Temporal Variability and Trend Analysis of Monthly Snow Cover in a Himalayan river basin

Snow cover variability in high-altitude Himalayan river basins plays a critical role in regulating regional hydrology, water availability, and climate-cryosphere interactions. Understanding its temporal behaviour is particularly important for snow-fed basins such as the Budhi Gandaki Basin, Nepal, where seasonal meltwater significantly influences downstream flow regimes. Previous studies often overlooked detailed monthly-scale time series analysis and robust trend diagnostics of snow cover variability. In this study, we investigate the temporal dynamics of monthly snow cover area in the Budhi Gandaki Basin (area: 3857.85 km²) using continuous observations from 2020 to 2024 using the Moderate Resolution Imaging Spectroradiometer (MODIS) 8-day snow cover products (MOD10A2) with 500 m resolution. A comprehensive time-series framework was applied, incorporating descriptive statistics, linear trend analysis, seasonal climatology, anomaly assessment, and non-parametric trend detection. The MOD10A2 snow cover composites were spatially averaged over the basin and temporally aggregated to monthly resolution using a proportional day-overlap weighting scheme to estimate basin-averaged monthly snow covered area (SCA). Monthly snow cover data were then transformed into a continuous time series to evaluate overall variability and long-term behaviour. Seasonal characteristics were quantified through monthly climatology and interannual variability using mean and standard deviation metrics. Trend significance was examined using the Mann-Kendall test, while the magnitude of change was estimated using Sen’s slope. Results reveal a mean snow cover index of 0.56 with substantial variability showing a standard deviation of 0.16 snow cover fraction (SCF), indicating pronounced seasonal and interannual fluctuations. Monthly climatology shows maximum snow cover during winter months, peaking in February (mean: 0.762 SCF) and January (mean: 0.756 SCF), while minimum values occur during the summer monsoon period, particularly in July (mean: 0.424 SCF) and June (mean: 0.426 SCF). Linear trend analysis indicates a gradual declining tendency in snow cover at a rate of -0.0024 SCF per month. However, the Mann-Kendall test yields a Z statistic of -1.77 with a p-value of 0.076, suggesting that the observed decreasing trend is not statistically significant at the 95% confidence level. Anomaly analysis further highlights episodic deviations from the climatological mean, with maximum positive and negative anomalies of +0.204 SCF and -0.162 SCF, respectively, reflecting short-term climate-driven variability. Overall, the findings indicate a weak but persistent declining tendency in snow cover, modulated strongly by seasonal and interannual variability rather than a statistically significant long-term trend. This study provides an improved understanding of snow cover dynamics in the Budhi Gandaki Basin and offers valuable insights for hydrological modeling, climate impact assessments, and sustainable water resource management in snow-fed Himalayan river systems.

Keywords: Snow cover variability; Time series analysis; Himalayan river basin; Seasonal climatology; Trend detection