Remote Sensing of Canopy Temperature at Incomplete Cover1

AbstractA field study was conducted in a barley (Hordeum vulgare L.) canopy to assess the potential for extracting canopy temperature information from nadir radiometric measurements at incomplete cover. Composite temperatures consisting of emitted and reflected longwave radiation from the barley and the soil background were measured by a nadir‐viewing infrared radiometer. Canopy temperatures were measured by an infrared radiometer at a 30° angle from the horizontal. Soil temperatures were measured with thermocouples.Composite temperatures were 0.5 to 11.5 C higher than canopy temperatures with the largest difference occurring at low canopy cover. The correlation between composite and canopy temperature for data acquired throughout the growing season was not significant. A model which considered emitted radiation from both the canopy and the soil background, and which included reflected longwave sky irradiance was used to predict crop temperatures from nadir measurements. Predicted temperatures agreed with observed values (r2 = 0.88), and the prediction accuracy was independent of canopy cover. When emissivity corrections were not applied, prediction accuracy varied with percent cover with largest errors occurring at low cover. Prediction accuracy also varied with canopy cover when appropriate emissivities were used but sky irradiance was ignored. Results indicate that canopy temperatures can be estimated from nadir measurements at incomplete cover if percent cover, soil temperature, soil and canopy emissivities, and sky irradiance are known.