Simulating the Impact of Climate Variability on Rainfed Wheat Production: Models Evaluation and Application.

Crop growth in process-based crop models is controlled by several different parameters. Calibration of these models is necessary for their application to new cultivars and environments. In this study, we applied a manual method to calibrate APSIM-Wheat and CERES-Wheat for flowering day, maturity day, leaf area index, biomass and grain yield of five spring wheat cultivars under rainfed conditions in Pakistan. Five wheat cultivars of diverse origin (Tatara, NARC-2009, Sehar-2006, SKD-1 and F-Sarhad) were planted on November 19th in Islamabad annually, between 2008 and 2011. The experiment was laid out in a Randomized Complete Block Design (RCBD) and replicated three times for individual plot sizes of 5 m × 3 m. APSIM-Wheat and CERES-Wheat were calibrated for all five wheat cultivars using genetic coefficients, and were then validated with validation skill scores. Wheat cultivars observed anthesis days after sowing ranged from 99 to 126, and physiological maturity ranged from 139-171, similar to the simulated cultivar. The observed leaf area index (LAI) for all cultivars ranged from 3.4 to 5.1 m2 m-2 during all years. Temporal changes in LAI accumulation for all cultivars indicate that both measured and simulated values match. Results show that the performance of both models was realistic, as indicated by accurate simulations of crop phenology, LAI, biomass and grain yield against measured data. Climate variability results indicate that temperature increases resulted in a 50% decline in yield of different wheat cultivars. These results also show that increased CO2 increased the yield up to 12%. This was similar to the combined effect of increased temperature and CO2. Therefore, we propose that new cultivars should be calibrated to increase accuracy in the simulation outcomes of models in terms of climate variability.