Leaf Temperature Infrared Thermometry & Crop Water Stress Index (CWSI) Sensing

1. Why Ambient Air Temperature Is Insufficient

In high-wire greenhouse steering, crops are active thermodynamic heat exchangers. Under intense radiation (\(> 600 \text{ W/m}^2\)), active transpiration evaporatively cools the foliage, causing leaf temperature (\(T_{leaf}\)) to be 2°C to 4°C cooler than ambient air (\(T_{air}\)). Conversely, when roots experience water stress or high root zone EC, stomata close, eliminating latent heat loss and elevating \(T_{leaf}\) up to 3°C above air temperature.

2. Stefan-Boltzmann Law & Infrared Radiometry

CWSI scales strictly between 0 and 1:

• CWSI = 0.0 – 0.2 (Optimal Hydration): Maximum transpiration cooling (\(\Delta T_{lower}\) baseline); stomata fully open.
• CWSI = 0.3 – 0.5 (Mild Generative Stress): Controlled mild deficit steering, stimulating fruit sugar accumulation without vegetative collapse.
• CWSI > 0.6 (Critical Drought Stress): Stomatal closure (\(\Delta T_{upper}\) threshold); immediate irrigation trigger required.

3. Integrating IR Leaf Sensors with VPD Control

Calculating Leaf-to-Air VPD (VPD_leaf) using actual contact/IR thermopile data provides 100% true physiological feedback, preventing catastrophic over-ventilation during sudden sun breaks in spring and autumn.