Assessing the effect of climate change and irrigation management on yield and water use efficiency of canola cultivars in Khuzestan province

Introduction

Climate change is one of the most pressing challenges for global agriculture, particularly in arid and semi-arid regions such as southwestern Iran. Increases in temperature, alterations in rainfall distribution, and the rise in atmospheric CO₂ concentrations strongly affect crop growth, yield formation, and water use efficiency (WUE). Rapeseed (Brassica napus L.), as a C₃ plant and a major oilseed crop, is highly sensitive to both climatic fluctuations and water availability. In Khuzestan province, rapeseed has become increasingly important in recent years, but its productivity is constrained by limited water resources and the growing threat of climate stress. Therefore, it is crucial to evaluate the combined effects of cultivar choice and irrigation strategies under both current and projected climate conditions to support sustainable rapeseed production. This study aimed to assess the performance of various rapeseed cultivars and irrigation regimes under future climate conditions.

Materials and Method

The research was conducted using the APSIM model to simulate the growth and yield of rapeseed at five sites in Khuzestan Province (Shush, Shushtar, Izeh, Dezful, and Behbahan) under both baseline and future periods. Climate projections for the future periods were obtained from five Global Circulation Models (GCMs: MPI-ESM1-2-LR, ACCESS-ESM1-5, MRI-ESM2-0, HadGEM3-GC31-LL, and CNRM-CM6-1) under three SSP scenarios (SSP1-2.6, SSP2-4.5, and SSP5-8.5). The LARS-WG software (version 8) was employed for statistical downscaling to generate future climate data. Three widely cultivated rapeseed cultivars, namely Hyola401, Hyola308, and RGS003, together with five irrigation treatments, were considered to simulate their responses to future climate conditions in terms of yield, WUE, ETc, and to identify the optimal combinations for the study sites. Irrigation treatments were defined based on soil field capacity (FC) levels, including FC50, FC60, FC70, FC80, and FC90. These treatments corresponded to the replenishment of soil moisture whenever the available water content was depleted to 50%, 60%, 70%, 80%, and 90% of field capacity, respectively. Statistical analyses, including regression analysis and Duncan’s multiple range test, were applied at the 0.05 and 0.01 probability levels.

Results and Discussion

The results showed that, under the baseline period, the cultivar Hyola401 achieved the highest grain yield in the FC90 treatment (an average of 4631 kg ha⁻¹ across all regions). The highest WUE was obtained by Hyola401 under the FC50 treatment (1.1 kg m⁻³) in Izeh. Regression analysis revealed a positive and significant relationship between grain yield and WUE (R² = 0.91; p-value < 0.05). During the future period, mean temperature increased by 1.53 °C compared with the baseline, while the length of the growing season decreased by about 5%, leading to reductions in both yield and WUE in most regions. For instance, in Izeh, a slight improvement in yield was observed for the cultivar RGS003 under FC50 and FC60 irrigation (by 0.65% and 0.34%, respectively) under the SSP1-2.6 scenario. In Shushtar, however, yield improvements were associated with the cultivar Hyola401 across all scenarios and irrigation treatments, with yield increases ranging from 2.5% to 17%. The results highlight the trade-off between grain yield and water use efficiency under different irrigation levels. While FC90 maximized grain yield, it also increased ETc beyond 6000 m3, resulting in lower WUE. Conversely, deficit irrigation (FC50) improved WUE but caused a significant yield reduction. The optimal balance was achieved at intermediate irrigation levels (FC60–70), particularly when combined with high-performing cultivars such as Hyola401. Future climate scenarios suggested that the negative effects of increased temperature and shorter growth duration would outweigh the positive effects of CO₂ fertilization in most warm regions. The increase in temperature and its negative association with grain yield were observed in in regression analyses, where the relationship between mean temperature and grain yield was significantly negative (R² = -0.68; p-value < 0.05). The largest yield reductions were predicted in Shush, confirming that heat stress during the reproductive stage is the most critical constraint. Nonetheless, in cooler regions such as Izeh the positive impact of elevated CO₂ partially offset the yield decline, confirming the spatial heterogeneity of climate change impacts. These findings are consistent with previous reports indicating that elevated CO₂ may mitigate some of the negative consequences of rising temperatures in C₃ crops, but cannot fully compensate for severe heat stress.

Conclusion

This study demonstrated that both cultivar selection and irrigation management are key determinants of sustainable rapeseed production under climate change. Under baseline conditions, Hyola401 × FC90 achieved the highest grain yield, while Hyola401 × FC50 achieved the highest WUE. However, under future climate conditions, yield reductions of up to -9% were projected, particularly in warmer areas such as Shush under SSP5-8.5. In contrast, yield and WUE improvements were observed in Izeh and Shushtar, largely due to the positive effects of increased atmospheric CO₂. Overall, intermediate irrigation strategies (FC60–70) combined with resilient cultivars such as Hyola401 and Hyola308 were found to offer the most sustainable balance between yield and WUE under future climate conditions. These results emphasize the importance of climate-smart strategies, including adaptive irrigation management, the selection of stress-tolerant cultivars, and the adjustment of planting schedules, to mitigate the adverse impacts of climate change and ensure sustainable rapeseed production in Khuzestan.