Modification of maize simulation model for predicting growth and yield of winter wheat under different applied water and nitrogen

Model WSM (Wheat Simulation Model) was developed based on the previous model (MSM, Maize Simulation Model). The planted cultivar was Shiraz with five irrigation treatments (1.2, 1.0, 0.8, and 0.5 ratios of the potential irrigation requirement under surface irrigation system and rain-fed) and four applied nitrogen treatments (0, 46, 92, 136 kg N ha−1). The irrigation water requirements were estimated by measuring the differences between soil field capacity and measured soil water content at root depth in the full irrigation treatment before irrigation. In the WSM model, dynamic flow of water, nitrogen, and heat through the soil were simulated numerically in an unsteady state condition at soil profile. Water and nitrogen transfer in the soil are governed by the Richard's equation and the diffusion convection equation, respectively. Emergence time of seed after sowing was simulated using soil water content, temperature, sowing depth, and soil particle diameters using beta function. Plant growth stages were simulated considering photoperiod, vernalization and air temperature. Hourly simulation of actual evaporation from soil surface and transpiration were simulated using the Penman–Monteith method based on atmospheric conditions and soil water content at root depth. Nitrogen uptake was simulated through mass flow and diffusion processes during the growing season. Produced dry matter was simulated as a function of hourly corrected intercepted radiation (based on air temperature) by plant leaves, maximum and minimum plant top N concentration and the amount of N uptake. Wheat grain yield was simulated by the ratio of grain N uptake and grain N concentration that were estimated by an empirical equation as a function of simulated top N uptake. Obtained experimental data in 2009–2010 were used to calibrate the model. The experimental results from 2010 to 2011 validated favorably the proposed model.