Spatial-Temporal Change Of Snow Cover And Its Response To Climate Factors Over Manasi River Basin

Snow covers an enormous portion of the Earth's surface and is very dynamic.Snow cover in mountainous areas is an important water resource in solid form,and snow accumulation and ablation are important parts in regional water resources cycle.Understanding its change and characteristics is imperative to climate change,weather forecasting,and water resource variation and redistribution.Factors of different scales that have influence on the spatial-temporal distribution of snow cover include terrain factors(elevation,slope,slope direction)and meteorological factors(temperature,precipitation)and other factors.The analysis of snow cover distribution under different terrain conditions and its relationship with meteorological factors has significance meanings for snow cover characteristics study and accurate predictive analysis.The study in this paper was supported by the National Science and Technology Major Project of China(Grant No.E0405/1112/05).Using MODIS snow and land surface temperature data,also in situ temperature and precipitation data,the 8-day and monthly snow areal extent percentages,snow cover days,8-day and monthly temperature and the inter-annual change of those variables were calculated.The spatial-temporal distribution of snow cover over the basin and its relationship with meteorological factors were investigated at various characteristic elevation zones after.The main contents and conclusions are as follows:(1)The temporal distribution characteristics of snow cover.Using the MODIS snow product MOD10A2 and altitudinal zonation,slope and slope direction of the basin,the snow areal extent percentage annual variations in different elevation zones,slope and slope direction,the inter-annual change of snow areal extent in typical months(January,April,July and October)were calculated.The results indicated that snow cover showed a single peak distribution below the elevation of 4000m,and a bimodal distribution above 4000m,and the annual variations of snow cover in different zones showed distinct seasonal characteristics.The impact of slope direction on snow cover distribution was greater than slope,and snow areal extent percentages in the north,northeast and northwest was the highest,east and west the second,south,southwest and southeast the lowest.The impact of slope was mainly based on elevation.Inter-annual change of snow cover in typical months indicated,January and July showed decline trend in all elevation zones,April showed decline trend in low altitude area and increase trend in high altitude area,October was on the contrary.(2)The spatial distribution characteristics of snow cover.The spatial distribution of snow cover days and its inter-annual change,the vertical distribution of snow cover in different months,the inter-annual change of snowline were analyzed by overlay the results of snow cover days and altitudinal zonation,slope and slope direction.Snow cover days did not show much difference in zones below 3200m but increase with elevation in area above 3200m.The snow cover days in 40.3%of the basin showed decrease trend from 2001 to 2014,and 12.1%decreased more than 18 days,59.7%showed increase trend and 11.4%increased more than 24 days,the area with increasing snow cover days mainly located within the elevation 800-3600m.The vertical variation of SAE indicated four different patterns in a year,each pattern have different features in the five elevation zones.The difference of snow cover in different slope direction was much more significant in Jan,Apr and Oct than Jul.The snowline elevation showed an increase trend of 2.25m per year from 2001 to 2014.(3)Correlation between snow cover and climate factors.Correlation between snow cover and temperature in annual and inter-annual,horizontal and vertical scales,snow cover and temperature and snowfall in annual variation scale,snowline and temperature in inter-annual variation scale were analyzed using the MODIS land surface temperature data and in situ temperature and snowfall data of 3 hydrological stations.The results indicated the correlation between snow cover and temperature in annual variation scale diminished with increase of elevation.The average fitting RMSE for snow cover annual variation was 0.12 using temperature and snowfall.The vertical variations of temperature were well correspondent to snow cover in Mar-Nov,with correlation coefficient above 0.6,while the correlativity was much poor in Dec,Jan and Feb,as a result of temperature inversion in winter months.Inter-annual change of snow cover days and average winter temperature from 2001 to 2014 had a strong spatial consistency,with average correlation coefficient of-0.84 below 3600m,and this negative correlation was apparent in 1600-1700m,where snow cover days increased by 26 days and average winter temperature decreased by 1.8?.Average July temperature from 2001 to 2014 in area around the snowline and in situ temperature from 2001 to 2012 recorded in the 3 stations both showed increase trend,which were consistent with the increase trend of snowline elevation.