Measurement of Crop Temperature by Leaf Thermocouple, Infrared Thermometry and Remotely Sensed Thermal Imagery1

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Abstract
Crop temperature of alfalfa (Medicago sativa L.) was measured with an infrared thermometer (TIR) and with leaf thermocouples (TTC). TTC of corn (Zea mays L.) was also measured. Thermal imagery of the alfalfa research sites and neighboring fields was also obtained. The study was undertaken to determine daily patterns of crop temperature, to compare crop and air temperature, to determine whether or not alfalfa and corn are consumers or generators of sensible heat in the climate of the central Great Plains, and to determine the utility of using remotely sensed thermal imagery to measure crop temperature.TTC and TIR were often closer than 0.5 C but the agreement was not consistently better than 1 to 2 C. Measurements indicated that daytime thermal inversions existed over alfalfa fields during several hours on the days studied. Alfalfa was often 5 to 7 C cooler than air at the 200 cm level in mid and late afternoon. The intensity and duration of the daytime inversions observed in this study indicate that significant quantities of advected sensible heat are supplied to the alfalfa for consumption in evapotranspiration (ET). This finding supports ET results reported by earlier studies of the authors.The temperature of corn, alfalfa, and air were compared late in the growing season. Even though the corn was irrigated its temperature was consistently higher than that of a nearby alfalfa field. It was also warmer than the air except for short periods in the late afternoon. Unlike alfalfa, corn generated sensible heat and therefore, probably used significantly less water than did the alfalfa.Quantitative interpretation of the thermal imagery was not possible but imagery obtained in late spring indicated that wheat and alfalfa were at approximately the same temperature and both were cooler than pasture. If the net radiation and crop boundary layer resistance terms are similar for these three crops then the imagery suggests that wheat and alfalfa used water at about the same rate and that pasture used less water than either.