*5.1. Validity of the SOS Dates Detected by Different VIs*

The SOS dates detected by the five different VIs all captured the spatial pattern of the SOS, which occurred earlier in the eastern areas and later in the western areas (Figure 8). The SOS dates detected by five different VIs also have very similar data distributions, ranging from 105 to 175 with peak values of approximately 157. Both the spatial details and data values were highly consistent with each other and with previous studies [38,55,59], verifying the validity of the detected SOS dates.

Statistical analysis in Figure 15 revealed that both the mean and standard deviation values of the SOS detected in less snowy areas (e.g., SCDr < 20%) by different VIs were very close to each other. However, for more snowy areas, such as for SCDr > 40%, discrepancies occurred in the SOS detected by different VIs. The SOS derived from NDVI and EVI2 were earlier, while those from NDPI and NDGI were later and those from NIRv were in between. These discrepancies could be attributed to the different sensitivities of various VIs to preseason snow cover.

For the entire study area, the SOS dates detected by different VIs have different correlations with each other, such as the results of NDGI/NDPI, which have relatively low correlations with those of NDVI/EVI2/NIRv. However, for less snowy areas, very high correlations were found for the results of all five VIs. Figure 17 compares the different SOS results for the areas with SCDr < 20%. The R values between the different SOS results ranged from 0.818 to 0.982, indicating a very high consistency of the SOS detected by different VIs.

Although previous studies confirmed the accuracy of satellite-derived SOS dates under snow-free conditions [20,21,31,40], a direct comparison with field data was lacking in this study due to the unavailability of in situ SOS measurements. However, we referred to previous studies [20,38,55,59,60] and found a high consistency of our results with the previous in situ measurements and satellite-derived SOS dates in terms of value ranges and spatial distribution. Moreover, a high consistency of the SOS results detected by different VIs was confirmed for less snowy conditions, which was also consistent with previous findings [19,20]. Since the satellite-derived SOS dates represent the macroscale spring phenology and cannot be simply equated with the in situ measurement [61,62], the absence of a direct comparison with in situ measurements does not affect the validity of our results, yet we hope more evidence from in situ measurements to support our findings in the future.

**Figure 17.** Comparison of the SOS detected by different VIs for SCDr < 20%. 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.
