Climate-informed irrigation and nitrogen strategies for Boro rice sustainability: DSSAT-CERES modeling under CMIP6 projections

Boro rice underpins Bangladesh's food security, yet climate change, groundwater depletion, and nitrogen inefficiency threaten its long-term productivity. Prior studies evaluated water-saving irrigation or nitrogen management independently, without quantifying their synergistic interactions under contemporary and future conditions. To address these gaps, a two-season (2024–2025) randomized complete block design field experiment at Bangladesh Agricultural University, Mymensingh evaluated eight treatment combinations from a 2 × 4 factorial arrangement: continuous flooding (CI) and alternate wetting-drying irrigation with 15 cm threshold (AI), each crossed with four nitrogen rates (N0, 25%, 50%, and 100% of 90 kg N/ha). The DSSAT-CERES-Rice model was calibrated on 2024 data and validated on independent 2025 observations. Ten CMIP6 models under four SSP scenarios across three temporal horizons (2026–2100) were bias-corrected via quantile mapping, with transfer functions fitted over 2000–2014 and validated over 2015–2024. Calibration achieved R²= 0.851, NSE= 0.725, and RMSE= 0.364 t/ha; validation yielded R²= 0.858 and RMSE= 0.788 t/ha. The AWD strategy reduced seasonal water use by 35%, improving water productivity from 0.59 to 0.63–0.72–0.89 kg/m³ . The optimized AIN2 treatment (AI with 25% nitrogen reduction) achieved 6.13 ± 0.34 t/ha, representing 89–93% of maximum yield under CIN1 while consuming 35% less water and 25% less nitrogen. Near-century projections indicated modest yield gains of 1.5–8.4%, whereas far-century SSP5–8.5 scenarios induced catastrophic yield declines of 30.7–32.9%, with water productivity deteriorating by 33–34%. These findings confirm that while integrated irrigation-nitrogen management enhances near-term resource efficiency, heat-tolerant cultivars, phenological adjustment, and climate-informed decision support are imperative for Boro rice sustainability.