Snow Cover Variability in the Cantabrian Mountains (Spain): A Watershed-Level Study Using Satellite Records (2000–2024)

This study presents an analysis of the main snow cover dynamics in the Cantabrian Mountains (northern Spain) using satellite imagery, examining the snow cover dates of appearance and melting, extent, duration and persistence. The study area comprises 36 hydrographic watersheds. Using Google Earth Engine (GEE), 14,082 satellite images (2000-2024) from MODIS-Terra, Landsat 5-8, and Sentinel-2 were analysed to create daily snow cover classifications. Seasonal series of Snow Cover Fraction (SCF) were extracted by 500-meter elevation intervals in each watershed and analyzed to extract indicators and trends. Results reveal reductions in snow cover extent. In autumn, it is reduced at ~-2%/decade above 1,500 m. Notable and significant negative trends (~-10% and up to -16%/decade in some basins) were detected in winter, particularly on the southern slopes of the Cantabrian Mountains. In spring, most basins show negligible and homogeneous trends among watersheds, except above 2,000 m, where pronounced reductions in extent (2.5% per decade) are observed. A shortening snow season is detected, caused by earlier occurrences of the Last Ephemeral Snow Day (LESD), occurring 2.7 days by decade earlier vs 0.4 days by decade earlier in case of the First Ephemeral Snow Day (FESD). The duration of the first snow cover of the season decreased noticeably above 1,500 m (9 days/decade). Peak seasonal snow cover extent tends to occur slightly later, and above 2,000 m. These events are markedly shorter due to a delay in the Snow Onset Day (SOD) and earlier Snow Melt Out Day (SMOD). The maximum SCF occurs between January 22nd and February 5th, depending on altitude, and is shifting earlier, especially at lower elevations. The mean snow cover duration is 16.4 days, with notable altitudinal variability (6.6 days at 500–1000 m and 38.5 days above 2,000 m), decreasing by 1 day/decade, with reductions up to 5.8 days/decade above 2,000 m, where the duration of the longest snow cover has decreased 8 days/decade. Snow cover persistence has declined by 1.2%/decade, with sharper reductions (3.4%) above 1,500 m. Despite biases from prolonged periods of cloud cover, dense canopy cover in some watersheds or the occurrence of rapid snow accumulation and melting events undetected by the satellites, findings ultimately reveal decreases in the duration, extent and persistence of snow cover since the early 21st century, although some of these are not statistically significant. These results highlight shifts in seasonal snow cycles, emphasizing the need for further research with longer time series and alternative observational datasets.