<i>Diurnal dynamics of canopy temperature in management zones for a variable rate irrigation system</i>

Using infrared thermometers (IRTs) mounted on center pivot irrigation system to make the dynamic prescription maps has been demonstrated to be a competitive method for variable rate irrigation management. In this field study, the diurnal canopy temperature dynamics of summer maize and contour maps of canopy temperature were obtained from IRTs installed in the stationary positions and moving irrigation system to choose the reasonable period for capturing single-time-of-day canopy temperatures of large field areas. The 1.64-ha experimental site located in a highly variable alluvial flood plain was divided into four management zones according to the available water holding capacity ranging from 152 to 205 mm within the 1.0 m soil profile. In each management zone, the irrigation was triggered when the soil water content reached 70% of field capacity and irrigated with a variable rate center-pivot irrigation system. Three IRTs were installed in the locations representing the mean soil water content and connected to a datalogger. An array of 10 IRTs were also connected to the datalogger and mounted on the center pivot irrigation machine. Contour maps of daytime canopy temperature were created based on the measurements every 2 h by running the center pivot. Results showed that the canopy temperature had variation trend similar to the air temperature. The canopy temperature values were low from sunset until sunrise, while they were high and changed minor from 11:00 to 17:00 local time and the differences among zones were large in this period. With the increasing of water stress, the canopy temperature became higher than air temperature. The order of canopy temperature values in management zones kept consistent with their irrigation water amount under VRI management. Contour maps of daytime canopy temperature were not fully consistent even in the period from 11:00 to 17:00 local time. These results demonstrated that when generating the dynamic prescription maps for variable rate irrigation management using IRTs mounted on center pivot irrigation system, the canopy temperatures should be adjusted to the same time at peak stress because of the fluctuation of canopy temperature and the slow moving speed of center pivot. The integration of soil moisture sensors with IRTs system was highly desirable to choose the scanning date of canopy temperature using center pivot machine.