Canopy Temperature-Based Water Stress Indices: Potential and Limitations

Abstract

Water stress in plant is associated with reduced availability of soil moisture under higher ambient temperature and wider vapour pressure deficit for a considerable period of time. Instruments like pressure chambers and porometers are being used to quantify crop water stress under field conditions, but their use is limited because of the numerous time-consuming measurements that must be made. The application of thermal indices involving canopy temperature for monitoring crop water stress and irrigation scheduling has been demonstrated by several researchers in the last five decades since the evolution of portable infrared thermometers in the 1960s. As the temperature of plant canopy is a manifestation of canopy energy balance, a water-stressed canopy is hotter than a well-watered one under the same environmental conditions. Infrared thermometer integrates the thermal radiation from all exposed surfaces in the field of view of the instrument that included the plant surface and exposed soil surfaces into a single measurement and converts it into temperature unit applying the principle of Stefan-Boltzmann law. However, different plant physiological as well as microclimatic factors like solar radiation, turbulence, air temperature and humidity must influence the canopy temperature at the time of observation. Hence, stomatal conductance and transpiration rates cannot be estimated by canopy temperature alone. In other words, canopy temperature alone is not enough to make estimates of plant water status. For this reason many researchers have attempted to normalize the canopy temperature to account for the influence of other variable microclimatic parameters like vapour pressure deficit, air temperature, wind speed, solar radiation, etc.