Modeling diurnal canopy temperature dynamics using one-time-of-day measurements and a reference temperature curve

The application of the temperature-time threshold (TTT) method of irrigation scheduling to self-propelled irrigations systems requires a method of estimating the diurnal canopy temperature dynamics using only a one-time-of-day measurement. Other research efforts such as the crop water stress index (CWSI) and field canopy temperature mapping using moving irrigation systems could also be served by the use of this method. This was accomplished using a stationary reference measurement to capture the canopy temperature dynamics. Two different methods were developed for estimating a temperature curve for a remote location from a one-time-of-day measurement at that location. The first method (scaled method) uses the ratio between the reference temperature and the remote location temperature, referenced to the predawn temperature, to scale the reference curve to yield the predicted curve. In the second method (Gaussian difference method), a three-parameter Gaussian equation was empirically fitted to the temperature differences between the reference and the measured remote canopy temperature curves. To test these two methods, canopy temperature data, sensed using stationary infrared thermometers, from three different crops (corn (Zea mays L.), cotton (Gossypium hirsutum L.), and soybean [Glycine mar (L.) Merr.]) were analyzed. 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 0.5degreesC. For all 3 yr, the total irrigation for a season using the extrapolated temperatures were within 18 mm on average of those actually scheduled using the TTT method and measured data.