[A-HW24] Human- and Climate-induced variability in water cycle and (sub)surface water resources

[AHW24-08] Multiscale Slow-flow Dynamics in Mountainous Catchments of Nepal Himalayas.

*Sanjay Kumar¹, Pankaj Dey¹, Brijesh Kumar Yadav¹ (1.Department of Hydrology, Indian Institute of Technology Roorkee, India)

Keywords:Slow-flows, Snow contribution, Hydrologic signatures, Glacier and Mountain Hydrology

Streamflow is composed of fast-flow – runoff generated after the rain event, and slow-flow – the contribution of the delayed flow from the catchment. Slow-flow is a vital component, as it sustains the flow in the river during the dry season. The slow-flow contribution to streamflow in mountainous watersheds is influenced by landscape, geomorphology and climate. The slow-flow contributions are further impacted in glaciated and non-glaciated catchments, where changes in glacier melt and associated recharge processes are involved. In this work, the slow-flow dynamics and its contribution to streamflow are investigated in a mountainous catchment of Nepal Himalayas – West Seti River basin. Slow-flow was estimated from daily streamflow records using numerical techniques, including graphical methods and digital filters. Slow-flow signatures – percentiles describing frequency distribution of slow-flow – are introduced to characterize the variability of observed slow-flow. The baseflow Index, seasonality ratio, concavity index and slope of the slow-flow duration curve were used to estimate seasonality, stability and sensitivity of slow flow. The results show that the annual slow-flow contribution to streamflow in glaciated catchments (65%) is lower than in non-glaciated catchments (72%). A declining trend in slow-flow signatures is observed during summer and winter seasons, particularly in moderate regimes (represented by the 1^st, 33^rd, and 50^th percentiles of slow-flow frequency distribution), which is attributed to decreasing snow-cover induced by climate change in glaciated catchments. However, glaciated catchments exhibit more stable slow-flows with sustained low-flow periods due to the consistent release of meltwater from glaciers, providing a buffer against seasonal low flows. In contrast, the consistency of slow-flow in non-glaciated catchments varies significantly with their shape (quantified by elongation ratio) and location within the lesser and higher Himalayan geological formations. Glaciated catchments are characterized by summer-dominant slow-flow, driven largely by snow and ice melt, whereas, non-glaciated catchments experience winter-dominant slow-flows, which are primarily governed by delayed subsurface flow. Overall, the study highlights the importance of monitoring slow-flow dynamics to improve water resource management in mountainous regions, where the interplay of natural and anthropogenic factors is leading to increasingly variable streamflow patterns.