Introduction: Crops yield can be considered in different situations, which is called different production levels. Potential yield level is that determined by radiation, temperature and cultivar traits without limitations of biotic or abiotic factors; while attainable yield level is that limited by water or nutrient supply, and actual yield level is that determined in the present of the limiting (water or nutrient) and reducing factors (such as weed, harmful insects and plant diseases). The yield gap is the difference between the potential yield level and actual yield level. Identifying the yields at different production levels and quantifying the yield gaps through field experiments may involve many years of data collection on which to make meaningful inferences. Crop simulation models are an alternative tool for analyzing interactions between water and nitrogen availabilities on yield generation. The objectives of the present study was to estimate potential yields and yield gaps for irrigated wheat using the CERES-Wheat model, and other limiting and reducing factors in the main wheat growing regions of Kermanshah province.
Materials and Methods: The study was conducted at 3 locations in Kermanshah province, which is located in west of Iran. Historical weather data for 1999 to 2016 were obtained from the Iran Meteorological Organization for the 3 locations. The weather data included daily solar radiation (MJ m^-2 d^-1), daily maximum and minimum temperatures (^°C), and daily rainfall (mm). The CERES-Wheat model was calibrated for Pishgam cultivar of wheat and validated for a main wheat growing regain to estimate yield gaps in some locations of Kermanshah province in west Iran. In order to calibrate and validation of the model, in another study, leaf area index, phenological growth stages, total dry weight yield and grain yield were used. The validated model was used to simulate long-term yield under three management conditions (potential, water-limited, nitrogen-limited). The experiments were conducted in the Campus of Agriculture and Natural Resources Field at Razi University, Kermanshah, Iran (34◦19´N, 47◦50´E, altitude 1320 m) on soil classified as Inceptisol typic during 2014-2015 and 2015-2016. The treatments were included 4 levels of nitrogen fertilizer application (90, 180, 300 and 360 kg ha^-1 urea).
Results and Discussion: The simulation results indicated that averaged simulated potential yield was 8.9 t ha^-1, while water and nitrogen limitation yields were 8.0 and 7.1 t ha^-1, indicating 10.3% and 20.2% reduction in wheat yield, respectively. The potential yields changed spatially due to changes in temperature and solar radiation. The average actual yield was 5.5 t ha^-1 which was 3.4, 2.7 and 1.8 t ha^-1 less than potential, water limitation and nitrogen limitation yields, respectively. There was fairly large gap between the actual and the potential yields (about 3.4 t ha^-1). When averaged over years, total yield gap obtained for Kermanshah was 2.7 t ha^-1, for Kangavar was 3.7 t ha^-1, and for Ravansar was 3.8 t ha^-1. Across locations, contribution of yield gap from water limitation was 26%, for nitrogen limitation was 51.3% and for other limiting and reducing factors was 22.7%.
Conclusion: It can be concluded that the management timing of nitrogen application might reduce yield gap across locations. On the other hand, improved irrigation methods might lead to improved actual yield through preventing of the water and nitrogen leaching which it can reduce the total yield gap. In this study, only the role of water and nitrogen limitation was evaluated. The data collected from questionnaires showed that the yield gap of wheat was affected by the unsuitable of crop management practices, including optimum planting date, and pest and weed management that were not studied in the simulations.