2.1. Study area, soil and weather conditions The experiments were conducted at the research fields of Irrigation and Water Management Division of Bangladesh Agricultural Research Institute, Gazipur (latitude: 23°99′N, longitude: 90°41′E), and Agricultural Research Station, Benarpota, Shatkhira (latitude: 22°43′N, longitude: 89°05′E), during 2014 and 2015. These areas are characterized by subtropical monsoon climate, with average annual rainfall of about 1,898 and 1,895 mm, respectively. The soil characteristics of the experimental fields are sandy clay loam and silty clay loam in texture, respectively. The field capacity, permanent wilting point, and bulk density were 0.295 cm3 cm−3, 0.141 cm3 cm−3, and 1.50 g cm−3 for Gazipur, while at Satkhira it was 0.4088 cm3 cm−3, 0.16 cm3 cm−3, and 1.50 g cm−3, respectively.
2.2. Agronomic practices and measurements The sunflower (Variety: BARI Surjomukhi-2) was sown on 25 November 2013, and 16 November 2014 at Gazipur and 15 December 2013, and 18 November 2014 at Satkhira on a total surface area of 641.7 m2 of a rectangular shape (31 × 20.7 m). The plant density was of 5.71 plants per m2 with maintaining a spacing of 0.70 × 0.25 m. Proper land preparation and recommended fertilization was applied. The emergence, pre-flowering, flowering, and heading of the plant were noticed at about 10–12, 40–45, 50–55, and 70–75 days after sowing. About 25–30 days after head formation, it reached its physiological maturity and harvesting was done on 18 March 2014, and 15 March 2015 at Gazipur, and 27 March 2014, and 5 March 2015 at Satkhira, respectively.
Root length density, bio-mass, and canopy coverage data were collected according to different days after sowing from plant establishment up to maturity. Three representative plants were selected from destructive plot (3.5 × 2 m) to collect the growth data. Each experimental plot was composed by five sunflower lines (0.70 m between rows) that were 2 m in length. Middle three rows were used for the collection of sample to minimize border effect. Depending on the crop growth, soil core samplers were inserted in to the soil of maximum 9 numbers by using the conventional soil coresampling (COR) method (0–0.12, 0.12–0.24 and 0.24–0.36 m from the row). In this method, soil samples were collected using soil core samplers with a known volume of 0.000516 m3 including a dimension of 0.074 m diameter and 0.12 m long by following Azevedo, Chopart, and Medina (2011). Roots were extracted from the soil by washing with tap water, following separation using a 1 mm mesh sieve.
Root lengths were calculated by following the modifying method (Habib, 1988). In this method, roots were separated according to diameter (L1 (5–10 mm diameter), L2 (1–2 mm diameter), L3 (1– 0.05 mm diameter), and L4 (<0.05 mm diameter) which was measured by vernier calipers with 0.0001 m division and different mesh sized sieve. In each category, the length of 30 randomly selected roots were measured by using a scale and then dried and weighted. Finally, root length density was calculated by the ratio of root length and volume of core sampler. The proportion of ground covered with green leaves was measured in different growth stages by following a grid as suggested by Burstall and Harris (1983). In this experiment, a wooden frame divided into 100 equal sections of dimensions 0.70 × 0.50 m of the planting pattern was used. It was placed half way on each side of the row to sample one plant and on top of the plant canopy, and which section filled more than half leaves was counted to minimize parallax error. This data was collected during sunny day at noon time. Then canopy cover was calculated as the ratio of the area of grids counted to the area of the ground allocated to the crop and is expressed as a percentage. Plants from 1 sqaure meter were randomly selected from each replicated plot for recording yield contributing data during and after harvest. Then total yield from each plot was recorded in keeping seed moisture content at about 8.5%.
Soil salinity was measured as electrical conductivity using EC meter, and the electrical conductivity of soil (ECs) in the nine experimental plots was measured by mixing the soil from different layer with the required proportion of fresh water. Canal water was used for irrigation in Satkhira and its salinity was measured by following Chen et al. (2009), while in Gazipur groundwater (GW) is used for irrigation.
2.3. Experimental design and treatments The layout of the experiments was completely randomized block design with three replications, with additional spare plot of 7 m2 area. Nine irrigation regimes were imposed and these were: FI-100 at vegetative, pre-flowering and heading stages (T1), DI-80 at vegetative, pre-flowering and heading stages (T2), DI-60 at vegetative, pre-flowering and heading stages (T3), FI-100 at vegetative, and pre-flowering stages (T4 ), DI-80 at vegetative, and pre-flowering stages (T5 ), DI-40 at vegetative, and pre-flowering stages (T6 ), FI-100 at vegetative, and heading stages (T7), DI-80 at vegetative, and heading stages (T8), and DI-60 at vegetative, and heading stages (T9 ). DI-80 and DI-60 means that irrigating 80 and 60% of root zone deficit up to field capacity.
2.4. Economic analysis Economic analysis was done under water-limiting condition by following Ali et al. (2007) and English (1990). In this case, water applied less than the actual amount and saved water can be used to productive another land. The maximum increase in farm income from additional area coverage by saved water is an opportunity cost of irrigation water. Total cost was calculated by adding total operating cost, interest on operating cost (seasonal basis at the rate of 5%) and land use cost.