Spatiotemporal Variation Of Snow Cover On The Northern And Southern Slopes Of Karakoram Mountains

Snow is one of the most active natural elements in the cryosphere.The distribution and change of snow cover affect the energy exchange,water cycle and climate change of the earth-atmosphere system.With high mountains and rugged terrain,the Karakoram is located in the arid area of Central Asia.It is the region with the highest concentration of mountain glaciers in Asia,from which the melted ice and snow supply the runoff of the upper Indus River on the southern slope of Karakoram and the Yarkant River on the northern slope,and is an important source of fresh water.It is of great significance to reveal the spatiotemporal characteristics of the snow cover on the southern and northern slopes of the Karakoram and analyze the main factors influencing the snow cover change.Because it is important for water resources management,agricultural development,and disaster prevention such as floods,mudslides,and snow disasters on the southern and northern slopes of the Karakoram.Based on remote sensing and GIS technology,this study constructed a dataset of snow phenology characteristics in the western part of the upper Indus River basin(WUIB)and the Yarkant River(YRB)basin during 2002-2018,using MODIS snow cover data.Combined with snow water equivalent data,it studied the snow cover area(SCA),snow phenology and snow water equivalent(SWE)of the western of upper Indus basin on the southern slope of the Karakoram and the Yarkant River basin on the northern slope.Using atmospheric circulation data and meteorological data,the influences of atmospheric circulation and climate change on snow cover variation were discussed.(1)From 2002 to 2018,the SCA in the whole study area showed a slight decrease,the SCA in the Yarkant River basin on the northern slope of the Karakoram showed a slight decrease,while the SCA in the western part of the upper Indus River basin on the southern slope showed a slight increase.Different from the trend in the whole study area,the SCA at the glacier area of the whole study area displayed an increasing trend;the SCA at the glacier area of the WUIB also showed an increasing trend,while that of the YRB showed a decreasing trend,which indicates that the snow cover change in the glacier area dominates the snow cover change of the basin scale.In terms of spatial change,a clear elevation dependence was found for SCP(greater with higher elevation)in the WUIB and YRB.The SCA of the WUIB and YRB is approximately normal at altitude.The SCA of the extremely high-altitude areas which is above 6000 m in the two basins remains relatively stable,while that of the low altitude areas is decreasing.The SCA of the high-altitude area shows an increasing trend.Changes of the minimum SCA in the two basins are relatively stable,mainly distributed in the glacier area.While there exists great difference in the maximum SCA between the WUIB and YRB.Compared with the WUIB,the SCA in YRB fluctuated more greatly.(2)From 2003 to 2011,the SWE of the WUIB and YRB both showed a slow volatility decrease.Affected by the westerly circulation,the maximum snow water equivalent in the YRB appeared in winter,and the minimum occurred in autumn;the SWE in the WUIB has two peaks during one year,which appeared in winter and summer,respectively.This is because the WUIB is also affected by the Indian Summer Monsoon in summer.In terms of interannual changes,the SWE of the WUIB and YRB showed decreasing trends during the westerlies and monsoon periods from 2003 to 2011.In terms of spatial distribution,affected by climate and topography,the SWE of the WUIB generally decreased in a northeast-to-southwest direction,while that of the YRB decreased in a southwest-to-northeast direction.The SWE was densely distributed and large in the mountainous areas with higher elevations in the basins,while the SWE in the lower reaches of the basin was relatively evacuated and small.In terms of the spatial change trend,from 2003 to 2011,the change rate of annual average SWE in the WUIB and YRB basically remained between-4 and 4 mm/a.The change rate of the average SWE in the westerlies period is maintained between-6 and 6 mm/a,and the average SWE in the monsoon period is basically maintained between-10 and 2 mm/a.Except that parts of areas of the southern slope of Karakoram,the west of Kunlun and the east of Pamirs showed an increasing trend of SWE,most parts of the study areas showed a decreasing trend of SWE.The change trend of the annual average SWE and the average SWE in the westerlies period showed basically the same spatial distribution pattern,indicating that the change of the SWE in the westerlies period basically controlled the change of the annual average SWE.(3)The SCD displayed a non-significant decrease trend in the YRB,and an increase in the WUIB.In spatial distribution,clear elevation dependences were found for SCD,SCMD,and SCOD in the YRB and the WUIB.SCD(SCOD,SCMD)in the WUIB decreased(delayed,advanced)from northeast to southwest,whereas in the YRB,they decreased(delayed,advanced)from southwest to northeast.Spatially,most of the study area showed delayed SCOD,advanced SCMD,and decreased SCD.Some areas in the Karakoram and eastern Pamir displayed lengthened SCD,advanced SCOD,and delayed SCMD during 2002-2018.Trend analysis of snow cover at altitudes showed increased SCD at elevation zone of 5000-6000 m in the YRB and 3000–4000 m in the WUIB,which were because of delayed SCMD.(4)Atmospheric circulation and climate change influence the changes of snow cover.In abundant snow years of the westerlies period,the intensity of the westerlies is greater than that in the less snow years of the westerlies period;also in abundant snow years of the monsoon,the intensity of monsoon is greater than that in the less snow years of the monsoon.Snow cover showed a remarkable negative relationship with temperature,and a remarkable positive relationship with precipitation.Sensitivity analysis showed that SCOD was positively sensitive to₍(6) with 0.37 days/?C in the WUIB and 1.6 days/?C in the YRB,while negatively sensitive to₍(6)(-0.07 days/mm in the WUIB and-0.13 days/mm in the YRB).Besides,negative sensitivity of SCMD to_{(6)(-0.92 days/?C in the WUIB and-1.62 days/?C in the YRB)and8}(-7.94days/?C in the WUIB and-5.82 days/?C in the YRB),positive to₍(6) were found(0.06days/mm and 0.13 days/mm in the WUIB and the YRB).