Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 27, 2004
Publication Date: November 4, 2004
Citation: Peters, R.T., Evett, S.R. 2004. Modeling diurnal canopy temperature dynamics using one-time-of-day measurements and a reference temperature curve. Agronomy Journal. 96:1553-1561. Interpretive Summary: It has been shown that the temperature-time threshold (TTT) method of scheduling irrigations based on crop leaf temperatures works as good as or better than soil water based scheduling. Since the TTT method is automatic, it is also simpler and less expensive for farmers to use. In order to apply the TTT method to center pivots, the crop leaf temperatures, over a whole day cycle, have to be determined from a one-time-of-day measurement. This was done using a stationary crop leaf temperature sensor to capture the temperature dynamics due to weather changes throughout the day. Two different methods were then used to scale these reference temperatures using the one-time-of-day measurement. Crop leaf temperature data from corn, cotton, and soybeans were used to show that one or both of these methods for predicting canopy temperatures worked with reasonable accuracy. Cumulative irrigations predicted for the three crops for the entire season using the predicted temperatures were within 3/4 in. on average of those actually scheduled using measured data.
Technical Abstract: It has been shown that the temperature-time threshold (TTT) method of automatic irrigation scheduling is a viable alternative to traditional soil water based irrigation scheduling. The TTT method of irrigation automation involves using infrared thermometers to examine canopy temperatures over the course of a day, and determining if the threshold temperature is exceeded for a threshold time, in which case an irrigation is scheduled. Applying the TTT method to infrared canopy temperature sensors mounted on moving irrigation systems such as center pivots or linear move systems requires the extrapolation of the diurnal canopy temperature dynamics using a one-time-of-day canopy temperature measurement. This was accomplished using a stationary reference to capture the canopy temperature dynamics and by two different methods of extrapolating a curve from a one-time-of-day measurement. The first method assumes that the ratio of the difference between the measured canopy temperature and the reference canopy temperature will remain the same throughout the whole day. This ratio is then used to scale the reference (scaled method) to yield the predicted curve. In the second method, a three parameter Gaussian equation was empirically fitted to the temperature difference (Gaussian difference method) between the reference and the predicted canopy temperature curves. Canopy temperature data, measured by stationary infrared thermometers, from three different crops (corn, cotton, and soybeans) were analyzed to validate these methods. For a few hours after dawn and before sunset, the scaled method was generally more accurate, while during the middle of the day the Gaussian difference method was more accurate. The average absolute value of the error between the predicted and actual temperatures from the best of both methods during daylight hours was roughly 0.5 deg C. One or both of these methods for predicting canopy temperature from a one-time-of-day measurement can be used with reasonable accuracy. Irrigations predicted over three seasons using the extrapolated temperatures were within 18 mm on average of those actually scheduled using the TTT method and measured data.