Modeling Runoff Components and Assessing Future Water Availability Due to Climate Change in the Karnali Basin Using the SPHY Model

Abstract

This study evaluates the hydrological behavior and future water availability of the Karnali basin under changing climatic conditions using a distributed hydrological modeling approach. Despite the basin’s importance for water resources and hydropower development in Nepal, there is no significant study of the relative contributions of different runoff components and their response to future climate change. The Spatial Processes in Hydrology (SPHY) model was applied to simulate rainfall runoff, snowmelt, glacier melt, and baseflow contributions. The model was calibrated for the period 2005–2014 and validated for 2015–2022. The model performance was found to be satisfactory, with Nash–Sutcliffe Efficiency (NSE) values of approximately 0.79 during calibration and 0.83 during validation, coefficient of determination (R2) values exceeding 0.75, and Percent Bias (PBIAS) within acceptable limits (-5.1%- +7.19%). The results indicate that rainfall runoff is the dominant contributor, accounting for approximately 53% of total streamflow, followed by baseflow (16.4%), while snowmelt and glacier melt contribute around 27% and 3.3%, respectively. Snow and glacier melt contributions are particularly significant during the pre-monsoon and early monsoon periods. Further, future streamflow projections were also carried out using bias-corrected CMIP6 climate data under SSP245 and SSP585 scenarios. The results suggest an increase in mean annual streamflow by 24.2% and 58.3% for mid-future climate and by 28.4% and 62.1% for far-future climate under SSP245 and SSP585 respectively relative to the baseline period (1985–2014). Seasonal analysis indicates a potential increase in peak monsoon flows by 25.92% and 46.2% under SSP245 and SSP 585 respectively by the end of the century, while changes in dry season flows remain comparatively moderate. Uncertainties associated with climate projections, model structure, and input data remain, particularly in representing cryosphere processes. However, the findings provide important quantitative insights into water resource planning and hydropower development in the Karnali Basin under future climate conditions.