Summary
A team of researchers from the
University of Leeds and the University of Sheffield recently completed a four
week field campaign on the Khumbu Glacier in Nepal. The Khumbu Glacier is the
highest in the world and every year a small section of the upper glacier
becomes the home to Everest Basecamp in Nepal.
Access to the Khumbu valley was
by a five day walk with two additional acclimatisation days along the Everest
Basecamp trail. Our team camped just off-glacier, a short walk from a small
number of trekking lodges at Lobuche. Logistical support and research
permissions were organised by Himalayan Research
Expeditions. Our guides were invaluable on the glacier and the kitchen team were always ready with hot food on our return!
Data collection involved
Structure-from-Motion ice cliff surveys, GCP georeferencing, and supraglacial
pond depth surveys and instrumentation.
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| Heading along the trail |
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| Our campsite following snowfall |
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| Looking towards the Khumbu Glacier |
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Background
It is widely known that Himalayan
Glaciers in this region are losing mass year on year, though the presence of
rocky debris on the surface of glaciers prolongs their response to climate
change. The debris cover, which is generally thickest at the terminus of a
glacier and becoming thinner at higher elevations, changes the spatial
distribution of maximum surface lowering, which occurs where debris is thinner
owing to the insulating effect of a thick rock cover. The ablative role of
supraglacial ponds and ice cliffs, which are widespread on such glaciers, is
little quantified. This is predominantly owing to difficult and hazardous
access for collecting field data. Ponds and ice cliffs therefore form the basis
of my research on the Khumbu Glacier.
Ongoing remote sensing analysis
from fine-resolution satellite imagery is been used to reveal
multi-temporal supraglacial pond dynamics by semi-automatically classifying
water bodies. An increasing trend observed on other glaciers in the region is of interest and concern for several
reasons. Large glacial lakes forming at the terminus of debris-covered glaciers
can pose a potential outburst flood risk in some circumstances, requiring
monitoring and remediation efforts to avoid a high-magnitude flood which can
travel long distances downstream. Supraglacial water storage also has the
potential to mitigate increases in meltwater generated under a warming climate.
Ponded water also absorbs incoming solar radiation and this thermal energy is
transmitted to the ice below, although this may be through a saturated sediment
and debris layer. Exposed ice cliffs often exist adjacent to dynamic ponds and
may feature a thin debris layer, reducing their albedo and hence increasing their
capacity to melt. Capturing pond and ice cliff dynamics using satellite imagery
alone is not possible, owing to revisit times, potential cloud cover and
illumination issues, and cost of acquisition. Field access to the features
permits surveys and instrumentation to be left in situ to allow continuous
monitoring. This is particularly important in supraglacial ponds which exhibit
a diurnal thermal regime and can drain englacially, transmitting the stored
thermal energy into the glacial interior.
Field monitoring
My field strategy involved repeat
Structure-from-Motion (SfM) surveys of ice cliffs, dGPS ground control point
identification, and the deployment and retrieval of thermistor strings and
pressure transducers in several supraglacial ponds.
Ice cliffs
SfM is a way of generating
fine-resolution 3d models of a surface using photographs from a standard camera
which are taken at different positions. The technique was implemented using
ground surveys around the ice cliff, although airborne surveys are equally
possible and are more time efficient. In this case we did not have access to an
aerial platform and helicopter traffic to Everest Basecamp would likely restrict permissions for
deployment. A range of cliff sizes, aspects, and locations was captured to
allow comparisons of melt rate and morphological evolution. Each survey
required a distribution of GCPs around the ice cliff before the photographic
survey could be undertaken. GCP markers were distributed and georeferenced with
a dGPS on the first ‘lap’ of the ice cliff. Photographs would then be taken
during one or two more circuits of the cliff to allow a range of vantage points
including high and low viewpoints. GCPs would then be collected on a final
circuit. The surface of the dynamic areas of the glacier studied were generally
rugged and unstable which limited surveys to two cliffs on a given day.
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| One of the ice cliffs and ponds surveyed |
GCP georeferencing
Velocity measurements of glaciers
are generally conducted using remotely sensed imagery. On debris-covered
glaciers this can be with optical or radar imagery. Typically the availability of appropriate imagery means
velocities below 10 m per year cannot be resolved and these regions are defined as
‘stagnant’. Recently it was shown using fine-resolution imagery from an
unmanned aerial vehicle that this categorisation may only loosely be applied,
since notable surface motion may still occur. During the
Khumbu field campaign I identified a number of boulders distributed in the
lower ablation area of the glacier which were georeferenced with a dGPS. A
repeat survey in May and October 2016 will reveal both horizontal and vertical
displacement, which can be used to validate remotely sensed observations since
the precision is far greater (on the order of mm - cm).
Pond surveys
Pond surveys were tailored to
assessing water storage dynamics and thermal characteristics. Thermistor
strings with temperature loggers at 1 m intervals were used to monitor temperature changes, in addition to a pressure transducer to capture water level
change. Most ponds encountered were partially frozen at the start of the field
campaign, limiting measurements of depth, which were taken with a plumb line. In
May 2016 a robotic surface water vehicle will be deployed with the aim of
obtaining fully distributed depth and temperature measurements.
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| Conducting a pond survey |
Most ponds were frozen on the surface by the end of the campaign,
requiring access though up to 10 cm of ice for instrument retrieval.
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| Instrument retrieval on a frozen pond |
Scott
