Canopy Temperatures: the Usefulness and Reliability of Remote Measurements1

Abstract

AbstractRemote measurements of leaf temperature offer a quick means of measuring individual leaf or canopy temperatures. The spatial area measured depends on the viewing angle of the infrared thermometer. This study was designed to evaluate the measurement of leaf temperature with attached vs remote measurements for a field row crop and to determine the relationship between angular and vertical temperature measurements for the seasonal growth of durum wheat. Leaf temperatures, measured with attached thermistors and thermocouples, were compared with those remotely observed with infrared thermometers to determine the accuracy of a remote measurement of leaf temperature.Small thermistors or thermocouples were attached to the underside of a leaf to measure leaf temperature and remotely measured with a Barnes PRT‐5 infrared thermometer with 2° field of view and 10.5–12.5 μm waveband, sited on the same leaf. These data were continuously, recorded for several days. These measurements were made on selected leaves from a field grown Gloria Pink bean (Phaseolus vulgaris L.) canopy in August 1976. Agreement was good for an individual leaf when the infrared measurement and attached thermistor measured the same location on an individual leaf. When the canopies or a composite of leaves were observed with infrared thermometers held vertically about one meter above the canopy and only selected leaves were measured with attached thermistors, the agreement was less good. Correction of infrared measurements to an emissivity of 0.96 decreased the difference between the two measurements in the afternoon (1330 hours PST) but increased the difference for the predawn readings. This suggests that one simple emissivity correction would not be sufficient. When angular measured canopy temperatures were compared with vertical measurements with the infrared thermometer throughout the growing season in durum wheat (Triticum durum Desf. ‘Produra’) canopies the difference between the two angular measurements was attributed to the amount of canopy seen by the infrared thermometer. When the canopy was sparse, the difference was minimal and increased to a maximum when the cover was between 20 and 50% and then decreased as the canopy became more full. Infrared temperature measurements provide reliable and accurate measurements of leaf temperature without physical attachments to the leaf. When the canopy is less than full, care has to be exercised to observe the canopy rather than a composite of soil and plants.