## 4 Results
### Interannual Variation in ROD  
To evaluate how snowmelt is affected by different land cover types, ROD changes were analyzed in the study area from 2017 to 2023. Since not every station can extract the ROD every year, we first filtered the stations to ensure the consistency in multi-year analysis, retaining only those that can extract the ROD in all years. The numbers of retained stations for tree cover, shrubland, grassland, and bare land were 65, 2, 7, and 2, respectively. Then, the average ROD was calculated using results from both station and Sentinel-1 data for each land cover type annually and we analyzed their trends (Figure 7). Figure 7 shows that the average ROD extracted from station data exhibits an advance in areas of tree cover, shrubland, and grassland, with the number of days advanced each year approximately 1.7, 2.2, and 0.6 d, respectively. However, the average ROD extracted from Sentinel-1 data shows a trend in the delay, with the number of days delayed each year being approximately 1.3, 4.9, and 1.7 d, respectively. In bare land, the average ROD extracted from the two methods both exhibit the advancing trend with similar rates of change. The average ROD extracted from station data advances by approximately 0.8 d per year, while that extracted from Sentinel-1 data advances by approximately 1.1 d per year. These results illustrate the impact of vegetation on ROD extraction from Sentinel-1 data, resulting in an opposite trend between the average ROD extracted from Sentinel-1 and that extracted at the stations. However, regardless of the ROD extraction method, the annual ROD trends for all land cover types from 2017 to 2023 are not significant, with all \(p\) values greater than 0.05. Figure 6: (**a**) Spatial distribution of the ROD extracted by Sentinel-1 under four land cover types in the study area in 2023 and (**b**) the area proportions of different land cover types at different runoff onset dates. Based solely on station observations, the annual advance in the average ROD for tree cover, shrubland, grassland, and bare land areas is approximately 1.7, 2.2, 0.6, and 1.1 d, respectively. In areas with higher vegetation density, the variation in average ROD is more obvious, primarily because vegetation traps blowing snow and increases radiation exchanges, thereby causing earlier snowmelt [39]. An advance in the ROD observed at stations is consistent with some recent research findings, indicating that snow in the western United States has been experiencing earlier melting in recent years [10; 11]. In 2023, there was a significant delay in the average ROD across all land cover types. Based on station observations, the average ROD for the four land cover types in 2023 was delayed by 28.2, 20.0, 19.1, and 28.5 d, respectively, compared to 2022. The variation in the average ROD extracted from Sentinel-1 data is similar. The phenomenon has significantly influenced the trend in the average ROD extracted by Sentinel-1 from 2017 to 2023. Especially in vegetated areas, the average ROD extracted by Sentinel-1 and that extracted at the stations show opposite trends. It is also the main reason why the trends in the average ROD extracted by the two methods from 2017 to 2023 are not significant (\(p>0.05\)). According to the station observations, an extreme snowfall event occurred in the study area in 2023 with the SWE in some stations reaching the highest on the record. To explore the impact of the unusual snowfall on the delay of the ROD, we calculate the annual average SWE for stations under different land cover types (Figure 8). These stations are consistent with those used in Figure 7.  
Figure 8 shows that in 2023, the average SWE for the four land cover types--tree cover, shrubland, grassland, and bare land--reached their maximum values of 141.6, 98.4, 120.6, and 248.8 cm, respectively. Correspondingly, the average ROD for these land cover types exhibited a significant delay in the same year (Figure 7).