I frequently see that people in the industry think of or even use NDVI (Normalized Difference Vegetation Index) as an indicator of plant water stress. I've also seen the confusion about NIR (near infrared: reflected radiation) and IR thermal infrared: emitted radiation from warm objects). I'm not going to get into the details of this, but I will mention a few helpful points:
1) NDVI is good for comparing dead/diseased vs healthy plants. It can detect severe plant water stress. It can’t be used for water management. Don’t try to correlate it with soil water content!
Figure 1. NDVI is good for comparing dead/diseased vs healthy plants. It can detect severe plant water stress.
2) Normalized thermal data in the form of CWSI (Crop Water Stress Index), on the other hand, is good for detecting plant water stress and irrigation scheduling. The CWSI can detect mild levels of plant water stress. If the right model is used it will show high correlation with soil water deficit.
The BINA Pro (DurUntash Lab, San Diego, CA) uses different types of NDVI to separate plant from the background and create a mask. This mask is then multiplied by the thermal image of the same plant (in real-time) to calculate its min, max, and average surface temperatures (Ts). Ts is then normalized (using microclimate data) to calculate the CWSI.