The field experiment was conducted during the winter season of 2018- 2019, between the months of December and March, at the research field of Irrigation and Water Management Division, Bangladesh Agricultural Research Institute (BARI) (Latitude 24.00o N, Longitude 90.25oE 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 0C, 56–89%, 0.76–10.87 km h-1 and 1.6–3.5 mm d-1, respectively. Total rainfall that occurred during crop growing season was recorded as 91 mm in which only 21 mm was effective. 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 the top 30 cm of the soil were 28.5%, 13.72% and 1.46 g cm-3. The concentrations (kg ha-1) of N, P2O5and K2O were 51.1, 12.5 and 265.6, respectively. The soil had an organic matter content of 1.04%. The experiment was set up in a split-plot design with three onion varieties and five different irrigation treatments that were replicated thrice. Sprinkler irrigation with five different water levels was applied compensating crop coefficient (Kc) and potential evapotranspiration (ETo) based predicted evapotranspiration loss (ETc). Each of the onion varieties experienced five levels of sprinkler irrigation as follows: Onion varieties-
V1= BARI Piaj-1
V2= Taherpuri Super (Metal)
V3= Taherpuri King (Lal Teer)
V4= BARI Piaj-4
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
I5= Sprinkler irrigation at 140% ETc
Onion varieties were kept in the main plots and irrigation levels in the sub-plots. The treatments with the same irrigation regime were arranged in a line covering all four varieties 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 onions (cv. BARI Piaz- 1, Taherpuri Super, Taherpuri King and BARI Piaj-4)) were planted at 15 cm × 10 cm spacing on 30 December 2018. During land preparation, farmyard 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 potassium and the full dose of phosphorus were applied at planting and the rest half of the nitrogen and potassium was applied in two equal splits at 25 and 50 days after planting. Just after transplanting, common irrigation was applied to all plots for establishing the plants. Thereafter, irrigation treatments started at 12 DAT and subsequent applications were applied according to the treatment 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 sunshine 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 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 at transplanting and at harvest and after each rainfall event by gravimetric method.