Using PlanetScope images to investigate the evolution of small glaciers in the Alps

Small glaciers (having an area smaller than 1 km2) dominate in terms of total number in the Alps and it is important to study their evolution. In this study we assessed the potential of daily, high resolution (3 m pixel size) PlanetScope images for investigating small Alpine glaciers. We compared the evolution of five small glaciers, covering an area lower or slightly higher than 1 km2, located in different areas of the Alps and having different characteristics and meteorological conditions. The investigated glaciers are: Bors (0.645 km2, Piedmont, Italy), Seewjinen (1.339 km2, Valais, Switzerland), Pizzo Ferre (0.442 km2, Lombardy, Italy), Gran Zebra (0.579 km2, Lombardy, Italy) and Solda Orientale (0.829 km2, South Tirol, Italy). We manually mapped the glacier outline and the highest position of the transient snowline (TSL) through PlanetScope images from the end of the ablation season for each year from 2017 to 2021 and each glacier. The snow cover area ratio (SCAR) was retrieved using the TSLs. We found that the glaciers lost between 4 and 14% of their area between 2017 of 2021, with a mean area loss of -1.5% per year. The strongest shrinkage (14%) is recorded at the smallest glacier (Pizzo Ferre). The highest TSL showed a variability from year to year. For two glaciers, a supervised classification of the surface facies (snow, ice and supraglacial debris) was performed on all the available cloud-free images from the 2017-2021 ablation seasons. PlanetScope images, with their high spatial and temporal resolution proved to be a valuable tool for studying small glaciers. They permitted to map small glaciers with small uncertainties (between 2.2 and 5.1% of the glacier area), much lower than with medium resolution satellite data (e.g. Landsat 15/30 m, Sentinel 10 m) and to increase the number of images available for analysis. The daily repeat cycle is well suited for increasing the probability to find images from the end of the ablation season with minimum snow cover, in order to identify the real end-ofsummer snowline.