Spatial distribution of supraglacial debris thickness on glaciers of the China-Pakistan Economic Corridor and surroundings

Debris-covered glaciers,characterized by the presence of supraglacial debris mantles in their ablation zones,are widespread in the China-Pakistan Economic Corridor(CPEC)and surroundings.For these glaciers,thin debris layers accelerate the melting of underlying ice compared to that of bare ice,while thick debris layers retard ice melting,called debriscover effect.Knowledge about the thickness and thermal properties of debris cover on CPEC glaciers is still unclear,making it difficult to assess the regional debris-cover effect.In this study,thermal resistance of the debris layer estimated from remotely sensed data reveals that about 54.0%of CPEC glaciers are debris-covered glaciers,on which the total debris-covered area is about 5,072 km2,accounting for 14.0%of the total glacier area of the study region.We find that marked difference in the extent and thickness of debris cover is apparent from region to region,as well as the debris-cover effect.53.3%of the total debris-covered area of the study region is concentrated in Karakoram,followed by Pamir with 30.2%of the total debris-covered area.As revealed by the thermal resistance,the debris thickness is thick in Hindu Kush on average,with the mean thermal resistance of 7.0×10^-2((m^2∙K)/W),followed by Karakoram,while the thickness in western Himalaya is thin with the mean value of 2.0×10^-2((m^2∙K)/W).Our findings provide a basis for better assessments of changes in debriscovered glaciers and their associated hydrological impacts in the CPEC and surroundings.

Glacier mapping based on Chinese high-resolution remote sensing GF-1 satellite and topographic data

The precise glacier boundary is a fundamental requirement for glacier inventory,the assessment of climate change and water management in remote mountain areas.However,some glaciers in mountain areas are covered by debris.The high spatial resolution images bring opportunities in mapping debris-covered glaciers.To discuss the capability of Chinese GaoFen-1 satellite lacking the short wave infrared band and thermal infrared band in mapping glaciers,this study distinguished supraglacial terrain from surrounding debris by combining GaoFen-1(GF-1)wide-field-view(WFV)images,the ratio of the thermal infrared imagery and morphometric parameters(DEM and slope)with 30 m resolution.The overall accuracy of 90.94%indicated that this method was effective for mapping supraglacial terrain in mountain areas.Comparing this result with the combination of GF-1 WFV and low-resolution morphometric parameters shows that a high-quality DEM and the thermal infrared band enhanced the accuracy of glacier mapping especially debris-covered ice in steep terrain.The user's and producer's accuracies of glacier area were also improved from 89.67%and 85.95%to 92.83%and 90.34%,respectively.GF data is recommended for mapping heavily debris-covered glaciers and will be combined with SAR data for future studies.

Study of thermal properties of supraglacial debris and degree-day factors on Lirung Glacier,Nepal

The extensive debris that covers glaciers in the ablation zone of the Himalayan region plays an important part in regulating ablation rates and water availability for the downstream region. The melt rate of ice is determined by the amount of heat conducted through debris material lying over the ice. This study presents the vertical temperature gradients, thermal properties in terms of thermal diffusivity and thermal conductivity, and positive degree-day factors for the debris-covered portion of Lirung Glacier in Langtang Valley, Nepal Himalaya using field-based measurements from three different seasons.Field measurements include debris temperatures at different debris thicknesses, air temperature, and ice melt during the monsoon(2013), winter(2013), and pre-monsoon(2014) seasons. We used a thermal equation to estimate thermal diffusivity and thermal conductivity, and degree-day factors(DDF) were calculated from cumulative positive temperature and ice melt of the measurement period. Our analysis of debris temperature profiles at different depths of debris show the daily linear gradients of-20.81 °C/m, 4.05 °C/m, and-7.79 °C/m in the monsoon, winter, and pre-monsoon seasons, respectively. The values of thermal diffusivity and thermal conductivity in the monsoon season were 10 times greater than in the winter season. The large difference in these values is attributed to surface temperature and moisture content within the debris. Similarly, we found higher values of DDFs at thinner debris for the pre-monsoon season than in the monsoon season although we observed less melting during the pre-monsoon season. This is attributed to higher cumulative temperature during the monsoon season than in the pre-monsoon season. Our study advances our understanding of heat conductivity through debris material in different seasons, which supports estimating ice melt and discharge from glacierized river basins with debris-covered glaciers in the Himalayan region.

Characteristics and changes of the Himalayas glacial area in China during 1990-2015

In recent decades,the continuously changed glaciers in the Himalayas not only affected process of atmospheric flow and water cycle in the plateau but also increased the frequency of secondary disasters,such as ice collapse and outburst floods.Therefore,the monitoring of Himalayas glacial change is of great significance in the aspects of climate change and disaster prevention and reduction.The Himalayas glacier outlines in China were extracted by ratio threshold and visual interpretation based on the Landsat TM/ETM+/OLI data and glacier catalogue data.Based on the ASTER GDEM data,the distribution and change characteristics at different altitudes were studied,and the surface moraine was identified to study the influence of glacial area variations.In addition,Glaciers were divided into marine and continental glaciers in this study,and the distribution and changes of the two types of glaciers were analyzed.Also,a comprehensive analysis of a long time series was performed.The results showed that:(1)From 1990 to 2015,the glaciers in the study area showed an overall trend of melting;the annual melting speed gradually accelerated from 0.48%/a to 0.75%/a.The total melting glacial area was 828.16 km,and the melting rate of the glaciers in the western section was the highest,at 0.63%.(2)The total area of continental glaciers was larger than that of marine glaciers,and its reduction was relatively larger,too.(3)The average size of debris-covered glaciers was 10 times that of debris-free glaciers,but their rate of change(8.1%)was 1/2 of that of debrisfree glaciers(17.8%).(4)The number of glaciers gradually decreased as the size of glaciers increased.The glaciers with grades of>50 km,0.5~1 km,and 1~2 kmhad large change rates,which were 20.1%,19.1%,and 18.5%,respectively.In summary,the Himalayas glaciers in China were melting at an accelerated rate and their numbers were gradually decreasing from 1990 to 2015.The location,type,elevation,size and debris cover of a glacier are all important factors influencing glacier change.It has been found that the lower the elevation or the smaller the size of the glacier,the greater the likelihood of glacier ablation,and the debris can inhibit the melting of glaciers to a certain extent.

Effect of debris on seasonal ice melt (2016−2018) on Ponkar Glacier, Manang, Nepal

Supraglacial debris is widely present on glaciers in alpine environments and its distribution greatly affects glacier melt.The present study aims to determine the effect of debris on glacier ice melt on Ponkar Glacier,Manang District,Nepal.We estimated ice melt under various debris thickness using Energy Balance(EB)model and conductive heat flux methods,which are compared with in-situ observations.Four stakes are installed on the glacier at different debris thickness of 11−40 cm.Meteorological data from March 2016 to May 2018 are obtained from the Automatic Weather Station(AWS)installed on the glacier surface at an elevation of 3,881 m a.s.l.for the energy balance calculation.Debris surface temperature and different debris depths are also measured on the glacier.The calculated ablation rates from the conductive heat flux method are 0.9,1.62 and 0.41 cm/d on pre-monsoon,monsoon and post-monsoon,respectively,with mean debris thermal conductivity 1.04 W/(m∙K).The net radiation shows little variation between the seasons,while turbulent heat flux varies in the season.Sensible heat flux was found to be highest in post-monsoon season due to a larger temperature gradient between surface and air.

Viscous creep of ice-rich permafrost debris in a recently uncovered proglacial area in the Tianshan Mountains, China

Since the Little Ice Age and as a consequence of climate warming,many recently deglaciated forefields have become and will continue to evolve into large ice-debris complexes exposed to periglacial processes and environment.Such transitional processes have significant implications for geomorphologic shaping and water supply for the downstream communities,especially in arid regions,but our understanding of their evolutionary processes and their potential geomorphic and hydrological impacts is stil limited.A landform transition from partly debris-covered glaciers to ice-rich permafrost debris undergoing slow viscous creep was revealed in the Aerzailaikunai Valley in the eastern Tianshan Mountains in China based on the results of in-situ observations and measurements(boreholes,ground temperature monitoring,electrical resistivity tomography surveys,and continuous global positioning system measurements,among others).The internal structure of ice-tll mixture contains pure ice layers,supersaturated frozen sands with ice lenses,and ice-bearing blocks with maximum volumetric contents of heterogeneous ice at 35%-60%.Beneath an 1.5-m-thick active layer,permafrost reached far into the underlying bedrock with the mean annual ground temperature of-2.1℃ at the depth of 20 m.The higher surface velocities(with an accumulative displacement of 65 mm from October 2019 to May 2020)and extremely high electrical resistivity(several million Ω m)of the debris-covered glacier margin were in sharp contrast to those of the progressively stabilizing ground surface(up to 16 mm)and the lower zones with relatively smaler electrical resistivity(several thousand Ω m).Combined with the borehole stratigraphy(higher rock content),monitored ground temperatures(permafrost environment),lower electrical resistance(ice-rich moraine),and continuous global positioning system results(viscous creeping),this study documents a transition from glacial to periglacial conditions,materials and processes characteristic of cold-dry ice-clad mountains,and reinforces the theory of the transition from debris-covered glaciers into morainically originated rock glaciers.