The experiment was carried out during the cropping season of 2017-18 at Irrigation and Water Management Division field, Joydebpur, Gazipur, Bangladesh Agricultural Research Institute (BARI). The average temperature and annual rainfalls are 25.8 °C and 2036 mm, respectively for Gazipur.
Treatments and Experimental Design
The treatments were:
T1: Irrigation after 15 days interval allowing drainage
T2: Irrigation after 20 days interval allowing drainage
T3: Irrigation after 25 days interval allowing drainage
T4: Irrigation after 30 days interval allowing drainage
The experiment was laid out in a Randomized Complete Block Design.
Management Practices
The texture of soil was silty clay loam having a bulk density of 1.5 g/cc and field capacity (FC) of 29%. BARI Chinabadam-8 was used as a test crop for this study. Seeds of groundnut were sown on 3rd December 2018 in 1m x 1m in Micro-lysimeter keeping spacing of 30 cm x 15 cm. The same crop was sown adjacent to the lysimeter tank so that the same environmental condition exists around the tank. The plot size of this adjacent crop growing area including the lysimeter tank was 3 m x 4 m. Each treatment was separated by 1 m from each other to prevent the water from going to other plots and affecting it. Fertilizers were applied at the rate of N12, P32, K42, S48 and B2 kg/ha. The entire amount of other fertilizers and half of the urea were applied as basal and were incorporated into the soil during final land preparation. The rest amount of urea was applied after 45 days of sowing when flowering was initiated. Light irrigations (27 mm) were applied at the time of sowing and up to plant establishment by following the existing moisture condition. Intercultural operations such as weeding, earthing up were done as and when necessary. The crop was harvested on 7th June 2018 and collected all the relevant data for analysis.
Crop Evapotranspiration
The micro–lysimeter contains 4-tanks, with an area of 1 m x 1 m and a depth of 1 m under the surface which contains a brick wall on four sides along with a drainage outlet at the end. This system was designed by Khan et al. (1992) where the crop grows allowing drainage water from the experimental tank. This system was used by Islam & Hossain (2010); and Biswas et al. (2014) in estimating the Crop coefficient value of hybrid maize at Gazipur. According to different treatments, a measured quantity of water was applied to the tank allowing drainage, and the adjacent plot outside of the tank was also irrigated based on existing soil moisture. Drainage water from each of the lysimeter tanks was collected at the drainage chamber and measured with a graduated cylinder. Crop evapotranspiration (ETc) was calculated by following the water balance equation (Eq. 1). By collecting soil samples, soil moisture contents were measured before each irrigation periodically for different treatments to calculate total stored soil moisture within the tank during the period (ΔSs). 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, mm
P = Drainage water, mm
ΔS = Stored soil moisture, mm
Reference crop evapotranspiration
Reference crop evapotranspiration (ETo) was estimated by using FAO-developed CROPWAT software by collecting and using location-specific geographical data (latitude, longitude, and elevation) and climatic data (maximum and minimum temperature, relative humidity, sunshine hour, and wind speed).
Crop coefficient (Kc)
Finally, crop coefficient (Kc) was estimated by using the following formula.
ETc = Kc × ETo ------------------------- (2)
Data collection
Plant height, days to 50% flowering, no. of branches per plant, no. of pods per plant, No. of seeds per plant, 100 pod weight, 1000 seed weight, yield per plant, etc. were recorded during the growing period. Soil moisture, field capacity, climate data from the nearby station, water requirement data etc. were recorded and collected when needed.