## 3 Results
### Analysis of Spatial Correlation between Snow Cover and NDVI
#### 3.3.3 Spatial Variation in Seasonal Lag Response  
Obviously, the lag effect also has spatial characteristics. The lagged seasonal effects of snow cover on vegetation and its spatial distribution were examined based on the correlations between the snow in spring and NDVI in summer, snow in winter and NDVI in spring, and snow in the cold season and NDVI in the warm season for 40 years from 1981 to 2020. Figure 11a shows the correlations between the snow in winter and NDVI in the following spring. The correlations were mainly positive between the snow cover in winter and NDVI in the following spring over the whole plateau, where the positive correlations were significant in the northern QTP, some parts of the Qinghai Plateau, and \"Yijiang lianghe\" region. Few areas had significant negative correlations. Figure 11b shows the correlations between the snow cover in the spring and NDVI in the summer, which indicated that positive correlations were found in most parts of the plateau. The correlations between the snow cover in spring and NDVI in summer were weaker compared with those between the snow cover in winter and NDVI in the subsequent spring. The distribution also changed, with positive correlations in the central, southern, and northern Qaidam Basin, and Hengduan Mountains. The positive correlations were significant in the central Himalayan region, central QTP, and Qaidam Basin. Negative correlations were found between the snow cover in spring and NDVI in summer in the higher elevation mountainous regions of the Gangdis and Kunlun Mountains, and the three river source regions of the Qinghai Plateau in the west. The correlations were less significant in all regions, except for the \"Sanjiangyuan\" region, central QTP, and western Himalayas, thereby indicating that the effect of snow in the spring on the NDVI in the summer was weaker compared with the effect of snow in the winter on the NDVI in the following spring, i.e., the lag was not strong.  
Figure 10: Spatial relationship between forest desert and snow cover vegetation on the QTP: (**a**) spatial distribution of forest and desert steppe; (**b**) spatial distribution of significant negative correlation.  
Based on previous studies [34] and the actual snow cover on the plateau, we selected six months from October to March as the cold season and April to September as the warm season. The snow depths in the cold season from 1981 to 2020 were selected to determine their correlations with the NDVI in the warm season, and the results are shown in Figure 12. The snow cover in the cold season generally had positive correlations with the NDVI in the warm season in most of the northern part of the plateau. In contrast, negative correlations were mainly found in southern Tibet and western Sichuan, and a significant negative correlation along the Gangdishan-Tanggula-Hengduan mountain line. The lag effect was more significant in areas with significant correlations between the snow cover and NDVI. According to the correlations between the snow depth in the cold season and NDVI in the warm season, more obvious lags were found in northern and southern Tibet, northern Qinghai, Xinjiang, and Gansu. In contrast, the lags were less significant in the Ali plateau, Qinghai plateau, and northern Sichuan regions. In general, the snow cover during the cold season significantly affected the NDVI during the warm season in most areas of the plateau.  
Figure 11: Seasonal snow cover and NDVI correlation: (**a**) winter snow cover—following spring NDVI; (**b**) spring snow cover—summer NDVI.  
Figure 12: Cold season snow cover—warm season NDVI correlation.