Nested Validation of Aquacrop Model for Simulation of Winter Wheat Grain Yield, Soil Moisture and Salinity Profiles under Simultaneous Salinity and Water Stress

Improvement of water management and water use efficiency at field scale is highly important under simultaneous salinity and water stress. The models that can translate the effects of different quantity and quality of water to crop yield, are useful tools for water management and improving water productivity (WP). AquaCrop (v4.0) simulates yield, water requirement, WP, soil water content and salinity under different conditions in the field, including water‐limiting and different quality of irrigation water. This study was carried out as split plot design (factorial form) in Birjand, in the east of Iran, in order to evaluate the AquaCrop model. Treatments consisted of three levels of irrigation water salinity (S₁, S₂, S₃ corresponding to 1.4, 4.5, 9.6 dS m‐¹) as the main plot, two wheat varieties (Ghods and Roshan), and four levels of irrigation water amount (I₁, I₂, I₃, I₄ corresponding to 125, 100, 75, 50% water requirement) as subplot. The model was nested, calibrated and validated separately for each salinity treatment, and also simultaneously for all three salinity treatments. The overall model accuracy was higher for estimating soil moisture profile, as compared to soil salinity profile. For simulation of soil water content, the average values of RMSE, d, CRM and R² in both calibration and validation were 11.8%, 0.79, 0.05 and 0.61 respectively, while for simulation of soil salinity they were 24.4%, 0.72, 0.19 and 0.57 respectively. The AquaCrop successfully simulated yield, biomass and WP for two wheat varieties under salinity and water‐limiting treatments with high accuracy. Average values of NRMSE, d, CRM, and R² for both calibration and validation of simulated grain yield were 7.1%, 0.97, 0.001 and 0.9 respectively, for the Roshan variety, while these measures were 8.2%, 0.98, −0.004 and 0.87 respectively, for the Ghods variety. Crop transpiration coefficient (KcTᵣ), normalized crop water productivity (WP*), reference harvest index (HIO), volumetric water content at field capacity (θFC), soil water content at saturation (θₛₐₜ), and temperature were the most sensitive parameters. Copyright © 2016 John Wiley & Sons, Ltd.