Canopy temperature for simulation of heat stress in irrigated wheat in a semi-arid environment: A multi-model comparison

Even brief periods of high temperatures occurring around flowering and during grain filling can severely reduce grain yield in cereals. Recently, ecophysiological and crop models have begun to represent such phenomena. Most models use air temperature (T_air) in their heat stress responses despite evidence that crop canopy temperature (T_c) better explains grain yield losses. T_c can deviate significantly from T_air based on climatic factors and the crop water status. The broad objective of this study was to evaluate whether simulation of T_c improves the ability of crop models to simulate heat stress impacts on wheat under irrigated conditions. Nine process-based models, each using one of three broad approaches (empirical, EMP; energy balance assuming neutral atmospheric stability, EBN; and energy balance correcting for the atmospheric stability conditions, EBSC) to simulate T_c, simulated grain yield under a range of temperature conditions. The models varied widely in their ability to reproduce the measured T_c with the commonly used EBN models performing much worse than either EMP or EBSC. Use of T_c to account for heat stress effects did improve simulations compared to using only T_air to a relatively minor extent, but the models that additionally use T_c on various other processes as well did not have better yield simulations. Models that simulated yield well under heat stress had varying skill in simulating T_c. For example, the EBN models had very poor simulations of T_c but performed very well in simulating grain yield. These results highlight the need to more systematically understand and model heat stress events in wheat.