## 2 Materials and Methods
### Rice Phenology  
Rice phenology can be divided into three main phases: vegetative, reproductive, and maturity, according to the widely-used Biologische Bundesanstalt, Bundessortenamt und CHemische scale (a German scale used to identify the phenological development stages of cereals, BBCH) [13; 21; 38]. The principal phases and corresponding numerical ranges are shown in Table 1. Most of the Asian rice cultivars are largely grown under the transplanting procedure, which marks the beginning of a cultivation cycle in fields. Transplanting induces a development stoppage followed by a recovery period that can last about 19 days on average in this study area. Besides, the backscattering properties of a rice paddy during this period can be quite different from other stages in the vegetative phase, as shown later in this study. Therefore, the transplanting phase was extracted from the vegetative phase and specially analyzed.  
Four phenological phases, including the transplanting (from stage 0 to 19), vegetative (from stage 20 to 49), reproductive (from stage 50 to 69), and maturity (from stage 70 to 99) phases, are studied. They are distinguishable based on field observations (Figure 2) according to the BBCH scale. Fields at a sample site reach a particular BBCH stage when more than 50% of their plants reach that stage [6]. This criterion is considered in the analyses of the ground measurements and radar images.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Principle Phase** & **BBCH** & **Name** \\ \hline Vegetative & 00–09 & Germination \\  & 10–19 & Leaf development \\  & 20–29 & Tillering \\  & 30–39 & Stem elongation \\  & 40–49 & Booting \\ \hline Reproductive & 50–59 & Heading \\  & 60–69 & Flowering \\ \hline Maturity & 70–79 & Development of fruit \\  & 80–89 & Ripening \\  & 90–99 & Senescence \\ \hline Transplanting & 00–19 & Transplanting, recovery (rice only) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Description of the phenology of cereals using the BBCH Scale.  
Figure 2: Four rice phenological phases in the study area in 2016. (**a**) transplanting, (**b**) vegetative, (**c**) reproductive, and (**d**) maturity.  
According to field observations, the average durations of the transplanting, vegetative, reproductive, and maturity phase were 19, 45, 16, and 30 days, respectively, for fields in 30 sample sites in 2016. The 24-day revisit period of RADARSAT-2 might be too coarse for the monitoring of the phase with 16 or 19 days. However, because of the different cultivation management (e.g., transplanting time and fertilization) and rice varieties, the evolution pace of rice phenology varied at different sites. Accordingly, each phenological phase had an extended span and the corresponding phase can be observed. For example, the reproductive phase lasted about 16 days for most of the fields, but field observations showed that the reproductive phase started from June 22 and ended on July 26 in 2016 (Figure 3).  
Table 2 shows the typical growth rhythm extracted from the field measurement. About 63%, 100%, 47%, and 67% of sample sites were, respectively, in transplanting, vegetative, reproductive, and maturity phases on the dates of RADARSAT-2 acquisitions in 2016 (Table 2). It should be noted that on 15 May 2016, fields where three sample sites were located were not planted yet.