Introduction:
Climate change and increasing constraints on water resources have emerged as major challenges for agricultural sustainability worldwide. In arid and semi-arid regions such as Iran, these challenges are particularly acute, necessitating strategic interventions to ensure food security and resource efficiency. Among various approaches, improving water productivity, defined as the ratio of crop yield to water consumed, has become a central objective in national and regional agricultural planning. Wheat, as a strategic crop in Iran, plays a vital role in the country’s food security. Irrigated wheat, in particular, is highly sensitive to climatic variations and water availability.Understanding its response to future climate scenarios is therefore essential for developing effective adaptation strategies. Modeling crop yield and water productivity under different climate conditions can provide valuable insights into potential risks and guide decision-making for sustainable agricultural development. This study focuses on Alborz Province, a key agricultural region in Iran, and investigates the quantitative responses of irrigated wheat to projected climate conditions in the 2040 horizon. Using the AquaCrop 7.1 model for crop simulation and the LARS-WG 8 model for climate downscaling, the research evaluates wheat yield and water productivity under three Shared Socioeconomic Pathways (SSPs): SSP1-2.6 (low emissions), SSP2-4.5 (intermediate emissions), and SSP5-8.5 (high emissions). The primary objective is to assess the effects of climate change on wheat water productivity across different irrigation levels and to propose practical solutions for optimizing water use and enhancing agricultural resilience.

Materials and Methods:
To simulate future climate conditions, the study employed the LARS-WG 8 model, a stochastic weather generator that downscales daily climate data from global circulation models (GCMs) to local station scales. Specifically, climate projections from the HadGEM3-GC31-LL model were used as input. This model is known for its robust representation of atmospheric processes and provides reliable data for scenario-based climate analysis. Climate data for the year 2040 were generated and validated under three SSP scenarios: SSP1-2.6, SSP2-4.5, and SSP5-8.5. These scenarios represent different trajectories of greenhouse gas emissions, socioeconomic development, and mitigation efforts. SSP1-2.6 assumes strong mitigation and low emissions, SSP2-4.5 reflects moderate mitigation and intermediate emissions, and SSP5-8.5 represents a high-emissions pathway with limited climate policy intervention. Following climate data generation, the AquaCrop 7.1 model was used to simulate wheat yield and water productivity. AquaCrop, developed by the Food and Agriculture Organization (FAO), is designed to simulate crop responses to water availability and is particularly suitable for analyzing water productivity under varying irrigation regimes. The model calibration was conducted in multiple stages to ensure accuracy, including the removal of noise treatments and adjustment of parameters. Alborz Province was selected as the study area due to its significant role in irrigated wheat production. The region has a cultivated area of 9587 hectares dedicated to irrigated wheat and an annual production of 48260 tons during the 2022–2023 crop year. Simulations were performed under three irrigation levels—100 percent, 80 percent, and 60 percent of full irrigation—and two irrigation intervals (14 and 7 days). These treatments allowed for a comprehensive analysis of water stress effects.Water productivity was calculated as the ratio of crop yield (kg) to water volume consumed (m³), providing a standardized metric for comparing performance across scenarios. The base year (2023) served as a reference point for evaluating changes in yield and productivity under future climate conditions.

Results and Discussion:
The simulation results revealed substantial impacts of climate change on both wheat yield and water productivity in Alborz Province. Under full irrigation conditions, water productivity declined from 1.85 kg/m³ in the base year to 1.70 kg/m³ in the SSP2-4.5 scenario, indicating a negative effect of intermediate emissions and moderate warming. In contrast, the SSP5-8.5 scenario, characterized by high CO₂ concentrations and more pronounced warming, resulted in an increase in water productivity to 1.93 kg/m³. This improvement is likely due to the fertilization effect of elevated CO₂, which enhances photosynthesis and biomass accumulation under optimal water conditions. However, the SSP2-4.5 scenario led to a significant yield loss of more than 4600 tons, representing approximately 10 percent of the province’s annual wheat production. At reduced irrigation levels (80 percent and 60 percent), the results were mixed. In the SSP5-8.5 scenario, crop yield increased by 4 percent compared to the base year, suggesting that CO₂ enrichment may partially offset water stress. Conversely, in the SSP2-4.5 scenario, yield decreased by 8 percent, highlighting the compounded effects of warming and reduced water availability. Interestingly, the modeling showed that yield reductions in the SSP2-4.5 scenario were relatively uniform across all irrigation levels. This uniformity implies that climatic factors, especially temperature increases, have a more dominant influence on yield than irrigation rates alone. The findings suggest that beyond a certain threshold, irrigation adjustments may not fully compensate for climate-induced stress. The accuracy of the AquaCrop model was also evaluated under different conditions. The model performed well under full irrigation, with high correlation between simulated and observed yields. However, its accuracy declined significantly under low irrigation levels and longer irrigation intervals. This limitation points to the need for structural improvements in the model, particularly in simulating crop responses to water stress and variable climate inputs.

Conclusion:
This study demonstrates that future climate scenarios exert differentiated impacts on wheat yield and water productivity in Alborz Province. While the SSP5-8.5 scenario offers potential yield gains due to CO₂ fertilization, the SSP2-4.5 scenario presents a more concerning outlook with notable yield losses and reduced water productivity. These findings have important implications for agricultural planning and water resource management in Iran.To enhance resilience, the study recommends several adaptation strategies, including: • Genetic improvement of wheat varieties to increase tolerance to heat and drought • Optimization of planting dates to align with favorable climate windows • Development of climate-smart policies that integrate crop modeling with water allocation planning • Investment in irrigation technologies that improve efficiency and reduce water loss.  Moreover, the study highlights the value of combining climate and crop models for integrated risk analysis. Such tools can support decision-makers in evaluating trade-offs, prioritizing interventions, and designing adaptive strategies that safeguard food security under changing climate conditions. Finally, the limitations observed in AquaCrop’s performance under water stress conditions suggest the need for further model refinement. Enhancing the model’s sensitivity to drought dynamics and incorporating feedback mechanisms between climate variables and crop physiology could improve its utility in future scenario analysis.