*4.2. Spring Phenology Derived from Different VIs*

Figure 8 shows the SOS detected from five different VIs. Nongrassland pixels (e.g., water bodies, forests, etc.) and the pixels whose SOSs were poorly detected were flagged as no data. SOS trends derived from different VIs were generally spatially consistent with the elevation variations. The SOS dates in the eastern part of the study area were earlier than those in the western parts. As shown in Figure 8, the SOS dates detected by NDVI were very similar to the SOS dates detected by EVI2, while those from NDPI and NDGI were highly consistent and those detected by NIRv were in between. The histograms revealed that the SOS dates detected by the five VIs were generally similar in terms of the value ranges and peak values. The detected SOS dates ranged from 105 to 175, while the average SOS dates derived from the five VIs followed the order of NDPI (DOY 154) > NDGI (DOY 152) > NIRV (DOY 150) > EVI2 (DOY 145) > NDVI (DOY 141), which was very consistent with previous studies in this study area [38,55,59]. This indicates that the SOS dates detected by the five VIs were generally consistent and reliable.

**Figure 8.** The SOS detected by (**a**) NDVI, (**b**) EVI2, (**c**) NDPI, (**d**) NDGI, (**e**) NIRV, and (**f**) the corresponding histograms for 2020.

Scatterplots in Figure 9 compare the SOS dates detected from different VIs on a pixelby-pixel basis. According to the consistency between the detected SOS, the five different VIs were aggregated into two groups. One group included NDGI and NDPI (R = 0.849 and RMSE = 5.957), and the other group included EVI2, NDVI, and NIRv. The SOS dates detected by EVI2 were highly correlated with those detected by NDVI (R = 0.934 and RMSE = 6.416) and NIRv (R = 0.938 and RMSE = 6.870), while the SOS detected by NDVI and NIRv had lower correlations (R = 0.776 and RMSE = 12.722). The SOS detected by the VIs from different groups showed large discrepancies. For example, the SOS dates from NDVI and NDPI were poorly correlated (R = 0.445) and had a large bias (RMSE = 17.823). In addition to the differences in VI calculations, this different performance may also be attributed to the different effects of snow cover on different VIs.

**Figure 9.** Comparison of the SOS detected by different VIs. The color from light gray to dark gray indicates increasing sample densities. (**a**–**j**) are scatterplots of the SOS dates detected from different VIs on a pixel-by-pixel basis.
