Warming waters of the Central Karakoram: modeling streamflow responses of the Shigar Basin, northern Pakistan to climate change

Glacial mountain watersheds are among the most climate-sensitive hydrological systems, where even minor shifts in temperature or precipitation can trigger significant changes in water availability, flood potential, and ecosystem stability. This study quantifies the projected impacts of climate change on the hydrology of the Shigar River Basin, Central Karakoram, Pakistan, using the Soil and Water Assessment Tool (SWAT) coupled with CMIP6 climate scenarios. Historical hydro-meteorological data from 1985 to 1994 were used for model calibration and validation, yielding robust performance indicators (R² = 0.78, NSE = 0.70, and PBIAS = 20.1% during calibration; R² = 0.69, NSE = 0.60, and PBIAS = 11.5% during validation). Future projections (2020–2050) were analyzed using the GFDL-ESM4 and MIROC6 models under three Shared Socioeconomic Pathways (SSP-2.6, SSP-4.5, and SSP-8.5). Results show a basin-wide warming trend of 0.3–0.6 °C per decade in mean annual temperature, with more pronounced increases in minimum temperatures and a corresponding shift from snowfall to rainfall dominance. Under the high-emission SSP-8.5 scenario, annual precipitation is projected to increase by 12–18% relative to the baseline, while MIROC6 predicts a 25–40% increase in annual precipitation. The simulated streamflow shows an upward shift in both magnitude and variability: median discharge rises from 400 m³ s⁻¹ (historical) to approximately 1,000 m³ s⁻¹ under SSP-8.5, with extreme events surpassing 2,400 m³ s⁻¹. Seasonal analysis reveals earlier and stronger peaks in April–June, indicating accelerated snowmelt and enhanced flood potential.