## 2 Materials and Methods
### Data Collection and Processing  
In this study, the DJI M600 Pro UAV equipped with a Rikola hyperspectral imager (Rikola Ltd., Oulu, Finland) (Figure 2) was used to acquire hyperspectral images of rice. The wavelength range of the imager is 500-900 nm, with 62 bands. The imager's default band range is 500-900 nm, and the number of bands that can be set is 0-380. In this study,  
Figure 1: The general location of the field experiment.  
the number of bands was set to 62, and the center wavelength and full width at half maxima of these bands were as shown in Table 1[37].  
The flight time of UAV was around 11:00 to 12:00 at noon (Beijing local time), when the weather condition was relatively clear and sunny, wind-free and cloud-free. The flight altitude was 200 m, with spatial resolution of hyperspectral image of 13 cm. Hyperspectral images with 1010 by 1010 pixels were obtained at the booting stage, heading stage, filling stage and ripening stage. In order to obtain hyperspectral data before and after flowering, intensive data collection campaigns were carried out from the late booting stage to the end of the flowering stage, with frequency of once every one to two days to ensure the complete time series data before and during the rice flowering stage.  
All hyperspectral images were processed with vignetting correction, lens distortion correction, and black level calibration by using the corresponding module functions in a Rikola Hyperspectral Imager V2.1.4 (Rikola Ltd., Oulu, Finland) software to solve the problems of image chromatic aberration, geometric deformation, and temperature-sensing dark current effects. DN values of raw images were transformed to radiance values [38]. Reflectance calculations were carried out using ENVI 5.1 software (Harris Geospatial Solutions, Inc., Boulder, CO, USA) by choosing the gray cloth (diffuse reflection area) in the processing image as the standard reference board and calculating the reflectance of each  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**WL** & **FWHM** & **WL** & **FWHM** & **WL** & **FWHM** & **WL** & **FWHM** & **WL** & **FWHM** \\ \hline
504 & 8.18 & 592 & 7.24 & 680 & 9.80 & 733 & 6.39 & 816 & 9.10 \\
512 & 8.18 & 596 & 7.72 & 685 & 5.82 & 736 & 6.68 & 824 & 7.69 \\
520 & 8.51 & 600 & 7.76 & 688 & 6.06 & 740 & 6.14 & 832 & 13.62 \\
528 & 7.58 & 604 & 7.26 & 692 & 6.50 & 744 & 6.23 & 840 & 14.49 \\
536 & 8.55 & 608 & 9.24 & 696 & 7.45 & 748 & 5.87 & 848 & 13.53 \\
544 & 7.63 & 616 & 10.21 & 700 & 6.99 & 752 & 6.59 & 856 & 13.61 \\
552 & 7.77 & 624 & 9.20 & 704 & 5.89 & 760 & 5.96 & 864 & 12.42 \\
560 & 7.98 & 632 & 8.93 & 709 & 6.00 & 768 & 6.09 & 872 & 12.75 \\
568 & 7.27 & 635 & 9.29 & 712 & 6.44 & 776 & 8.71 & 880 & 12.83 \\
576 & 8.30 & 650 & 8.36 & 716 & 6.91 & 784 & 7.38 & 888 & 10.51 \\
580 & 8.16 & 656 & 9.37 & 720 & 6.86 & 792 & 8.51 & & \\
584 & 7.24 & 664 & 8.75 & 724 & 6.22 & 800 & 8.72 & & \\
588 & 6.51 & 672 & 9.43 & 728 & 5.81 & 808 & 8.16 & & \\ \hline \end{tabular} Note: green band (WL = 504–560 nm), yellow band (WL = 568–616 nm), red band (WL = 624–680 nm), red-edge band (WL = 685–760 nm), NIR band (WL = 768–888 nm).  
\end{table}
Table 1: Wavelength (WL, nm) and full Width at Half Maxima (FWHM, nm) of all the bands of Rikola.  
Figure 2: UAV-based hyperspectral platform.  
pixel (Formula (1)), and then obtaining the average reflectance curve of each plot through the Statistics for All ROIs function of ENVI.  
\[Ref_{img}\ =\ \frac{Rad_{img}\times Ref_{grey}}{Rad_{grey}} \tag{1}\]  
where \(Rad_{grey}\) is the radiation of the reference board, \(Rad_{img}\) is the radiation of image, and \(Ref_{grey}\) is the reflectivity of the reference board.