Interannual Variability of Ephemeral Snow and Its Water Equivalent in a Mexican Mediterranean Mountain Region

Increasing temperature and decreasing precipitation threaten the extent, persistence, and dynamics of snow across spatial scales, particularly ephemeral snow in Mediterranean mountain regions. This study estimates ephemeral snow cover and snow water equivalent (SWE) in the Sierra de San Pedro Mártir, Baja California, Mexico, using open-access datasets and remote sensing. Camera trap images and limited in situ data were used to calibrate the normalized difference snow index (NDSI) for snow detection and to estimate SWE and topographic effects on SWE from 2002 to 2023, encompassing wet, dry, and normal years. The optimal NDSI threshold for snow detection was 6.4 for MODIS Terra and 5.3 for MODIS Aqua, substantially lower than thresholds commonly reported for seasonal snowpacks in forested regions. In wet years, snowfall contributed up to 20% of annual precipitation, compared with ~13% in dry years. In normal years, the average SWE is 70 mm (24% of annual precipitation). SWE increased by 30% (91 mm) during wet years and decreased by 21% (55 mm) during dry years. Eastness (aspect) was the only statistically significant topographic predictor of SWE for MTerra, with higher SWE values observed on west-facing slopes. This study provides the first quantitative assessment of ephemeral SWE dynamics in a Mexican Mediterranean mountain system and establishes a framework for monitoring marginal snowpacks under increasing climatic variability.