4.4.2. Effect of Snow Cover on SOS Detection

Figure 15 compares the statistical distribution of SOS detected from the five VIs at different SCDr and elevation zones. At the same SCDr, an increase in elevation caused a delay in the SOS for all five VIs. For less snowy areas with SCDr < 20%, an increase in elevation from < 3500 m to ≥ 4500 m caused a delay of approximately 19–25 days. Within the same elevation zone, the increase in SCDr caused different effects on the SOS detected by different VIs. At elevations ≥ 4500 m, an increase in SCDr from < 20% to > 60% delayed the SOS by 3 and 5 days for NDGI and NDPI, respectively, but advanced the SOS by 11, 9, and 4 days for NDVI, EVI2, and NIRv, respectively. Generally, the SOS detected by NDGI, NDPI, and NIRv was less affected by snow, while the SOS detected by NDVI and EVI2 was more affected by snow.

Based on the simulation results, the effect of snow on SOS detection depends on SCDc, ESS, and snow-free SOS. Figure 16 shows the statistical distribution of the SOS for different SCDc and ESS cases in each elevation zone using real satellite data. To ensure sufficient pixels for the statistical analysis, we considered three SCDc cases, including SCDc < 48, 48 ≤ SCDc < 80, and SCDc ≥ 80, and divided the ESS values into 12 cases with 16-day intervals from DOY 32 to 208. In the zone with DEM < 3500 m, the intervals of

48 ≤ SCDc < 80 and SCDc > 80 had a maximum of two pixels and were excluded from the statistical analysis.

**Figure 15.** Error bars of the SOS detected by five different VIs for different SCDr intervals at each elevation zone. Error bars show the mean and standard deviation of the SOS in each case. (**a**–**d**) are the statistical distribution of SOS detected from the five VIs at different SCDr and elevation zones.

**Figure 16.** Changes in SOS with increasing ESS under different SCDc cases derived from satellite data. (**a**), (**b**–**d**), (**e**–**g**), and (**h**–**j**) are statistics for elevation zones of DEM < 3500 m, 3500–4000 m, 4000–4500 m, and ≥ 4500 m, in each of which three cases of SCDc < 48, 48 ≤ SCDc < 80, and SCDc ≥ 80 were plotted. SCDcMean and SCDcSTD are the mean and standard deviation of SCDc in each elevation zone.

As shown in Figure 16, the effect of snow on the detected SOS followed the order of NDPI/NDGI < NIRv < EVI2 < NDVI. Generally, the larger the SCDc was, the larger the effect of snow cover on the detected SOS. For short snow (i.e., SCDc < 48), the fluctuation in the SOS with varying ESS was the smallest for all five Vis, indicating a negligible effect of snow. Using the SOS detected for short snow (i.e., SCDc < 48) as a benchmark, the SOS detected under medium snow (i.e., 48 ≤ SCDc < 80) was generally advanced for NDVI and EVI2 and was slightly delayed for NDPI, NDGI, and NIRv for ESS later than DOY 144. For long snow (i.e., SCDc ≥ 80), the detected SOS dates were advanced for ESS earlier than DOY 160 and delayed for ESS later than DOY 160. For short and medium snows, the effect of snow on SOS detection was generally small for NDPI, NDGI, and NIRv. Generally, an earlier ESS results in earlier estimates of the SOS, while a later ESS results in later estimates of the SOS. These findings were highly consistent with the simulation results and verified the validity of the simulation experiments.
