Comparison of scaled canopy temperatures with measured results under center pivot irrigation

Remote sensing of crop canopy temperatures using infrared thermometers is being used in conjunction with several new developments in irrigation scheduling and control. These include the time-temperature-threshold (TTT) method of irrigation scheduling, the crop water stress index (CWSI), and the creation of field level canopy temperature maps using infrared temperature sensors mounted on self-propelled irrigation systems. A method of estimating the canopy temperature dynamics throughout the day using only a one-time-of-day canopy temperature measurement is useful in the application of many of these technologies to self-propelled irrigation systems such as center pivots or lateral moves. Two different algorithms developed by Peters and Evett (2004) for doing this were tested using data collected under center pivot irrigation. These algorithms use the canopy temperature dynamics captured at a stationary location to create a reference curve. Sixteen different infrared thermometers were positioned in stationary locations throughout a field with four different irrigation level treatments; 100%, 66%, and 33 % of the irrigation requirements, and a non-irrigated, or dryland treatment. One-time-of-day canopy temperature measurements were taken from each of these treatments at various times of day and were scaled using the reference curve to estimate diurnal canopy temperature curves. These curves were then compared with the actual measurements and the errors were analyzed. Mean errors using one-time-of-day measurements early in the day (before 0800 h) and late in the evening (after 2200 h) were unacceptably high. However, the absolute mean errors using measurements taken during the middle of the day were approximately 1°C with a standard deviation of about 1°C. The stress level of the reference curve crop did not make a significant difference in the absolute mean errors