A team of researchers from the University of Leeds and the University of Sheffield recently completed a four week field campaign on Khumbu Glacier in Nepal. 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.
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 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.
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.
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.
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 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.
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.