Changes of surface hydrology of a mountain glacier studied with very high resolution aerial and satellite images and machine learning

The Preludium Project “Changes of surface hydrology of a mountain glacier studied with very high resolution aerial and satellite images and machine learning” was dedicated to studying changes on the surface of the Universidad glacier in Andes in central Chile. The project’s findings are based on satellite and airborne images and digital elevation models. Data from between 2013 and 2022 were used. This period is a time of a historically long drought, which impacted the country’s water resources and glaciers’ state. Based on the images from the Landsat 8 satellite an assessment of sediment accumulation on the glacier’s surface could be performed. These images do not allow for discerning of small objects on the Earth’s surface, but are available for the entire period included in the study. Landsat 8 also provides infrared images recorded in wavelengths not commonly used among satellite cameras. A very accurate airborne image from 2013 was used to create a map of sediment presence on the surface of the glacier. This map, as well as Landsat 8 images from years 2013 to 2022, served to study how the sediment coverage was changing across the years. Analysis carried out across the entire glacier and its several fragments has shown that the sediment cover on the glacier is expanding. This change is correlated with yearly sums of precipitation – in the dry years a higher amount of sediment was detected than in the more humid years. The reason behind the correlation is that rainfall and snowfall wash out and cover accumulating sediments. The process of debris accumulation is particularly intensive on small, secondary glacial cirques and on the glacier’s tongue’s end which is the lowest part of the glacier. In order to create the glacier’s surface maps satellite images sufficiently accurate to recognise individual crevasses on the glacier’s surface were used. Maps of snow cover, sediment cover and bare ice were created using machine learning methods. These maps validated the results of the Landsat 8-based analysis. The part of the glacier in which more snow is accumulated from snowfall than is lost to melting is called the accumulation zone. Visual analysis of images taken at the end of each autumn between 2013 and 2022 allowed to determine the extent of the glacier in each year and assess the size of the accumulation zone. The project has shown that during the multi-year drought the ratio of the accumulation area to the entire glacier’s area decreases. Areas, which earlier contributed to the increase of ice mass switch their role and join the area, where melting is stronger than accumulation. Because of this process the glacier shrinks and its margins retreat. Maps of the glacier’s extent revealed that the glacier’s edge is receding quickly up into the valley. Intensive retreat takes place also in the small cirques, which are in danger of disappearance in the near future. Water from snow and ice melting flows along the glacier’s surface and creates a network of streams similar to river networks on land. One of the goals of the project was to analyse this network on the surface of the Universidad glacier’s tongue. Digital maps of terrain elevation from two years: 2013 and 2022 were used to create maps of stream density (total length of streams divided by unit of area) and mean stream order (a measure of a network’s development). The analysis of these maps shows that on moraines, covered with thick debris, stream density decreased between 2013 and 2022. These are the areas, where streams are fed almost exclusively with rain and snowfall. On the other hand, on revealed glacial ice, where water is provided both by precipitation and melting ice, density of streams and network complexity increased. The conclusion is that in 2022, relative to 2013, the amount of water brought in with precipitation decreased, while the amount of water provided by ice melting has increased. This trend fits meteorological data, which show that in 2022 the precipitation sum was significantly lower, than in 2013, but temperatures were much higher. The project’s results bring a new understanding of the Universidad glacier’s state during a long lasting drought. The glacier’s location in central Chile makes it an important source of water for the region’s inhabitants. Because of that, monitoring of the glacier’s changes across the years brings information valuable for local communities and contributes to future research, including forecasts of future glaciological and hydrological situation.

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