When assessing the status of plants and crops, the crop water stress index (CWSI) is probably the best choice. Using a simple thermal image and ground measurements of a wet and a dry surface or air temperature, you can calculate an estimate of the plant water stress. But the CWSI QGIS3 Plugin incorporates three different workflows to assess the hydration status of your plants and crops. The most common workflow using a dry and wet bulb as field reference measurements, a workflow that only requires a ambient temperature as reference and a more experimental workflow that just requires a thermal map or image.
I created a tutorial to guide you through the whole workflow from thermal image, to adding the field reference data, to your final CWSI estimates and how to interpret them.