3. Threshold for detecting phenological stages
3.1. Heading date
According to time-series data of the spectral reflectance of paddy fields (Shibayama & Akiyama, 1989), the maximum NDVI appears around the heading date. The reason for this is that the rice changes its growth phase from vegetative growth to reproductive growth on reaching the heading date, and leaves begin to wither and die. Therefore, we assumed that the smoothed time profile of EVI also shows a peak on the heading date. We defined the date of the maximum EVI in the time profile as the estimated heading date.