| The Qinghai Tibet Plateau is the highest altitude plateau in the world and the area with the most snowfall in China.59%of the Qinghai Tibet Plateau is covered with snow in winter,especially in the southeast.The change of snow has a far-reaching impact on the global water cycle,energy balance and glacier melting.Taking Linzhi city in the southeast of Tibet as the research area,this paper downloads the Landsat remote sensing images from 1990 to 2020 on the GEE(Google Earth engine)platform,and obtains the remote sensing image data of the research area in1990,1995,2000,2005,2010,2015 and 2020 after preprocessing.Then,based on the decision tree classification,the normalized snow index is combined with the brightness threshold technology method,and the visual interpretation is used as an aid to construct the model for extracting the snow information in Linzhi city.The snow information extracted by the model passes the accuracy evaluation.The kappa coefficient of the extraction result is 87.89%,and the other accuracy is more than 90%.Finally,the extracted snow information is superimposed with topographic and climatic factors to study the spatial distribution and change characteristics of snow cover in Linzhi city from 1990 to 2020.The main conclusions are as follows:(1)The snow cover area of all districts and counties in the study area from 1990to 2020 and its percentage in the total area of districts and counties.The largest snow cover area is Chayu county and the smallest is Lang county;The highest percentage value is basically in Bomi County,and the lowest percentage value is basically in Motuo County.(2)The stability of snow cover in the study area shows polarization.The percentage of snow with strong stability in the study area is 41.41%,and the percentage of snow with weak stability in the study area is 30.54%,indicating that the snow cover in the study area has changed greatly from 1990 to 2020.Among the seven districts and counties in the study area,Bomi county has the best stability and gongbujiangda county has the worst stability.Using the formula of time series data change trend rate to calculate the change characteristics,the snow cover area in the study area shows a decreasing trend as a whole.(3)In the five study periods,the snow cover area changed greatly from 1995 to2010,decreased by 9641.51 km~2;From 2015 to 2020,it is the only time period when the snow cover area is growing as a whole,with an increase of 6271.04 km~2;From1990 to 1995 and from 2010 to 2015,the snow cover area changed slightly,reducing722.38 km~2 and 611.47 km~2 respectively;From 1990 to 2020,the snow cover area decreased by 4704.32 km~2,mainly in gongbujiangda County,the west of Milin county and the south of Chayu county.(4)The snow cover area in the study area is greatly affected by the elevation change.The snow cover is mainly distributed in extremely high mountains and high mountains,that is,the area with an altitude of more than 3500m;Snow cover in the study area is mainly distributed on steep slopes,steep slopes and slopes,and relatively less on flat and gentle slopes;In different slope directions,the snow cover area in the study area is the largest in the shady slope,followed by semi shady slope and semi sunny slope,and the sunny slope is the smallest.(5)In different longitude zones:the snow cover in the study area is mainly distributed in longitude zone 2,longitude zone 3 and longitude zone 4,and the snow cover area of longitude zone 1 and longitude zone 5 is small;In different latitudes:the snow cover in the study area is mainly concentrated in latitudinal zone 1 and latitudinal zone 2,and the snow cover in latitudinal zone 3 is less.(6)The correlation coefficient between snow cover area and annual average temperature is-0.918,showing a high negative correlation,and that between snow cover area and annual average precipitation is 0.714,showing a moderate correlation.Compared with precipitation,snow cover in the study area is more sensitive to temperature change. |
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