3.2. Planting date
In general, paddy fields are plowed and flooded before rice planting. The VI of paddy fields decreases during this period and then increases again after rice planting. Thus, for detecting the planting date from the smoothed time profile of EVI, we defined the following procedure for determining the planting date: (i) the minimal point (the first derivative equals 0 and changes from negative to positive at this point) or inflection point (the second derivative equals 0 and changes from negative to positive at this point) earlier than 60 days from the estimated heading date was identified; then, (ii) the later one of these two points was automatically taken as the estimated planting date. The reason why two such discriminative points were used together for detecting the planting date is that the amount of biomass in the planting season is much less than that at the other phenological stages and it was difficult to detect subtle changes in EVI by only one discriminative point.