Model for simulation of winter wheat yield under dryland and irrigated conditions

The ability to accurately model crop water requirements can improve irrigation management. Forecasting of wheat yield at the time of planting under different irrigation management programs is possible with a wheat growth simulation model calibrated for the study area. In this study, a model was developed for simulation of wheat growth and yield. The model involves a water balance and a crop yield submodels. The model can provide daily estimate of soil water content, potential and actual evapotranspiration, runoff, deep percolation, leaf area index (LAI), dry matter (DM) and finally grain yield. Soil properties and daily rainfall and pan evaporation are model inputs. The sowing date and soil water content at sowing date should be specified. The wheat growing season was divided into three periods; pre-anthesis, anthesis and post-anthesis. During the first stages, the crop assimilate was devoted to biomass increment. While, in the last stage, crop assimilate increased the grain mass. The leaf area index was related to total dry matter and pan evaporation since sowing. Available energy was partitioned between evaporation and transpiration according to leaf area index. Actual transpiration was dependent on the available soil water in the root zone. The root zone consisted of two soil layers. The actual transpiration and transpiration efficiency were used to estimate the daily dry matter increment. Grain number was dependent on total dry matter at anthesis. Finally, grain yield was obtained as production of grain mass and grain number. The model was calibrated for winter wheat yield in Badjgah area, Fars province, Islamic Republic of Iran with data from an experiment with five irrigation treatments. The model produced good estimates of dry matter and grain yield. The accuracy of the model was verified with data from another 12 irrigation treatments in study area. The model was applied to estimate dryland winter wheat yield for a 17-year period. The estimated grain yields were reasonable. The results indicated that soil water content and irrigation at sowing date could increase the total dry matter. However, they could not influence the grain yield considerably. (C) 2003 Elsevier Science B.V. All rights reserved.