Remote Sensing for Crop Water Stress.

Water stress is one of the major causes for loss of crop productivity.  Remote sensing can be used in variable rate irrigation applications to detect crop water stress through thermal infrared or microwave sensing.  Thermal infrared sensing can be used to measure canopy temperature and either crop water stress or canopy level stomatal conductance.  Measurement of canopy temperature, when combined with reflectance measurements in the red and NIR regions, can  be used to construct reflectance index-temperature space graphs that lead to identification of field locations where nutrient and or water stress occur.  Thermal infrared sensing can also be used to infer crop water stress by measuring a Crop Water Stress Index (CWSI) that is proportional to the difference between canopy and air temperatures, but also depends on the atmospheric vapor pressure deficit.  CWSI values are estimated relative to the canopy and air temperatures for a non-stressed (well watered) crop.  This method works well for full crop canopies in close proximity to a well water section of the crop.  A simplified approach for estimating CWSI involves thermal infrared measurements of canopy temperature relative to the temperature of a nearby artificial reference surface consisting of a wet, white fabric covering polystyrene floating in a container of water.  Care must be taken to segment thermal images in fields with partial canopy cover in order to eliminate errors due to high soil temperatures.