Assessing climate change impact on glacier runoff in the Upper Yukon Watershed: A Comparative study of CMIP5 and CMIP6 projections

As climate change continues to drive hydrological shifts, understanding the evolving contributions of glaciers to discharge becomes crucial for effective water resource management. Most previous studies in mountainous regions have relied on models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) to analyze future discharge. However, there is a lack of focus on comparing these hydroclimatic assessments with those driven by recent CMIP6 models in these regions. The present study project the future contribution of glaciers to discharge using outputs of CMIP5 and CMIP6 models over the glacierized Upper Yukon Watershed (UYW). First, the glacier model parameters of a semi-distributed hydrological model were calibrated using observed daily discharge over the period 2013–2018. Then, the glacier mass losses from the hydrological model were compared with estimates from elevation changes. Finally, climate change projections of glacier contribution to discharge were performed using ten models based on the Representative Concentration Pathways RCP4.5 and RCP8.5 (CMIP5), and fourteen simulations based on the Shared Socioeconomic Pathways SSP2-4.5 and SSP3-7.0 (CMIP6). The considered temporal horizons were 2011–2040, 2041–2070 and 2071–2100, with 1988–2018 as the reference period. During the calibration period, glacier mass changes from the hydrological model compared well with those from elevation changes, while simulated discharge for the reference period from RCP4.5, RCP8.5, SSP2-4.5 and SSP3-7.0 scenarios closely matched those observed. However, glacier contributions from CMIP6 scenarios over the reference period were more aligned with previous estimates compared to those from the CMIP5 scenarios. Despite a decrease in glacier contributions from CMIP6 scenarios, the long-term annual average volumes of discharge increased over time due to simultaneous increase in temperature and precipitation. These results highlight the consistency of using CMIP6 simulations and demonstrate that decreasing glacier contributions will not affect the annual volume of water in the UYW.