Canopy temperature and heat stress are increased by compound high air temperature and water stress, and reduced by irrigation – A modeling analysis

Abstract. Crop yield is reduced by heat and water stress, and even more when they co-occur. Yet, compound effects of air temperature and water availability on crop heat stress are poorly quantified: crop models, by relying at least partially on empirical functions, cannot account for the feedbacks of plant traits and response to heat and water stress on canopy temperature. We developed a fully mechanistic model coupling crop energy and water balances, to determine canopy temperature as a function of plant traits, stochastic environmental conditions and their variability; and irrigation applications. While general, the model was parameterized for wheat. Canopy temperature largely followed air temperature under well-watered conditions; but when soil water potential was more negative than −0.14 MPa, further reductions in soil water availability led to a rapid rise in canopy temperature – up to 10 °C warmer than air at soil water potential of −0.62 MPa. More intermittent precipitation led to higher canopy temperatures and longer periods of potentially damaging crop canopy temperatures. Irrigation applications aimed at keeping crops under well-watered conditions could reduce canopy temperature, but in most cases were unable to maintain it below the threshold temperature for potential heat damage; the benefits of irrigation became smaller as average air temperature increased. Hence, irrigation is only a partial solution to adapt to warmer and drier climates.