Investigating the Gap Frequency of a Maize Canopy using Night TIR Data

Gap frequency is the probability that a light penetrates through the canopy unintercepted and reaches the surface under the vegetation. Previous study revealed that 22h local time, the brightness components of maize field were stable, so the they were easy to be distinguished in TIR images. To investigate the directional variation of gap fraction over a medium dense maize canopy (LAI=3.64) by night TIR data, an experiment was carried out on August 23th 2005 in Huailai county of Hebei Province. The acquisition of TIR images was conducted by a system consisting of a broadband TIR camera, and a truck platform whose height is about 2.5 m. The gap frequency can be calculated by threshold value method. The result showed the main characteristics of row crop that very little azimuth variation appears for gap frequency except for the observations along row direction, in the row direction the gap frequency is 0.35 and declines slowly with the zenith angles and converges to a unchanged value 0.14 along row direction; gap frequency declines from the nadir slowly; in other azimuth directions, gap frequency declines sharply. Put the measured parameters into a GORT model, the directional gap frequency can be simulated. Compared the measured gap frequency with the simulated one, the two had good agreement. The differences between them were due to reasons, such as the disagreement of the objects, the angle variation was not controlled accurately in experiment, the selection of projection function and clumping parameter et al.