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Daytime mean VPD ranged from 0 to 5.3 kPa. For more than 75% of the time its variation was less than 20%, and the maximum variation was up to 50%.

Actual vapour pressure of the air ranged from 0.5 to 2.5 kPa. The accuracy of such VPD estimations was assessed using data collected every 15 min near Wagga Wagga in New South Wales, Australia. In APSIM (Agricultural Production Systems Simulator), VPD is estimated from daily maximum and minimum temperatures with the assumption that the minimum temperature equals dew point, and there is little change in vapour pressure or dew point during any one day. Data required to calculate the mean VPD on a daily basis are rarely available, and most models use approximations to estimate it.

We concluded that to bring accuracy in the simulation outcomes of models, new cultivars should be calibrated to minimize uncertainty to allow judicious recommendations in response to climate variability.Vapour pressure deficit (VPD) has a significant effect on the amount of water required by the crop to maintain optimal growth. Climate variability results depicted that an increase in temperature from 0 ☌ to 5 ☌ resulted in a 60% average decline in the yield of wheat cultivars while increased CO 2 increased yield similar to the combined effect of increased temperature and CO 2. Evaluation with the measured data showed that performance of both models was realistic as indicated by the accurate simulation of crop phenology, LAI, biomass and grain yield against measured data. The temporal changes in maximum LAI accumulation for all cultivars indicate that both measured and simulated values match each other. Both models were able to accurately simulate anthesis and maturity days, maximum leaf area index, biomass and grain yield, with normalized root mean square error (RMSE) less than 10%, D-index greater than 0.80 and model efficiency above 80% in most cases. APSIM-Wheat and CERES-Wheat were calibrated for all five wheat cultivars using genetic coefficients estimated based upon measured data during 2008–09 cropping year and validated with independent data sets (experimental data of 2009––11 cropping seasons) which were not used for models calibration. The experiments were laid out in Randomized Complete Block Design (RCBD) replicated four times with individual plot size of 5 m × 3 m. Five wheat cultivars of diverse origin namely Tatara, NARC-2009, Sehar-2006, SKD-1 and F-Sarhad were planted on 19th November, at Islamabad during the years 2007–2011. We applied a manual method to calibrate APSIM-Wheat and CERES-Wheat for the flowering day, maturity day, leaf area index, biomass and grain yield of five spring wheat cultivars under rainfed conditions in Pakistan. Model calibration is necessary for application to new cultivars and environment. Crop growth in process based crop models is controlled by different parameters.