Temporal variations of directional brightness temperature over a maize canopy in South France

A field experiment has been conducted from May to August in 1999 over a maize canopy at INRA Avignon branch in France. The experiment covered the whole growth period of maize plants in order to observe directional brightness temperature (TBD) variations of row structure canopy as a function of measuring time and date. The TBD was extracted from thermal infrared (TIR) images captured by a TIR camera mounted upon a crane. The results show that TBD were highly date and time specific. Comparing the variations of DBT near the noontime with different biomass, the measurements over a middle dense canopy revealed an evident row-direction-oriented hot stripe in the DBT polar map, where a hot spot appeared along the solar direction. Similarly, for a low cover condition, a wider hot stripe appeared in the polar map, the hottest area in the band is around the solar position. However, for a high biomass condition when the field was nearly wholly covered by the leaves, the lowest temperature appeared along the row direction, which formed a cool strip in the polar map. Comparing the variations of DBT on the same day, for a measurement over middle dense canopy, in the morning and afternoon, with a large solar zenith, the range of DBT variation decreased, hot strip features weakened and turned wider, solar position was out of the hot strip. As a conclusion of the field experiment, the maize TBD feature is dominated by sun-sensor geometry, complex effects of canopy structures coupled with spatial distributions of canopy brightness temperatures. A row structure effect appeared clearly in the TBD polar map throughout the whole measurement. Quantitative explanations are expected in further research on the physical models.