3.3. Harvesting date
After the heading season, VI in paddy fields begins to decrease as leaves wither and die. It then decreases abruptly because of harvesting. Therefore, the date of an inflection point (the second derivative equals 0 and changes from positive to negative) later than 30 days after the estimated heading date was defined as the estimated harvesting date.
4. Results and discussion
4.1. Validation with statistical data
Table 1 shows the performance of the method using the wavelet and Fourier transforms for detecting the phenological stages and growing period. There was no significant difference between using the wavelet and Fourier transforms in the heading and harvesting dates. However, in determining the planting date, the performance of the method using the wavelet transform was superior to that using the Fourier transform. The maximum difference of the root mean square error (RMSE) for estimating the planting date was 5.1 days between them. For estimating the growing period, the maximum difference was 7.0 days. This difference in estimating the growing period resulted mainly from the difference in estimating the planting date. Among the mother wavelets tested here, Coiflet 4 gave the best estimate.
Table 1. Comparison of root mean square error (RMSE) of the estimated phenological date and growing period against the statistical data