Sciences in Cold and Arid Regions ›› 2018, Vol. 10 ›› Issue (5): 357-368.doi: 10.3724/SP.J.1226.2018.00357

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

Mohan Bahadur Chand1,2,*(),Rijan Bhakta Kayastha2   

  1. 1 Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan
    2 Himalayan Cryosphere, Climate and Disaster Research Center (HiCCDRC), Kathmandu University, Dhulikhel 45200, Nepal
  • Received:2018-02-13 Accepted:2018-06-28 Online:2018-10-01 Published:2018-11-19
  • Contact: Mohan Bahadur Chand E-mail:mohanchand06@gmail.com

Abstract:

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.

Key words: Lirung Glacier, Himalaya, debris-covered glacier, degree-day factor, thermal conductivity, temperature gradient

Figure 1

(a) Geographic context of the study area within Nepal; (b) The Lirung catchment, with the Lirung debris-covered glacier showing the location of thermistors, ablation stakes, and AWS. Backdrop is Sentinel-2A false-color composite from October 15, 2017"

Table 1

Position of the thermistor sensors in each season and their respective elevation on the glacier"

Elevation (m a.s.l.) Monsoon (2013) Winter (2013) Pre-monsoon (2014)
Site 1 Site 2 Site 1 Site 2 Site 1 Site 2
4,093 4,156 4,093 4,196 4,093 4,196
Depth from surface (m) 0.1 0.05 0.0 0.0 0.0 0.0
0.2 0.15 0.1 & 0.2 0.1 & 0.2 0.1 0.1
0.3 0.35 0.4 0.4 0.4 0.4
Total debris thickness (m) 0.38 0.42 0.4 0.4 0.4 0.4

Figure 2

Supco thermistors installed at three different debris thickness (0.0 m, 0.2 m, and 0.4 m) in the winter season. The left panel shows the position of the thermistor at the AWS site and the right panel shows the position of the thermistor above the AWS site"

Table 2

Ablation stakes used for ice melt measurements at different debris thickness and elevation of the glacier in different seasons"

Stake No. Season Debris thickness (m) Elevation (m)
1 Monsoon and pre-monsoon 0.003 (Dirty ice) 4,097
2 Winter 0.02 4,095
3 Monsoon, Winter and Pre-monsoon 0.05 4,070
4 Monsoon and winter 0.10 4,070
5 Pre-monsoon 0.16 4,094
6 Monsoon and pre-monsoon 0.20 4,093
7 Winter 0.23 4,136
8 Monsoon 0.25 4,157
9 Winter 0.28 4,109
10 Pre-monsoon 0.35 4,154
11 Monsoon 0.38 4,093
12 Monsoon 0.42 4,154

Figure 3

Debris temperature profiles for three different seasons of 2013 (monsoon and winter) and 2014 (pre-monsoon). Measurement location near AWS was at 4,093 m a.s.l. (Site 1) for each season (represented by a, b, c), while location was at 4,156 m a.s.l. for the monsoon season (Site 2, represented by d) and at 4,196 m a.s.l. for the winter and pre-monsoon seasons (Site 2, represented by e and f)"

Figure 4

Variation of the debris temperature measured at 1-hour intervals by thermistors in three different seasons of 2013 and 2014 at the AWS site. Scatter plot show the variation of the debris temperature from the mean for (a) monsoon, (b) winter, and (c) pre-monsoon seasons and red-colored line represents the mean temperature at each measurement point within the debris"

Figure 5

Mean daily cycles of debris temperatures at various layers of the debris on Lirung Glacier during 2013 (a) monsoon, and (b) winter, and 2014 (c) pre-monsoon seasons. Temperature measurements from both sites, i.e., AWS site and above AWS site were plotted for the monsoon season and data from the AWS sites were plotted for the winter and pre-monsoon seasons "

Figure 6

Averaged debris temperature profiles for different time of the day at the different debris thicknesses that was measured at 0.10, 0.20 and 0.30 m in the (a) monsoon season and at 0 m (surface), 0.10 and 0.40 m during the (b) winter and (c) pre-monsoon seasons"

Figure 7

Scatter plots of the first derivative of hourly temperature with time (dT/dt) against the second derivative of hourly temperature with depth (d2T/dz2). The total debris thickness of debris was 0.38 m for the monsoon season and 0.40 m for the winter season at the AWS site. The time period for the assessment of the derivatives: September 24 to October 3 2013 for (a) monsoon, and December 1 to 4 2013 for (b) winter. The slope of the best fitting line gives an approximation of the thermal diffusivity "

Figure 8

Degree-day factors for the monsoon, winter, and pre-monsoon seasons. The time period for the estimation of degree-day factor was from September 20 to October 3, 2013, November 30 to December 12, 2013, and from April 7 to 16, 2014 for the monsoon, winter, and pre-monsoon seasons, respectively"

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