Future hydrology of a Himalayan basin shaped by elevation dependent shifts in water balance components

Pranisha Pokhrel , Philip D. A. Kraaijenbrink, Jasper Griffioen , Thom Bogaard, Walter Immerzeel

Mountainous basins in the Himalayas serve as critical ``water towers'' sustaining downstream livelihoods, yet their hydrological response to climate change is complex due to extreme elevation gradients. This study investigates the future hydrology of the Karnali basin (situated in Western Nepal) by disaggregating water balance components along its elevation profile. Using the fully distributed hydrological model SPHY forced by five CMIP6 climate models under three emission scenarios (SSP1-2.6, SSP3-7.0, and SSP5-8.5), simulations were conducted for mid century (2041--2070) and late century (2071--2100) periods relative to a 1991--2020 reference. Projections indicate a warmer and wetter future, with mean annual precipitation increasing by 19\% to 38\% and temperatures rising by 1.5 $^\circ$C to 5.1 $^\circ$C by the late century. A widespread shift from snowfall to rainfall is projected, with the most significant declines in snowfall fraction occurring between 2,000 and 4,000 m. While total annual discharge at the basin outlet is projected to increase by 35\% to 59\% by 2100, this masks a critical internal reorganization of the water balance. Snowmelt contributions are projected to decline and peak up to 43 days earlier under high-emission scenarios, reducing water yield in climate sensitive zones between 3,000 and 5,000 m. These losses are offset by substantial increases in rainfall runoff and baseflow at lower elevations, specifically between 1,000 and 2,000 m. This elevation dependent shift transforms the basin into a more rainfall dominant system, weakening its natural buffering capacity and increasing sensitivity to precipitation variability. These findings suggest heightened risks of hydrological extremes and highlight the necessity of elevation based analysis to capture the spatial redistribution of water components in mountainous basins.