Climatic Signals of Drying in a Himalayan Watershed: A Multi-grid Precipitation Analysis of Bhilangana River Basin

Abstract

Regional-scale assessments often reveal that climate responses vary sharply depending on elevation, land use, and topographic influences emphasizing the need for high-resolution, basin-specific evaluations. To detect and quantify long-term climatic trends, non-parametric statistical methods are commonly used due to their robustness and distribution-free nature. This study examines precipitation trends over the Bhilangana river basin in the Garhwal Himalaya using reanalysis gridded data from TerraClimate for 35-year duration from 1990 to 2024. This study initially examines the presence of serial correlation in all data series using the lag-k autocorrelation coefficient (rk) at a significance level of 0.05 for a two-tailed test. A number of statistical methods, including the Mann-kendall test, Modified MK test, and Sen’s slope estimator were applied to monthly, seasonal, and annual time series across four representative grids at different elevation ranges. While no trends were statistically significant at the 95% confidence level, several consistent patterns emerged. The most pronounced declines were observed in the winter and pre-monsoon seasons, with Sen’s slope estimates revealing annual decreases up to -3.34 mm/year, particularly in high elevation grids. Strong negative Z-values in March and November (Z < -1.5) further support a shift toward drier conditions during transitional months. Meanwhile, post-monsoon and late monsoon months exhibited weak but spatially coherent positive trends, hinting at possible shifts in rainfall distribution. The absence of significant autocorrelation confirms the robustness of the trend analysis. Spatially, precipitation shows a clear elevation gradient and strong seasonal contrasts, with monsoon rainfall contributing over 60% of the annual total. Overall, while trends remain below the threshold of statistical significance, the results point to a gradual and emerging drying pattern with implications for hydrological sustainability, climate change, and water resource planning in the study area. The findings suggest a transition toward a drier and more variable precipitation regime, with critical implications for snow accumulation, glacier mass balance, baseflow generation, and water availability during the dry season. As such, these observed patterns, though not statistically conclusive, are scientifically robust and demand closer monitoring to support adaptive water resource planning and climate-resilient hydrological management in this ecologically sensitive Himalayan basin.