Future projections of cryosphere degradation and its hydrological impacts in the upper Bulgan River basin, Altai Mountains

Study region The nested Baitag and Duchinjil catchments in the upper Bulgan River basin, Altai Mountains, western Mongolia, have small glacier coverage and widespread permafrost. Study focus FLEX-Cryo model was used to assess future cryosphere and runoff changes. The model was evaluated using runoff and cryospheric data. Bias corrected projections from 13 CMIP6 models were used separately to drive the model under SSP2–4.5 and SSP5–8.5. New hydrological insights for the region Ensemble mean annual runoff shows no significant trend during 2024–2100. Winter runoff increases, summer runoff decreases, and runoff timing advances in both catchments. Under SSP2–4.5 and SSP5–8.5 scenario, glaciers persist as remnants in Baitag at the end of the century but disappear in Duchinjil around 2060. At the Duchinjil station, annual maximum snow depth increases while snow cover duration decreases under both scenarios. Permafrost area decreases in Baitag under both scenarios. Mean active layer thickness increases from approximately 2.25 m to 2.95 m under SSP2–4.5 and 3.42 m under SSP5–8.5 by 2099. Summer runoff decreases approximately twice as fast in upstream Duchinjil as in Baitag. Under SSP5–8.5, peak runoff advances by 18 days in Baitag and 38 days in Duchinjil between 2030–2034 and 2090–2094. Hydrological assessments in similar cold and arid mountain catchments should emphasize seasonal runoff changes and runoff timing.