Climate Change Impacts on Tributary-Scale Water-Balance Partitioning and Hydrological Resilience in the Koshi River Basin, Himalaya

High-mountain river basins are critical for downstream water security, yet climate change is altering snow storage, precipitation, and runoff regimes across their steep elevation gradients. The Koshi River Basin (KRB), a large transboundary Himalayan system, exemplifies this challenge, but how CMIP6-driven forcing reorganizes tributary-scale water-balance partitioning and hydrological resilience across the KRB remains poorly understood, since existing studies mostly rely on pre-CMIP6 scenarios or basin-outlet responses, with little attention to internal partitioning or rainfall-versus-snow tributary contrasts. This study quantifies baseline water-balance structure and evaluates how future climate change modifies the partitioning of precipitation into evapotranspiration, snowmelt, and water yield across six major KRB tributaries and the basin outlet, using a multisite-calibrated ArcSWAT model at seven gauging stations forced with bias-corrected CMIP6 projections from ACCESS-CM2 and MPI-ESM1-2-LR (SSP2-4.5, SSP5-8.5; 2035-2060 relative to 1984-2009). Baseline conditions show a runoff-dominated, energy-limited regime, with water yield consistently exceeding evapotranspiration and seasonal snowmelt contribution in high-elevation tributaries. Future projections indicate robust warming and scenario-dependent precipitation responses, producing spatially heterogeneous changes in water yield, declining snow buffering, and increasing interannual variability. Precipitation elasticity of runoff often exceeds unity, and basin-outlet responses mask substantial tributary-scale contrasts, underscoring the need for tributary-focused water-balance assessment in Himalayan climate-adaptation planning.