A field study was conducted with onion (cv. BARI Piaj-1) under sprinkler irrigation system during the winter season (December–March) of 2015–2016 at the research field of Irrigation and Water Management Division, Bangladesh Agricultural Research Institute (BARI) (Latitude 24000_N, Longitude 90025_E and altitude 8.40 m msl), Gazipur. The average temperature, relative humidity, wind speed and pan evaporation rate during the crop growing season ranged from 14.5 to 26.4 C, 56–89%, 0.76–10.87 km/hr and 1.6–3.5 mm/d, respectively. Rainfall during crop growing season was recorded as 34 mm in two events. The percentage of sand, silt and clay in the experimental soil were 36.5, 35.4 and 28.1, respectively.Field capacity, wilting point and bulk density of top 30 cm of the soil were 28.5%, 13.72% and 1.46 g/cm3. The concentrations (kg/ha) of N, P2O5and K2O were 51.1, 12.5 and 265.6, respectively. The soil had an organic matter content of 01.04%. The treatment comprised of three levels of irrigation frequency and four levels of irrigation amount as follows:
Irrigation frequencies
F1= Sprinkler irrigation at 5 days interval
F2= Sprinkler irrigation at 10 days interval
F3= Sprinkler irrigation at 15 days interval
Irrigation levels
I1= Sprinkler irrigation at 60% ETc
I2= Sprinkler irrigation at 80% ETc
I3=Sprinkler irrigation at 100% ETc
I4= Sprinkler irrigation at 120% ETc.
Three levels of irrigation frequency and four levels of irrigation amount were arranged in split-plot design with irrigation frequency formed in the main plots and amount in the sub-plots. Each treatment was replicated three times and irrigated using a sprinkler irrigation system as per treatments. The treatments with the same irrigation regime were arranged in a line covering three replications for better management of irrigation. Since, the characteristics of the experimental land were homogeneous, there was little possibility of variation in results for such arrangements of the treatments. Each plot was of 4 m × 3.75 m size surrounded by 1.5 m wide buffer strip to restrict lateral seepage of water in-between adjoining plots. Forty days old seedlings of onion (cv. BARI Piaz- 1) were planted at 15 cm × 10 cm spacing on 22 December 2015. During land preparation, farm yard manure @ 5 t/ha was properly mixed with the soil. Fertilizers were applied @ 115 kg N, 60 kg P and 60 kg K per hectare. Half of the nitrogen and the full dose of phosphorus and potassium were applied at planting and the rest half of the nitrogen was applied in two equal splits at 25 and 50 days after planting.
Just after planting, a common irrigation was applied to all plots for establishing the plants. Thereafter, irrigation treatments started at 12 DAP and subsequent applications were applied according to the treatments design. Irrigation was applied through sprinkler system based on crop evapotranspiration (ETc). Reference evapotranspiration (ETo) was calculated on a daily basis from daily meteorological data by Penman-Monteith’s equation using CROPWAT computer programme. Daily meteorological data required for CROPWAT model including maximum and minimum air temperature, relative humidity, wind speed at 2 m height and sun shine hour were collected from a weather station about 1.0 km away from the study site. The actual crop evapotranspiration was computed by multiplying the reference evapotranspiration (ETo) with crop coefficient (Kc) for different growth stages of the crop. The daily irrigation requirement for the crop was calculated by subtracting the effective rainfall from the computed ETa. Time of operation of sprinkler system was calculated for different levels of irrigation dividing water requirement of the crop over irrigation intervals (6, 9 and 12 d) by discharge of a sprinkler nozzle. The duration of operation was controlled with gate valves provided at the inlet of each lateral. Soil water content measurements were made from 0-15, 15-30 and 30-45 cm depths before and after each irrigation as well as planting and harvest and after rainfall by gravimetric method. Crop water use (evapotranspiration, ETc) was estimated using the water balance method (Walker and Skogerboe, 1987) as:
ET= I + P - D - R ± SWS
Where P is precipitation (mm), I is irrigation (mm), D is the drainage (mm), R the run-off and SWS is the variation in water content of the soil profile. The change in soil water contents at 30–45 cm soil layer was considered to be deep percolation. Run-off was taken to be zero since it did not occur with the use of micro-sprinkler irrigation system.
The recommended plant protection measures were adopted as and when required. Irrigation was stopped 15 days before harvesting in all treatments. Ten plants from each plot were selected randomly and tagged for recording growth parameters viz., plant height, number of leaves and neck girth. Leaf area and above ground dry matter were also recorded on 10 plants at different phenological stages. Yield parameters viz., bulb diameter, bulb length, bulb unit weight were recorded from the plants used for recording observations. The bulbs were harvested at full maturity stage on 18 March 2016. After proper curing and neck cutting, yield and yield contributing characters were recorded. The bulb yield per hectare was calculated based on the plot yield. The analysis and interpretation of data were done using the DMRT method of analysis of variance technique as described by Gomez and Gomez (1984).