Hydrological Performance of Uncorrected CORDEX-SA Climate Model Outputs Across Glacier, Snow and Rain–Snow Regimes in the Upper Indus Basin

Six river basins of the Upper Indus Basin (UIB), Pakistan, representing glacier-dominated (Hunza and Shyok), snow-dominated (Gilgit, Chitral, and Astore), and rain–snow-mixed (Swat) hydroclimatic regimes are examined to evaluate the suitability of uncorrected CORDEX-SA climate data for hydrological modelling. Data scarcity and complex high-mountain topography make it difficult to obtain reliable inputs, and although bias correction is widely applied to regional climate model (RCM) outputs, it can distort the climate signal and introduce additional uncertainties. This study investigates the extent to which uncorrected outputs from the CORDEX South Asia ensemble can directly simulate daily streamflow in a data-scarce mountainous environment. Nine GCM–RCM combinations, comprising five global climate models and three regional climate models (COSMO-crCLIM-v1-1, REMO2015, and RegCM4-7), are used to drive the HBV-IANIGLA snow–glacier hydrological model over 1981–2005, with calibration from 1981 to 1999 and validation from 2000 to 2005. Several CORDEX-SA members reproduce daily streamflow with acceptable skill without bias correction, but performance is strongly regime-dependent. Snow-dominated basins perform best, followed by glacier-dominated basins, while the rain–snow-mixed basin remains most challenging. COSMO-crCLIM-v1-1 and RegCM4-7 outperform REMO2015 on average. The results indicate that uncorrected CORDEX-SA members can support hydrological assessments in snow-dominated catchments, whereas glacier-dominated and monsoon-influenced basins require additional treatment of climate forcing.