Abstract
The hydrological systems of heavily-downwasted debris-covered glaciers differ from clean-ice glaciers due to the hummocky surface and debris mantle of such glaciers, leading to a relatively limited understanding of drainage pathways. Supraglacial ponds represent sinks within the discontinuous supraglacial drainage system, and have been documented to sporadically drain englacially. To assess pond dynamics, pond water level measurements were made on Lirung Glacier during May and October of 2013 and 2014. The four field seasons coincided with aerial, satellite, and terrestrial orthomosaic images and digital elevation models, which provided snapshots of the ponds and their surroundings. We analysed the glacier's closed surface catchments to identify surface drainage pathways and englacial drainage points, and compared this to field observations of surface discahrge. The ponded area was higher in the pre-monsoon than post-monsoon, with individual ponds filling and draining seasonally associated with the surface exposure of englacial conduit segments. We recorded four pond drainage events, all of which occurred gradually (duration of weeks), observed diurnal fluctuations indicative of varying supply and discharge, and we documented instances of interaction between distant ponds. The DEM drainage analysis identified numerous sinks >3m across the glacier surface, few of which exhibited ponds (23%), while the field survey highlighted surface discharge only explicable via englacial routes. Taken together our observations provide evidence for widespread supraglacial-englacial connectivity for meltwater drainage paths. Results suggest that progressive englacial conduit collapse events, themselves likely driven by supraglacial pond drainage, enable the glacier surface to evolve into a configuration following relict englacial conduit systems. Within this system, ponds form in depressions of reduced drainage efficiency and link the supraglacial and englacial drainage networks.
Highlights
Debris-covered glaciers make up a minority of mountain glaciers worldwide, but debris-covered areas can account for a considerable portion of total glacier area regionally (Sasaki et al, 2016)
This is evident in High Mountain Asia, where inhabitants are strongly dependent on the hydrology of glacierized catchments for moderating seasonal and interannual variations in water availability (Bolch et al, 2012; Immerzeel and Bierkens, 2012; Ragettli et al, 2016b; Pritchard, 2017)
Rapid downslope transport of surface water occurs in areas of steep relief, but the surface debris may lead to the formation of shallow groundwater lenses in lowslope areas
Summary
Debris-covered glaciers make up a minority of mountain glaciers worldwide, but debris-covered areas can account for a considerable portion of total glacier area regionally (Sasaki et al, 2016). This is evident in High Mountain Asia, where inhabitants are strongly dependent on the hydrology of glacierized catchments for moderating seasonal and interannual variations in water availability (Bolch et al, 2012; Immerzeel and Bierkens, 2012; Ragettli et al, 2016b; Pritchard, 2017). The variable relief of heavily-downwasted debris-covered glaciers often leads to frequent intersection between the glacier surface and englacial conduits, whether active or disused (relict), resulting in discontinuous supraglacial and englacial hydrological networks, and exchange of surface and near-surface water across the glacier (Benn et al, 2012, 2017)
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