ETc = Crop evapotranspiration, mm
I = Irrigation, mm
R = Rainfall
P = Drainage water, mm
ΔS = Stored soil moisture, mm
Reference crop evapotranspiration (ETo) was estimated by using FAO developed CROPWAT software by inserting location information (like, latitude, longitude, and elevation) and climatic data (maximum and minimum temperature, relative humidity, sunshine hour, and wind speed).
Finally crop coefficient (Kc) was estimated by using the following formula.
ETc = Kc × ETo ------------------------- (2).