Continuously observed spectral reflectance imagery in the monitoring of rice leaf greenness: Effective bi-band ratio and observation time of day

We demonstrated a close-range sensing technique to seasonally track trends in canopy leaf greenness in rice paddies. A weatherproof narrow-band triplet digital camera system was positioned 3 m above a paddy rice field during the 2017 rice season in Kagoshima. The system automatically logged reflectance images at 10-min intervals from 8:00 to 16:00 in visible green (560 ± 10 nm), red (650 ± 10 nm), and near-infrared (855 ± 10 nm) bands. Two rice cultivars were grown at two fertilizer concentrations and seedling numbers in three replicates in concrete-framed plots. The camera was mounted on a platform at the top of an iron pole installed on a handcart built with scaffolding pipes. The east-looking camera captured oblique images of four plots (2 × 2 plots) with the camera's axis tilted 65° from vertical with the handcart positioned 3.2 m away from the west side of the plots. To capture images of a different group of plots each day, the handcart was manually moved to a randomly chosen set position among the five predetermined positions north and south along the field on the concrete-paved open side area every morning. Radiometric corrections for images utilized solar irradiance sensors and prior calibrations to calculate instantaneous reflectance in each band. The leaf greenness value (SPAD) and the leaf area index (LAI) were weekly collected for rice canopies, and daily values were obtained by interpolating the weekly data. From reflectance images, SPAD and LAI data were analyzed for 35-day period including the panicle formation stage. The canopy greenness index, defined as the product of (SPAD-30) and LAI (which potentially represents canopy nitrogen condition), highly correlated (r = 0.62, n = 4276) with the canopy surface averages of the bi-band ratio (NIR/Green), where NIR and Green indicate the reflectance values in the near-infrared and green band, respectively. Further investigations including daily-averaged or hourly-sampled reflectance and/or bi-band ratios revealed that the correlation coefficient for the daily-averaged reflectance was 0.71 (n = 128) and that for the hourly-sampled reflectance was 0.73 (n = 528) for the 15:00-16:00 period and 0.62 (n = 690) for the 11:00-14:00 period. We found that NIR/Green values obtained during the late afternoon time period best tracked the canopy greenness and differences in the cultivar and fertilization levels.