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Title: Spatio-temporal variability and energy-balance implications of surface ponds on Himalayan debris-covered glaciers
Author: Miles, Evan
ISNI:       0000 0004 6056 9258
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2016
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Surface ponds play a key role in transferring atmospheric energy to the ice for debris-covered glaciers, but as the spatial and temporal distribution of ponds is not well documented, their effect on glacier ablation is unknown. This thesis uses remote sensing and field methods to assess the distribution of supraglacial ponds in the Langtang Valley of Nepal, then develops and applies numerical models of pond surface energy balance to determine energy receipts at the pond, glacier, and basin scales. 172 Landsat TM/ETM+ scenes are analysed to identify thawed supraglacial ponds for the debris-covered tongues of five glaciers for the period 1999-2013. There is high variability in the incidence of ponding between glaciers, and ponds are most frequent in zones of low surface gradient and velocity. The ponds show a pronounced seasonality, appearing rapidly in the pre-monsoon as snow melts, reaching a peak area in the monsoon of about 2% of the debris-covered area, then declining in the post-monsoon as ponds drain or freeze. The satellite observations are supplemented by diverse field observations on Lirung Glacier in the Langtang Valley made in 2013 and 2014, confirming that overall pond area is markedly higher in the pre-monsoon than post-monsoon. Four ponds are observed in detail showing pond drainage, stability, and growth. The thesis then advances efforts to develop a model of mass and energy balance for supraglacial ponds, using field data from a small pond on Lirung Glacier. Sensitivity testing is performed for several key parameters and alternative melt algorithms. The pond acts as a significant recipient of energy, and participates in the glacier’s local hydrologic system during the monsoon. The majority of absorbed energy leaves the pond via englacial conduits, delivering sufficient energy to melt 2612 m3 of ice (~5.3 m ablation for the pond area). Energy receipts for all Lirung Glacier ponds for 2014 are then determined, using the full model and simpler approaches based on data availability. The partition of absorbed energy between pond-proximal and englacial melt is inconsistent between ponds, and the shortwave energy balance alone is not adequate to represent pond energy absorption. The model results suggest that ponds absorbed sufficient energy to account for ~10% of Lirung Glacier’s ablation in 2014.Finally, a simplified pond surface energy-balance model is applied to assess pond energy absorption for the entire Langtang catchment, using local meteorological data for 2013 and mean monthly pond distributions from the Landsat observations. Supraglacial ponds are found to absorb sufficient atmospheric energy to account for 5-16% (mean ~12%) of the debris-covered area’s volume loss in 2013 (equivalent to 0.11 m thinning for this area). Less absorption occurs in the pre-monsoon and post-monsoon than in the monsoon due to decreased latent heat exchange. Altitude is an additional control, but seasonal surface energy balance remains positive at the ELA of 5400 m. This research suggests that due to the efficiency of supraglacial ponds as vectors of atmospheric energy to the glaciers’ interior, they may account for a considerable portion of the debris-covered area’s ablation (~10%) in spite of their low aerial coverage (1-2%), and ponds must be accounted for in studies of debris-covered glacier ablation and evolution.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral