Knowledge of the expected canopy temperature in a given environment draws attention to orchard management when the canopy temperature is warmer than predicted. Canopy temperatures warmer than predicted can be due to insufficient available soil water and are remedied by irrigation. Insect and disease damage can also interfere with leaf physiology, stomatal function, and the resulting transpiration rate that increases leaf temperature. A crop water stress index (CWSI) was developed as a measure of the relative transpiration rate occurring from a plant at the time of measurement, using plant canopy temperature (measured with an IR thermometer) and the vapor pressure deficit (a measurement of the dryness of the air). Jackson et al. (1981) present the theory behind the energy balance, and it separates net radiation from the sun into sensible heat that warms the air and latent heat that evaporates water from the leaf, e.g., transpiration. When a plant is transpiring without limitations, the leaf temperature is at a theoretically low baseline limit (generally 1 to 7°C below air temperature), and the CWSI is 0. As the transpiration decreases due to insufficient water or other adverse factors, the leaf temperature rises and can be 4 to 6°C above air temperature. As the crop undergoes water stress, the stomata close, and transpiration decreases and leaf temperature increases. The CWSI is 1 when the plant is no longer transpiring and leaf temperature reaches an upper baseline limit. In general, the crop does not need to be watered until the CWSI reaches 0.1 to 0.2. In this range, the crop is transpiring at less than the optimal rate, and plant performance will start to decrease. The use of the CWSI in tree fruit crops is not as direct as in agronomic crops, however, because the tree canopy often has gaps exposing wood, soil, and sky. The canopy must be uniform to establish accurate baselines and reliable field measurements for irrigation scheduling.
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