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. In the past few decades, a number of thermal indices have been applied to estimate crop water stress under field condition. The difference between canopy temperature and air temperature (canopy-air temperature difference, CATD) was the first and one of the most commonly used thermal indices to quantify crop water stress. The summation of CATD over some critical period in the crop’s life cycle was termed as stress degree day (SDD). Similarly, the difference between canopy temperature of stressed and non-stressed plants has been used as an index called temperature stress day (TSD). The “canopy temperature variability” (CTV) takes into account the spatial variability of canopy temperature in crop field which was found to be higher in stressed plant than that of non-stressed plant. The temperature-time threshold (TTT) method assumes that the stress is not occurring in the crop until the canopy temperature reaches certain threshold value and calculates the amount of time that canopy temperature is greater than temperature threshold to quantify moisture stress. The crop water stress index (CWSI) further normalizes the canopy-air temperature difference with vapour pressure deficit of air. The calculation of CWSI quantifies the moisture stress of a plant as a comparison of its canopy temperature with that of a non-water-stressed plant and a maximum stressed plant with respect to their differences from the ambient air temperature at a given vapour pressure deficit. Conceptually, CWSI of a non-stressed and fully stressed (non-transpiring) plant is 0 and 1, respectively. The water deficit index (WDI) integrated the percent vegetation coverage and canopy temperature to compensate the effect of soil background that interferes in the remote measurement of canopy temperature through infrared thermometry. The “Biologically Identified Optimal Temperature Interactive Console (BIOTIC)” is an irrigation protocol that provides irrigation scheduling based upon measurements of canopy temperatures and the temperature optimum of the crop species of interest. But some critical issues like impact of rapid fluctuation in radiation and wind speed on crop water stress, crop to crop variability in stress tolerance and the requirement of stress at particular phenophases of some crops have not been duly focused. Thus the canopy temperature-based water stress indices have limited application in irrigation scheduling at field scale. However, with advancement of satellite-based optical and thermal remote sensing in recent years, there is a renewed interest in thermal indices for crop stress monitoring.