2.1. Model description In order to simulate crop growth and yield, the AquaCrop model accounts for soil physical and hydraulic processes, atmospheric conditions (rainfall, temperature, evapotranspiration, ET and carbon dioxide concentration), crop physiological and productivity parameters (phenology, crop cover, rooting depth, biomass production and harvestable yield) and field management components (irrigation, fertilizer and field agronomic practices) (Raes et al., 2009a; Steduto et al., 2009). AquaCrop simulates the water balance components of soil evaporation and crop transpiration separately based on the daily canopy cover and soil drying, using the daily ETo calculated from weather data (Raes et al., 2012). The crop responds to water stress through four stress coefficients (leaf expansion, stomata closure, canopy senescence, and change in harvest index, HI). Using the crop water productivity parameter, AquaCrop calculates daily aboveground biomass production (Hsiao et al., 2009; Steduto et al., 2009). The normalized crop water productivity (WP*) is considered to be nearly constant for a given climate and crop. The WP* for rice is set between 15 and 20 gm−2 (Raes et al., 2009b) and yield is obtained by multiplying biomass by the crop-specific HI. The internal adjustment of HI in relation to available water depends on timing, severity and duration of water stress (Hsiao et al., 2009; Raes et al., 2009a; Steduto et al., 2009). The HI is adjusted in response to five water stress coefficients, namely: the coefficients for inhibition of leaf growth, for inhibition of stomata, for a reduction in green canopy duration due to senescence, for a reduction in biomass due to pre-anthesis stress and for pollination failure (Raes et al., 2009a; Steduto et al., 2009). The HI for rice is typically found between 35 and 50 percent, depending on genotype and stress factors.
The parameters that determine the development of canopy cover (CC) are the canopy growth coefficient (CGC), the canopy decline coefficient (CDC), the initial CC (CCo), maximum CC, days to recovery after transplanting, and the start of canopy senescence. The CGC controls the rate at which the canopy expands and the CDC controls how fast the canopy dies off after the start of canopy senescence. The AquaCrop model contains several user-specified options to simulate irrigation practices, including timing, amounts to be applied, and a selection of irrigation methods. All the threshold and sensitivity parameters were used in the model according to Steduto et al., (2009). A summary of the adjusted crop parameters used is presented in the Results section.
2.2. Site description, experimental setup and procedures Crop trials were conducted on a 0.5 ha field of the research farm of the Bangladesh Rice Research Institute (BRRI) in Gazipur, Bangladesh during the dry (Boro) season of 2008–09 and 2009–10. Three irrigation treatments: continuous standing water (CSW), and surface irrigation applied after 3 days or 5 days of standing water disappearance (3DAWD or 5DAWD) were assigned in a randomized complete block design with four replications. A deep tube well supplied irrigation water. Unit plots (5 × 6 m) were separated by 20 cm wide compacted earth bunds. Polyethylene sheeting was embedded in the middle of the bunds down to 45 cm depth, in order to prevent losses of water to lateral water flow. Forty-five-day-old seedlings of the IR69515-KKN-4-2-1-1 genotype were transplanted – at the density of 3 seedlings per hill – on January 17 and 14, respectively, in 2009 and 2010 at 20 × 20 cm spacing and the harvest took place in late April in both years. Fertilizers at 140:30:75:18:5 kg ha−1 of NPKSZn were used as recommended by BRRI, in the form of urea, triple superphosphate, potassium chloride, gypsum and zinc sulfate. The amounts refer to the elemental form of each nutrient. All fertilizers were applied as basal fertilizers, except nitrogen. The urea was applied in three equal splits at 15–20, 30–35 and 40–45 days after transplanting (DAT).
Water balance within the field was calculated according to Cabangon et al. (2004):
I + R = ET + SP + D + ÂS
where I is irrigation, R is rainfall, ET is evapotranspiration, SP is seepage and percolation, D is over-bund drainage (all in mm d−1) and ÃS is the difference in soil water storage (mm).