Phenology Of Snow Cover In Hengduan Mountains

The hengduan Mountains connects the Qinghai-Tibet Plateau and the Yunnan-Guizhou Plateau.The area has many landcover types,such as plateaus,mountains,river valleys and basins.Located at the turning point of the first two steps of the terrain in China,the terrain dips from north to south.Snow are important elements of the hengduan mountains region ecological environment.At the same time snow are the biggest seasonal changes in the hengduan mountains of underlying surface.The ice and snow melt water is an important water of hengduan mountains region and its downstream.so by determining different underlying surface threshold,the improvement of traditional threshold value of snow leakage phenomenon,improve the identification accuracy of snow,and then more accurate study of hengduan mountains,snow,Is particularly urgent.The hengduan Mountains region is characterized by complex terrain and unique geographical location and characteristics.It is of great practical significance to explore the temporal and spatial variation characteristics and rules of local snow cover for revealing the impacts of local snow cover on climate evolution,biodiversity and water resources utilization.Firstly,the optimal threshold of Hengduan Mountain area is determined by using mo DIS V006 version data,and the snow recognition results under the optimal threshold are compared with the "true value" and the snow recognition results under the traditional threshold,and the qualitative and quantitative verification is carried out to determine the optimized threshold.Secondly,using the threshold method,the upper and afternoon star synthesis method,the snow over-altitude algorithm,and the conditional probability interpolation method of the spatio-temporal cube,the day-by-day cloudless binary snow products with a resolution of 500 meters were generated and the snow cover days（SCD）,snow start date（SCS）and snow end date（SCE）in the area were calculated by cell;finally,two test methods were used: M-K test and Sen trend analysis for comparative verification,and the spatio-temporal change of snow phenology and its law in Hengduan Mountain area was systematically analyzed.The results show that:（1）The NDSI sequence of cloud removal has a good correspondence with the snow depth of the site,and NDSI can accurately reflect the snow melting phenomenon of the pixel where the site is located.When NDSI threshold of non-forest area is 0.26,the accuracy of snow extraction is 94.46%.When NDSI threshold of forest area is 0.03,the accuracy of snow extraction is 94.23%.（2）Compared with the traditional NDSI threshold,the new NDSI threshold improves the accuracy of MODIS snow extraction.Using Landsat 8 OLI data as the "truth value",the accuracy of snow extraction results was verified.The accuracy of snow extraction corresponding to the new threshold and the traditional threshold was 84.21% and 82.86%,respectively.In the area with large snow cover,both thresholds can obtain better snow extraction results.In the region where the snow is scattered and broken,the new threshold can improve the snow leakage phenomenon of the traditional threshold to a certain extent,and improve the accuracy of snow recognition.（3）The distribution of snow cover in hengduan Mountains is very uneven,and the spatial-temporal variation of snow cover is significantly different.The snow is mainly distributed in the high altitude mountainous areas in the north and northwest.The days of snow cover in high altitude areas are generally high,reaching snow cover and even forming year-round snow areas,such as Gongga Mountain,Meili Snow Mountain and queer mountain.There is a significant spatial difference between the start date and end date of snow cover.The overall snow cover in high-altitude mountains and their surrounding areas starts earlier and ends later.The snow on high-altitude mountains mostly starts in October and melts after May.In the areas with relatively low altitude,the start date is later and the end date is earlier.（4）The snow cover days in Hengduanshan area generally show a decreasing trend,the snow start date generally shows a non significant advance trend,and the advance date is concentrated in 0-1 days.The start date of snow cover was delayed in most areas,and the end date of snow cover was advanced in most areas.According to the analysis of different underlying surfaces,on the whole,the days of snow cover in forestland area are higher than that in non-forestland area,the start date of snow cover is earlier than that in non-forestland area,and the end date of snow cover is later than that in non-forestland area.The influence of different underlying surfaces on the start date of snow cover is less than that on the end date of snow cover and the days of snow cover,indicating that the start date of snow cover is little affected by the underlying surface.In this paper,MODIS data were used to study the snow phenology in the hengduan Mountains area,which is a useful supplement to the literature on snow phenology in the region.Good snow recognition accuracy can ensure the reliability of snow phenology research results.Therefore,in this paper,the different lower cushion surface thresholds of the main body in the hengduan Mountain area（Qinghai-Tibet Plateau area）are optimized to ensure the accuracy of subsequent snow recognition and the reliability of snow phenology research results.It provides a certain scientific basis for the subsequent governance of the hengduan Mountain area.