## 4 Results
### Assessment of the Key Periods  
The assessment of the key periods is conducted on the study area B. The time efficiencies and result accuracies of two period strategies were compared. In Period I, all the available CCD images (eight CCD images in total) in study area B during the entire rice growth period were used for assimilation. In Period II, the dominant points were chosen for assimilation. The simulated WRT of two period strategies were obtained and compared with the measured WRT. The accuracy and efficiency results of two period strategies are shown in Table 6. For simulation accuracy, Period I resulted in higher precision than Period II; overall, both periods resulted in good precision. However, with respect to time efficiency, Period II shortened the run time of the model operation by more than 50%, which is a clear advantage over Period I. Period II greatly reduced the time efficiency of the model while maintaining the accuracy of the model. It was proven that the selected key period was reasonable and practical; furthermore, the selected key period also provided the key period for the experimental field observations.  
Figure 10: Assessment of the accuracy for the four dominant points. (**a**) shows the the fitted results of the LAI in study area C, (**b**) shows the ratio of the LAI in the two study areas.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample Plot**} & \multicolumn{4}{c}{**Serial Number of the Dominant Point**} \\ \cline{2-5}  & **1** & **2** & **3** & **4** \\ \hline
**Study Area C** & 184 (DOY) & 199 & 217 & 243 \\
**Ratio (LAIC/LAIA)** & 172 & 184 & 217 & 243 \\
**Result** & 184 & 199 & 217 & 243 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Position of the dominant points in two different curves and the determination of the dominant points.  
The LAI values calculated from the four CCD images based on the optimal time points were incorporated into the RS-WOFOST model, and thus, the spatio-temporal distribution of heavy metal stress in rice can be obtained by the RS-WOFOST model for each pixel for any day during the growing period. To clearly show the distribution of heavy metal stress in rice, four dates during each rice growth stage were chosen as being representative of the WRT distribution, which reflects the changes in the WRT during the entire growing season (Figure 11). As mentioned above, the irreversible effects of heavy metal suppress the normal growth of rice roots, which is clearly reflected in the negative correlation between heavy metal stress and the health of rice tissues. From the perspective of the growth mechanism, during the tillering stage and jointing-booting stage, rice tissues prioritize vegetative growth, and a large amount of water and nutrients are required, which is when many of the heavy metals enter into the root cells, which restrains and damages cell growth and affects the WRT; then, during the heading-flowering stage, rice tissues enter into the reproductive growth stage, which gives priority to grain growth; finally, during the ripening stage, the absorption of the roots declines, which reflects the ageing of the roots, and the effect of heavy metals on the rice decreases.