Canopy Temperature (CT) and Canopy Temperature Depression (CTD)

Canopy Temperature (CT)

• Definition: The temperature of the crop canopy (leaf surface temperature) measured relative to the surrounding air.
• Measurement tools:
  • Infrared thermometers (IRTs)
  • Thermal imaging cameras
  • Remote sensing (drones/satellites)
• Normal range: Generally 1–3 °C lower than air temperature under well-watered conditions.

Concept:

• When transpiration is active → evaporative cooling → canopy is cooler than air.
• Under water stress → stomata close → transpiration reduced → canopy temperature rises above air temperature.

Canopy Temperature Depression (CTD)

Definition: The difference between air temperature (Ta) and canopy temperature (Tc).CTD = Ta − TcCTD

Interpretation:

• High CTD → canopy is cooler than air → indicates good transpiration & water status.
• Low CTD or negative CTD → canopy warmer than air → indicates water stress, stomatal closure.

Significance of CT & CTD

• Indicator of Crop Water Status: Low canopy temperature (high CTD) = well-hydrated crop. High canopy temperature (low CTD) = water stress.
• Tool for Irrigation Scheduling: Canopy temperature can be used to determine when to irrigate. Related to Crop Water Stress Index (CWSI).
• Breeding for Stress Tolerance; CTD is used as a selection criterion in drought-tolerant wheat, sorghum, and pearl millet. Cooler canopies under stress = better genotypes.
• Relation to Yield: In wheat, genotypes with higher CTD often produce higher yields under drought or heat stress. CTD > 2 °C is generally associated with superior yield performance under water-limited conditions.

Key Facts & Values

• Infrared thermometer is most common tool for canopy temperature.
• CTD is higher in drought-tolerant genotypes (wheat, sorghum, chickpea).
• CTD value often ranges between 1–5 °C depending on water status.
• High CTD (2–5 °C): Good transpiration, cooler canopy, higher yield potential.
• Low CTD (0 to –1 °C): Water stress, stomatal closure, canopy hotter than air.
• Used by: CIMMYT & ICAR programs for drought screening in wheat and millets.

In short for ASRB NET:

• Canopy Temperature (CT): Leaf/canopy temperature relative to air, measured with infrared thermometer.
• Canopy Temperature Depression (CTD): Ta – Tc.
• Significance: High CTD = cooler canopy = better transpiration & drought tolerance; Low CTD = warmer canopy = stress.