*4.4. Effect of Snow Cover on Spring Phenology Detection from Satellite Data* 4.4.1. Effect of Snow Cover on VI

Figure 14 shows the statistical distributions of the maximum, minimum, and range of the time series of VIs for different SCDr intervals within the same elevation zone. To ensure a sufficient number of pixels for statistics, the SCDr intervals with too few pixels were excluded from the subsequent analysis, including the intervals of SCDr = 40–60% and SCDr > 60% in the zone of DEM < 3500 m. Generally, the decrease in minimum VI values with increasing SCDr was more significant than the decrease in maximum VI values. This is because snow always melts before the peak growth season and does not affect the maximum VI over time. Consequently, the range of VI values increased with the increasing SCDr for all five Vis. These findings were consistent with the simulation results. However, the reduction in VI values with increasing SCDr was not as significant as in the simulation results. This is probably because there were few snow-free pixels, and the minimum VI in the interval SCDr < 20% was affected by snow. Based on the decrease in the minimum VI value with increasing SCDr, the effect of snow on the VI value follows the order of NDPI/NDGI < NIRv < EVI2 < NDVI, which was consistent with the findings from the simulation experiments described in Section 4.3.1.

**Figure 14.** Statistical distribution of the maximum, minimum, and range of the time series of five VIs for different SCDr intervals and elevation zones. (**a**–**c**), (**d**–**f**), (**g**–**i**), and (**j**–**l**) are statistics for elevation zones of DEM < 3500 m, 3500–4000 m, 4000–4500 m, and ≥ 4500 m, in each of which the maximum, minimum, and range of the VI values over time were plotted.
