## 4 Results
### Phenological Metric Detection  
Based on the three VI time series, the SOS and EOS over the XUAR are mapped for the period 2001-2012. Their mean and standard deviation (SD) from the three VI datasets for each vegetation type are summarized in Table 4. The standard deviation of SOS ranges from 11.9 days to 37.6 days. The standard deviation of EOS ranges from 13.3 days to 47.0 days. The EOS from NDVI shows greater deviation than SAVI and EVI except for evergreen needleleaf forest. The SOS from NDVI time series show greater deviation than SAVI and EVI except for cropland.  
Histograms of the two phenology metrics for different vegetation types are illustrated in Figure 8. Despite the shift of peak, the histogram distributions of SOS and EOS from three VIs are in agreement. Obvious bimodal distributions are displayed for the SOS of grassland, SOS of cropland and EOS of cropland. The bimodal distributions of SOS and EOS of cropland indicate different phenological patterns of different crop types. Due to the influence of bare soil on vegetation index [47], grasslands of different densities may show inconsistent green-up behaviours, which causes the bimodal distributions of SOS. Discrepancies from three VIs are observed for the SOS of grassland, EOS of evergreen needleleaf forest and EOS of grassland. For grassland, SOS\({}_{\rm NDVI}\) has a large peak and secondary peak at DOY80 and DOY110, while SOS\({}_{\rm NDVI}\) has two similar peaks. Similar discrepancies are observed from EOS of grassland. For evergreen needleleaf forest, EOS\({}_{\rm EVI}\) has bimodal distribution compared to unimodel distribution of EOS\({}_{\rm NDVI}\) and EOS\({}_{\rm EVI}\). The values distributed near the tail of the histogram indicate overestimations in the EOS\({}_{\rm NDVI}\) of evergreen needleleaf forest, mixed forest open shrupland, woody savanna, savanna and grassland.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline  & \multicolumn{2}{c}{**SOS\({}_{\rm NDVI}\)**} & \multicolumn{2}{c}{**SOS\({}_{\rm SAVI}\)**} & \multicolumn{2}{c}{**SOS\({}_{\rm SAVI}\)**} & \multicolumn{2}{c}{**EOS\({}_{\rm SAVI}\)**} & \multicolumn{2}{c}{**EOS\({}_{\rm SAVI}\)**} & \multicolumn{2}{c}{**EOS\({}_{\rm SAVI}\)**} \\ \cline{2-13}  & Mean & SD & Mean & SD & Mean & SD & Mean & SD & Mean & SD & Mean & SD \\ \hline Evergreen needleleaf forest & 117.4 & 29.1 & 112.8 & 21.4 & 113.0 & 24.4 & 277.3 & 26.3 & 284.9 & 20.2 & 306.3 & 28.0 \\ Mixed forest & 127.0 & 26.6 & 119.5 & 15.0 & 114.9 & 15.8 & 274.3 & 28.1 & 286.8 & 15.2 & 311.3 & 19.3 \\ Open shrubland & 142.2 & 27.6 & 137.7 & 22.7 & 135.6 & 20.9 & 308.0 & 35.7 & 315.1 & 28.0 & 324.1 & 28.0 \\ Woody savannas & 120.2 & 37.6 & 113.8 & 24.2 & 112.5 & 26.8 & 288.4 & 45.1 & 294.7 & 24.1 & 310.6 & 26.6 \\ Savannas & 125.6 & 32.4 & 120.7 & 16.3 & 120.9 & 17.4 & 297.7 & 47.0 & 298.0 & 14.4 & 307.6 & 14.3 \\ Grassland & 117.7 & 35.4 & 115.9 & 28.0 & 117.1 & 29.0 & 311.2 & 42.6 & 311.6 & 29.3 & 314.4 & 28.2 \\ Cropland & 139.1 & 21.9 & 133.3 & 20.7 & 132.5 & 22.1 & 296.5 & 19.6 & 306.4 & 18.0 & 316.1 & 17.1 \\ Cropland/natural vegetation mosaic & 124.6 & 17.8 & 116.7 & 11.9 & 111.6 & 12.0 & 285.9 & 25.9 & 298.8 & 16.4 & 314.0 & 13.3 \\ \hline \end{tabular}
\end{table}
Table 4: Mean day of year (DOY) and standard deviation (SD) of phenological metrics detected from VIs for 2001–2012 for the XUAR.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Percentage** & **NDVI** & **SAVI** & **EVI** \\ \hline
0\%–10\% & 77.44 & 78.91 & 69.16 \\
10\%–20\% & 15.93 & 14.74 & 18.89 \\
20\%–30\% & 3.24 & 2.93 & 5.67 \\ \textgreater{}30\% & 3.39 & 3.42 & 6.28 \\ \hline \end{tabular}
\end{table}
Table 3: Percentage of mean annual VI variability for 2001–2012 for the XUAR.