Abstract
Abstract. We provide the first synoptic view of the drainage system of a Himalayan debris-covered glacier and its evolution through time, based on speleological exploration and satellite image analysis of Ngozumpa Glacier, Nepal. The drainage system has several linked components: (1) a seasonal subglacial drainage system below the upper ablation zone; (2) supraglacial channels, allowing efficient meltwater transport across parts of the upper ablation zone; (3) sub-marginal channels, allowing long-distance transport of meltwater; (4) perched ponds, which intermittently store meltwater prior to evacuation via the englacial drainage system; (5) englacial cut-and-closure conduits, which may undergo repeated cycles of abandonment and reactivation; and (6) a "base-level" lake system (Spillway Lake) dammed behind the terminal moraine. The distribution and relative importance of these elements has evolved through time, in response to sustained negative mass balance. The area occupied by perched ponds has expanded upglacier at the expense of supraglacial channels, and Spillway Lake has grown as more of the glacier surface ablates to base level. Subsurface processes play a governing role in creating, maintaining, and shutting down exposures of ice at the glacier surface, with a major impact on spatial patterns and rates of surface mass loss. Comparison of our results with observations on other glaciers indicate that englacial drainage systems play a key role in the response of debris-covered glaciers to sustained periods of negative mass balance.
Highlights
Debris-covered glaciers in many parts of the Himalaya have undergone significant surface lowering in recent times (Kääb et al, 2012), with net losses of several tens of metres since the 1970s (Bolch et al, 2008a, 2011)
The seasonal variations in ice velocities in the upper ablation zone are too large to be explained by changes in ice creep rates, which would require fluctuations in driving stress that are inconsistent with the observed surface elevation changes on the glacier (Thompson et al, 2016)
Much of the upper ablation area of Ngozumpa Glacier consists of icefalls with surface gradients up to 30◦, and fields of transverse crevasses occur across the entire width of the W branch down to an elevation of 5270 m (Fig. 9a)
Summary
Debris-covered glaciers in many parts of the Himalaya have undergone significant surface lowering in recent times (Kääb et al, 2012), with net losses of several tens of metres since the 1970s (Bolch et al, 2008a, 2011). Glacier thinning and reduced surface gradients have resulted in lower driving stresses and ice velocities, and large parts of many glaciers are stagnant or nearly so (Bolch et al, 2008b; Quincey et al, 2009) These morphological and dynamic changes have encouraged formation of supraglacial ponds and lakes and increased water storage within glacial hydrological systems (Quincey et al, 2007; Benn et al, 2012). Feature tracking on TerraSAR-X imagery is used to detect regions of the glacier subject to seasonal velocity fluctuations, as a proxy for variations in subglacial water storage Taken together, these methods provide the first synoptic view of the drainage system of a large Himalayan debris-covered glacier and its influence on glacier response to recent warming
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